EPICS R3.14 Channel Access Reference Manual

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Configuration

Why Reconfigure Channel Access

Typically reasons to reconfigure EPICS Channel Access:

Two independent control systems must share a network without fear of interaction

A test system must not interact with an operational system

Use of address lists instead of broadcasts for name resolution and server beacons

Control system occupies multiple IP subnets

Nonstandard client disconnect time outs or server beacon intervals

Specify the local time zone

Transport of large arrays

All Channel Access (CA) configuration occurs through EPICS environment variables. When searching for an EPICS environment variable EPICS first looks in the environment using the ANSI C getenv() call. If no matching variable exists then the default specified in the EPICS build system configuration files is used.

CA and Wide Area Networks

Normally in a local area network (LAN) environment CA discovers the address of the host for an EPICS process variable by broadcasting frames containing a list of channel names ( CA search messages ) and waiting for responses from the servers that host the channels identified. Likewise CA clients efficiently discover that CA servers have recently joined the LAN or disconnected from the LAN by monitoring periodically broadcasted beacons sent out by the servers. Since hardware broadcasting requires special hardware capabilities, we are required to provide additional configuration information when EPICS is extended to operate over a wide area network (WAN).

IP Network Administration Background Information

Channel Access is implemented using internet protocols (IP). IP addresses are divided into host and network portions. The boundary between each portion is determined by the IP netmask. Portions of the IP address corresponding to zeros in the netmask specify the hosts address within an IP subnet. Portions of the IP address corresponding to binary ones in the netmask specify the address of a host's IP subnet. Normally the scope of a broadcasted frame will be limited to one IP subnet. Addresses with the host address portion set to all zeros or all ones are special. Modern IP kernel implementations reserve destination addresses with the host portion set to all ones for the purpose of addressing broadcasts to a particular subnet. In theory we can issue a broadcast frame on any broadcast capable LAN within the interconnected internet by specifying the proper subnet address combined with a host portion set to all ones. In practice these "directed broadcasts" are frequently limited by the default router configuration. The proper directed broadcast address required to reach a particular host can be obtained by logging into that host and typing the command required by your local operating environment. Ignore the loop back interface and use the broadcast address associated with an interface connected to a path through the network to your client. Typically there will be only one Ethernet interface.

IP ports are positive integers. The IP address, port number, and protocol type uniquely identify the source and destination of a particular frame transmitted between computers. Servers are typically addressed by a well known port number. Clients are assigned a unique ephemeral port number during initialization. IP ports below 1024 are reserved for servers that provide standardized facilities such as mail or file transfer. Port number between 1024 and 5000 are typically reserved for ephemeral port number assignments.

IP port numbers

The two default IP port numbers used by Channel Access may be reconfigured. This might occur when a site decides to set up two or more completely independent control systems that will share the same network. For instance, a site might set up an operational control system and a test control system on the same network. In this situation it is desirable for the test system and the operational system to use identical PV names without fear of collision. A site might also configure the CA port numbers because some other facility is already using the default port numbers. The default Channel Access port numbers have been registered with IANA.

WAN Environment

When the CA client library connects a channel it must first determine the IP address of the server the channels Process Variable resides on. To accomplish this the client sends name resolution (search) requests to a list of server destination addresses. These server destination addresses can be IP unicast addresses (individual host addresses) or IP broadcast addresses. Each name resolution (search) request contains a list of Process Variable names.If one of the servers reachable by this address list knows the IP address of a CA server that can service one or more of the specified Process Variables, then it sends back a response containing the server's IP address and port number.

During initialization CA builds the list of server destination addresses used when sending CA client name resolution (search) requests. This table is initialized by introspecting the network interfaces attached to the host. For each interface found that is attached to a broadcast capable IP subnet, the broadcast address of that subnet is added to the list. For each point to point interface found, the destination address of that link is added to the list. This automatic server address list initialization can be disabled if the EPICS environment variable "EPICS_CA_AUTO_ADDR_LIST" exists and its value is either of "no" or "NO". The typical default is to enable network interface introspection driven initialization with "EPICS_CA_AUTO_ADDR_LIST" set to "YES" or "yes".

Following network interface introspection, any IP addresses specified in the EPICS environment variable EPICS_CA_ADDR_LIST are added to the list of destination addresses for CA client name resolution requests. In an EPICS system crossing multiple subnets the EPICS_CA_ADDR_LIST must be set so that CA name resolution ( search requests ) frames pass from CA clients to the targeted CA servers unless a CA proxy (gateway) is installed. The addresses in EPICS_CA_ADDR_LIST may be dotted IP addresses or host names if the local OS has support for host name to IP address translation. When multiple names are added to EPICS_CA_ADDR_LIST they must be separated by white space. There is no requirement that the addresses specified in the EPICS_CA_ADDR_LIST be a broadcast addresses, but this will often be the most convenient choice.

If a client needs to communicate with two servers that are residing at different port numbers then an extended syntax may be used with the EPICS_CA_ADDR_LIST environment variable. Each host name or IP address in the EPICS_CA_ADDR_LIST may be immediately followed by a colon and an IP port number without intervening whitespace. Entries that do not specify a port number will default to EPICS_CA_SERVER_PORT.

Routing Restrictions on vxWorks Systems

Frequently vxWorks systems boot by default with routes limiting access only to the local subnet. If a EPICS system is operating in a WAN environment it may be necessary to configure routes into the vxWorks system which enable a vxWorks based CA server to respond to requests originating outside it's subnet. These routing restrictions can also apply to vxWorks base CA clients communicating with off subnet servers. An EPICS system manager can implement an rudimentary, but robust, form of access control for a particular host by not providing routes in that host that reach outside of a limited set of subnets. See "routeLib" in the vxWorks reference manual.

Disconnect Time Out Interval

If the CA client library does not see a beacon from a server that it is connected to for EPICS_CA_CONN_TMO seconds then an state-of-health message is sent to the server over TCP/IP. If this state-of-health message isn't promptly replied to then the client library will conclude that channels communicating with the server are no longer responsive and inform the CA client side application via function callbacks. The parameter EPICS_CA_CONN_TMO is specified in floating point seconds. The default is typically 30 seconds. For efficient operation it is recommended that EPICS_CA_CONN_TMO be set to no less than twice the value specified for EPICS_CA_BEACON_PERIOD.

Prior to EPICS R3.14.5 an unresponsive server implied an immediate TCP circuit disconnect, immediate resumption of UDP based search requests, and immediate attempts to reconnect. There was concern about excessive levels of additional activity when servers are operated close to the edge of resource limitations. Therefore with version R3.14.5 and greater the CA client library continues to inform client side applications when channels are unresponsive, but does not immediately disconnect the TCP circuit. Instead the CA client library postpones circuit shutdown until receiving indication of circuit disconnect from the IP kernel. This can occur either because a server is restarted or because the IP kernel's internal TCP circuit inactivity keep alive timer has expired after a typically long duration (as is appropriate for IP based systems that need to avoid thrashing during periods of excessive load). The net result is less search and TCP circuit setup and shutdown activity suring periods of excessive load.

Dynamic Changes in the CA Client Library Search Interval

The CA client library will continuously attempt to connect any CA channels that an application has created until it is successful. The library periodically queries the server destination address list described above with name resolution requests for any unresolved channels. Since this address list frequently contains broadcast addresses, and because nonexistent process variable names are frequently configured, or servers may be temporarily unavailable, then it is necessary for the CA client library internals to carefully schedule these requests in time to avoid introducing excessive load on the network and the servers.

When the CA client library has many channels to connect, and most of its name resolution requests are responded to, then it sends name resolution requests at an interval that is twice the estimated round trip interval for the set of servers responding, or at the minimum delay quantum for the operating system - whichever is greater. The number of UDP frames per interval is also dynamically adjusted based on the past success rates.

If a name resolution request is not responded to, then the client library doubles the delay between name resolution attempts and reduces the number of requests per interval. The maximum delay between attempts is limited by EPICS_CA_MAX_SEARCH_PERIOD (see Configuring the Maximum Search Period ). Note however that prior to R3.14.7, if the client library did not receive any responses over a long interval it stoped sending name resolution attempts altogether until a beacon anomaly was detected (see below).

The CA client library continually estimates the beacon period of all server beacons received. If a particular server's beacon period becomes significantly shorter or longer then the client is said to detect a beacon anomaly. The library boosts the search interval for unresolved channels when a beacon anomaly is seen or when any successful search response is received, but with a longer initial interval between requests than is used when the application creates a channel. Creation of a new channel does not (starting with EPICS R3.14.7) change the interval used when searching for preexisting unresolved channels. The program "casw" prints a message on standard out for each CA client beacon anomaly detect event.

Configuring the Maximum Search Period

The rate at which name resolution (search) requests are sent exponentially backs off to a plateau rate. The value of this plateau has an impact on network traffic because it determines the rate that clients search for channel names that are miss-spelled or otherwise don't exist in a server. Furthermore, for clients that are unable to see the beacon from a new server, the plateau rate may also determine the maximum interval that the client will wait until discovering a new server.

Starting with EPICS R3.14.7 this maximum search rate interval plateau in seconds is determined by the EPICS_CA_MAX_SEARCH_PERIOD environment variable.

The CA Repeater

When several client processes run on the same host it is not possible for all of them to directly receive a copy of the server beacon messages when the beacon messages are sent to unicast addresses, or when legacy IP kernels are still in use. To avoid confusion over these restrictions a special UDP server, the CA Repeater, is automatically spawned by the CA client library when it is not found to be running. This program listens for server beacons sent to the UDP port specified in the EPICS_CA_REPEATER_PORT parameter and fans any beacons received out to any CA client program running on the same host that have registered themselves with the CA Repeater. If the CA Repeater is not already running on a workstation, then the "caRepeater" program must be in your path before using the CA client library for the first time.

If a host based IOC is run on the same workstation with standalone CA client processes, then it is probably best to start the caRepeater process when the workstation is booted. Otherwise it is possible for the standalone CA client processes to become dependent on a CA repeater started within the confines of the host based IOC. As long as the host based IOC continues to run there is nothing wrong with this situation, but problems could arise if this host based IOC process exits before the standalone client processes which are relying on its CA repeater for services exit.

Since the repeater is intended to be shared by multiple clients then it could be argued that it makes less sense to set up a CA repeater that listens for beacons on only a subset of available network interfaces. In the worst case situation the client library might see beacon anomalies from servers that it is not interested in. Modifications to the CA repeater forcing it to listen only on a subset of network interfaces might be considered for a future release if there appear to be situations that require it.

Configuring the Time Zone

Note: Starting with EPICS R3.14 all of the libraries in the EPICS base distribution rely on facilities built into the operating system to determine the correct time zone. Nevertheless, several programs commonly used with EPICS still use the original "tssubr" library and therefore they still rely on proper configuration of EPICS_TS_MIN_WEST.

While the CA client library does not translate inbetween the local time and the time zone independent internal storage of EPICS time stamps, many EPICS client side applications call core EPICS libraries which provide these services. To set the correct time zone users must compute the number of positive minutes west of GMT (maximum 720 inclusive) or the negative number of minutes east of GMT (minimum -720 inclusive). This integer value is then placed in the variable EPICS_TS_MIN_WEST.

Starting with version R3.14 the environment variable EPICS_CA_MAX_ARRAY_BYTES determines the size of the largest array that may pass through CA. Prior to this version only arrays smaller than 16k bytes could be transfered. The CA libraries maintains a free list of 16384 byte network buffers that are used for ordinary communication. If EPICS_CA_MAX_ARRAY_BYTES is larger than 16384 then a second free list of larger data buffers is established and used only after a client send its first large array request.

The CA client library uses EPICS_CA_MAX_ARRAY_BYTES to determines the maximum array that it will send or receive. Likewise, the CA server uses EPICS_CA_MAX_ARRAY_BYTES to determine the maximum array that it may send or receive. The client does not influence the server's message size quotas and visa versa. In fact the value of EPICS_CA_MAX_ARRAY_BYTES need not be the same in the client and the server. If the server receives a request which is too large to read or respond to in entirety then it sends an exception message to the client. Likewise, if the CA client library receives a request to send an array larger than EPICS_CA_MAX_ARRAY_BYTES it will return ECA_TOLARGE.

A common mistake is to correctly calculate the maximum datum size in bytes by multiplying the number of elements by the size of a single element, but neglect to add additional bytes for the compound data types (for example DBR_GR_DOUBLE) commonly used by the more sophisticated client side applications. Based on this confusion, one could arrive at the conclusion that EPICS_CA_MAX_ARRAY_BYTES might have been better named EPICS_CA_MAX_DATUM_BYTES, or that the software should be changed internally to round the users request up by the size of the maximum scalar datum (nothing has been done to address this issue so far).

Configuring a CA Server

The server configures its port number from the EPICS_CAS_SERVER_PORT environment variable if it is specified. Otherwise the EPICS_CA_SERVER_PORT environment variable determines the server's port number. Two servers can share the same UDP port number on the same machine, but there are restrictions - see a discussion of unicast addresses and two servers sharing the same UDP port on the same host .

Server Beacons

The EPICS_CAS_BEACON_PERIOD parameter determines the server's beacon period and is specified in floating point seconds. The default is typically 15 seconds. See also EPICS_CA_CONN_TMO and Dynamic Changes in the CA Client Library Search Interval .

CA servers build a list of addresses to send beacons to during initialization. If EPICS_CAS_AUTO_BEACON_ADDR_LIST has the value "YES" then the beacon address list will be automatically configured to contain the broadcast addresses of all LAN interfaces found in the host and the destination address of all point-to-point interfaces found in the host. However, if the user also defines EPICS_CAS_INTF_ADDR_LIST then beacon address list automatic configuration is constrained to the network interfaces specified therein, and therefore only the broadcast addresses of the specified LAN interfaces, and the destination addresses of all specified point-to-point links, will be automatically configured.

If EPICS_CAS_BEACON_ADDR_LIST is defined then its contents will be used to augment any automatic configuration of the beacon address list. Individual entries in EPICS_CAS_BEACON_ADDR_LIST may override the destination port number if ":nnn" follows the host name or IP address there. Alternatively, when both EPICS_CAS_BEACON_ADDR_LIST and EPICS_CAS_INTF_ADDR_LIST are not defined then the contents of EPICS_CA_ADDR_LIST is used to augment the list. Otherwise, the list is not augmented.

The EPICS_CAS_BEACON_PORT parameter specifies the destination port for server beacons. The only exception to this occurs when ports are specified in EPICS_CAS_BEACON_ADDR_LIST or possibly in EPICS_CA_ADDR_LIST. If EPICS_CAS_BEACON_PORT is not specified then beacons are sent to the port specified in EPICS_CA_REPEATER_PORT.

Binding a Server to a Limited Set of Network Interfaces

The parameter EPICS_CAS_INTF_ADDR_LIST allows a ca server to bind itself to, and therefore accept messages only over, a limited set of the local host's network interfaces (each specified by it's IP address). On UNIX systems type "netstat -i" (type "ipconfig" on windows) to see a list of the local host's network interfaces. Specifically, UDP search messages addressed to both the IP addresses in EPICS_CAS_INTF_ADDR_LIST and also to the broadcast addresses of the corresponding LAN interfaces will be accepted by the server. By default, the CA server is accessible from all network interfaces configured into its host. In R3.14 and previous releases the CA server employed by iocCore does not implemet this feature .

Ignoring Process Variable Name Resolution Requests From Certain Hosts

Name resolution requests originating from any of the IP addresses specified in the EPICS_CAS_IGNORE_ADDR_LIST parameter are not replied to. In R3.14 and previous releases the CA server employed by iocCore does not implemet this feature.

Client Configuration that also Applies to Servers

Building an Application

Required Header (.h) Files

An application that uses the CA client library functions described in this document will need to include the cadef.h header files as follows.

#include "cadef.h"

This header file is located at "<EPICS base>/include/". It includes many other header files (operating system specific and otherwise), and therefore the application must also specify "<EPICS base>/include/os/<arch>" in its header file search path.

Required Libraries

An application that uses the Channel Access Client Library functions described in this document will need to link with the EPICS CA Client Library and also the EPICS Common Library. The EPICS CA Client Library calls the EPICS Common Library. The following table shows the names of these libraries on UNIX and Windows systems.

If you do not use the EPICS build environemnt (layered make files) then it may be helpful to run one of the EPICS make files and watch the compile/link lines. This may be the simplest way to capture the latest system and compiler specific options required by your build environment. I have included some snapshots of typical build lines below, but expect some risk of this information becoming dated.

Typical Linux Build Options

/usr/bin/gcc -c -D_POSIX_C_SOURCE=199506L -D_POSIX_THREADS -D_XOPEN_SOURCE=500 -DOSITHREAD_USE_DEFAULT_STACK -D_X86_ -DUNIX -D_BSD_SOURCE -Dlinux -D_REENTRANT -ansi -O3 -Wall -I. -I.. -I../../../include/os/Linux -I../../../include ../acctst.c

/usr/bin/g++ -o acctst -L/home/user/epicsR3.14/epics/base/lib/linux-x86/ -Wl,-rpath,/mnt/bogart_home/hill/epicsR3.14/epics/base/lib/linux-x86 acctstMain.o acctst.o -lca -lCom

Typical Solaris Build Options

/opt/SUNWspro/bin/cc -c -D_POSIX_C_SOURCE=199506L -D_XOPEN_SOURCE=500 -DOSITHREAD_USE_DEFAULT_STACK -DUNIX -DSOLARIS=9 -mt -D__EXTENSIONS__ -Xc -v -xO4 -I. -I.. -I./../../../include/os/solaris -I./../../../include ../acctst.c

/opt/SUNWspro/bin/CC -o acctst -L/home/phoebus1/JHILL/epics/base/lib/solaris-sparc/ -mt -z ignore -z combreloc -z lazyload -R/home/disk1/user/epics/base/lib/solaris-sparc acctstMain.o acctst.o -lca -lCom

Typical Windows Build Options

cl -c /nologo /D__STDC__=0 /Ox /GL /W3 /w44355 /MD -I. -I.. -I..\\..\\..\\include\\os\\WIN32 -I..\\..\\..\\include ..\\acctst.c

link -nologo /LTCG /incremental:no /opt:ref /release /version:3.14 -out:acctst.exe acctstMain.obj acctst.obj d:/user/R3.14.clean/epics/base/lib/WIN32-x86/ca.lib d:/user/R3.14.clean/epics/base/lib/WIN32-x86/

Typical vxWorks Build Options

/usr/local/xcomp/ppc/bin/ccppc -c -D_POSIX_SOURCE -DCPU=PPC603 -DvxWorks -include /home/vx/tornado20/target/h/vxWorks.h -ansi -O3 -Wall -mcpu=603 -mstrict-align -fno-builtin -I. -I.. -I../../../include/os/vxWorks -I../../../include -I/home/vx/tornado20/target/h ../acctst.c

Other Systems and Compilers

Contributions gratefully accepted.

Command Line Utilities

acctst

acctst <PV name> [progress logging level] [channel duplication count] 
                 [test repetition count] [enable preemptive callback]

Description

Channel Access Client Library regression test.

The PV used with the test must be native type DBR_DOUBLE or DBR_FLOAT, and modified only by acctst while the test is running. Therefore, periodically scanned hardware attached analog input records do not work well. Test failure is indicated if the program stops prior to printing "test complete". If unspecified the progress logging level is zero, and no messages are printed while the test is progressing. If unspecified, the channel duplication count is 20000. If unspecified, the test repetition count is once only. If unspecified, preemptive callback is disabled.

catime

catime <PV name> [channel count] [append number to pv name if true]

Description

Channel Access Client Library performance test.

If unspecified, the channel count is 10000. If the "append number to pv name if true" argument is specified and it is greater than zero then the channel names in the test are numbered as follows.

<PV name>000000, <PV name>000001, ... <PV name>nnnnnn

casw [-i <interest level>]

Description

CA server "beacon anomaly" logging.

CA server beacon anomalies occur when a new server joins the network, a server is rebooted, network connectivity to a server is reestablished, or if a server's CPU exits a CPU load saturated state.

CA clients with unresolved channels reset their search request scheduling timers whenever they see a beacon anomaly.

This program can be used to detect situations where there are too many beacon anomalies. IP routing configuration problems may result in false beacon anomalies that might cause CA clients to use unnecessary additional network bandwidth and server CPU load when searching for unresolved channels.

If there are no new CA servers appearing on the network, and network connectivity remains constant, then casw should print no messages at all. At higher interest levels the program prints a message for every beacon that is received, and anomalous entries are flagged with a star.

caEventRate

caEventRate <PV name> [subscription count]

Description

Connect to the specified PV, subscribe for monitor updates the specified number of times (default once), and periodically log the current sampled event rate, average event rate, and the standard deviation of the event rate in Hertz to standard out.

ca_test

ca_test <PV name> [value to be written]

Description

If a value is specified it is written to the PV. Next, the current value of the PV is converted to each of the many external data type that can be specified at the CA client library interface, and each of these is formated and then output to the console.

Command Line Tools

caget

caget [options] <PV name> ...

Description

Get and print value for PV(s).

The values for one or multiple PVs are read and printed to stdout. The DBR_... format in which the data is read, the output format, and a number of details of how integer and float values are represented can be controlled using command line options.

When getting multiple PVs, their order on the command line is retained in the output.

Put value to a PV.

The specified value is written to the PV (as a string). The PV value is read before and after the write operation and printed as "Old" and "new" values on stdout.

The array variant writes an array to the specified PV. The first numeric argument specifying the number of array elements is kept for compatibility with the array data format of caget - the actual number of values specified on the command line is used.

Get and print channel and connection information for PV(s).

All available Channel Access related information about PV(s) is printed to stdout.

The -s option allows to specify an interest level for calling Channel Access' internal report function ca_client_status(), that prints lots of internal informations on stdout, including environment settings, used CA ports etc.

Verify that the broadcast addresses are identical on the server's host and on the client's host. This can be checked on UNIX with "netstat -i" or "ifconfig -a"; on vxWorks with ifShow; and on windows with ipconfig. It is normal for the broadcast addresses to not be identical if the client and server are not directly attached to the same IP subnet, and in this situation the EPICS_CA_ADDR_LIST must be set. Otherwise, if the client and server are intended to be on the same IP subnet, then the problem may be that the IP netmask is incorrectly set in the network interface configuration. On most operating systems, when the host's IP address is configured, the host's IP subnet mask is also configured.

Client Isn't Configured to Use the Server's Port

Verify that the client and server are using the same UDP port. Check the server's port by running "netstat -a | grep nnn" where nnn is the port number configured in the client. If you do not set EPICS_CA_SERVER_PORT or EPICS_CAS_SERVER_PORT then the default port will be 5064.

Unicast Addresses in the EPICS_CA_ADDR_LIST Does not Reliably Contact Servers Sharing the Same UDP Port on the Same Host

Two servers can run on the same host with the same server port number, but there are restrictions. If the host has a modern IP kernel it is possible to have two or more servers share the same UDP port. It is not possible for these servers to run on the same host using the same TCP port. If the CA server library detects that a server is attempting to start on the same port as an existing CA server then both servers will use the same UDP port, and the 2nd server will be allocated an ephemeral TCP port. Clients can be configured to use the same port number for both servers. They will locate the 2nd server via the shared UDP port, and transparently connect to the 2nd server's ephemeral TCP port. Be aware however that If there are two server's running on the same host sharing the same UDP port then they will both receive UDP search requests sent as broadcasts, but unfortunately (due to a weakness of most IP kernel implementations) only one of the servers will typically receive UDP search requests sent to unicast addresses (i.e. a single specific host's ip address).

Client Does not See Server's Beacons

Two conclusions deserve special emphasis. First, if a client does not see the server's beacons, then it will use additional network and server resources sending periodic state-of-health messages. Second, if a client does not see a newly introduced server's beacon, then it will take up to EPICS_CA_MAX_SEARCH_PERIOD to find that newly introduced server. Also, starting with EPICS R3.14.7 the client library does not suspend searching for a channel after 100 unsuccessful attempts until a beacon anomaly is seen. Therefore, if the client library is from before version R3.14.7 of EPICS and it timed out attempting to find a server whoose beacon cant be seen by the client library then the client application might need to be restarted in order to connect to this new beacon-out-of-range server. The typical situation where a client would not see the server's beacon might be when the client isnt on the same IP subnet as the server, and the client's EPICS_CA_ADDR_LIST was modified to include a destination address for the server, but the server's beacon address list was not modified so that it's beacons are received by the client.

A Server's IP Address Was Changed

When communication over a virtual circuit times out, then each channel attached to the circuit enters a disconnected state and the disconnect callback handler specified for the channel is called. However, the circuit is not disconnected until TCP/IP's internal, typically long duration, keep alive timer expires. The disconnected channels remain attached to the beleaguered circuit and no attempt is made to search for, or to reestablish, a new circuit. If, at some time in the future, the circuit becomes responsive again, then the attached channels enter a connected state again and reconnect call back handlers are called. Any monitor subscriptions that received an update message while the channel was disconnected are also refreshed. If at any time the library receives an indication from the operating system that a beleaguered circuit has shutdown or was disconnected then the library will immediately reattempt to find servers for each channel and connect circuits to them.

A well known negative side effect of the above behavior is that CA clients will wait the full (typically long) duration of TCP/IP's internal keep alive timer prior to reconnecting under the following scenario (all of the following occur):

An server's (IOC's) operating system crashes (or is abruptly turned off) or a vxWorks system is stopped by any means

This operating system does not immediately reboot using the same IP address

A duplicate of the server (IOC) is started appearing at a different IP address

It is unlikely that any rational organization will advocate the above scenario in a production system. Nevertheless, there are opportunities for users to become confused during control system development , but it is felt that the robustness improvements justify isolated confusion during the system integration and checkout activities where the above scenarios are most likely to occur.

Contrast the above behavior with the CA client library behavior of releases prior to R3.14.5 where the beleaguered circuit was immediately closed when communication over it timed out. Any attached channels were immediately searched for, and after successful search responses arrived then attempts were made to build a new circuit. This behavior could result in undesirable resource consumption resulting from periodic circuit setup and teardown overhead (thrashing) during periods of CPU / network / IP kernel buffer congestion.

Put Requests Just Prior to Process Termination Appear to be Ignored

Short lived CA client applications that issue a CA put request and then immediately exit the process (return from main or call exit ) may find that there request isn't executed. To guarantee that the request is sent call ca_flush followed by ca_context_destroy prior to terminating the process.

ENOBUFS Messages

Many Berkley UNIX derived Internet Protocol (IP) kernels use a memory management scheme with a fixed sized low level memory allocation quantum called an "mbuf". Messages about "ENOBUFS" are an indication that your IP kernel is running low on mbuf buffers. An IP kernel mbuf starvation situation may lead to temporary IP communications stalls or reduced throughput. This issue has to date been primarily associated with vxWorks systems where mbuf starvation on earlier vxWorks versions is rumored to lead to permanent IP communications stalls which are resolved only by a system reboot. IP kernels that use mbufs frequently allow the initial and maximum number of mbufs to be configured. Consult your OS's documentation for configuration procedures which vary between OS and even between different versions of the same OS.

Contributing Circumstances

The total number of connected clients is high. Each active socket requires dedicated mbufs for protocol control blocks, and for any data that might be pending in the operating system for transmission to Channel Access or to the network at a given instant. If you increase the vxWorks limit on the maximum number of file descriptors then it may also be necessary to increase the size of the mbuf pool.

The server has multiple connections where the server's sustained event (monitor subscription update) production rate is higher than the client's or the network's sustained event consumption rate. This ties up a per socket quota of mbufs for data that are pending transmission to the client via the network. In particular, if there are multiple clients that subscribe for monitor events but do not call ca_pend_event() or ca_poll() to process their CA input queue, then a significant mbuf consuming backlog can occur in the server.

The server does not get a chance to run (because some other higher priority thread is running) and the CA clients are sending a high volume of data over TCP or UDP. This ties up a quota of mbufs for each socket in the server that isn't being reduced by the server's socket read system calls.

The server has multiple stale connections. Stale connections occur when a client is abruptly turned off or disconnected from the network, and an internal "keepalive" timer has not yet expired for the virtual circuit in the operating system, and therefore mbufs may be dedicated to unused virtual circuits. This situation is made worse if there are active monitor subscriptions associated with stale connections which will rapidly increase the number of dedicated mbufs to the quota available for each circuit.

When sites switch to the vxWorks 5.4 IP kernel they frequently run into network pool exhaustion problems. This may be because the original vxWorks IP kernel expanded the network pool as needed at runtime while the new kernel's pool is statically configured at compile time, and does not expand as needed at runtime. Also, at certain sites problems related to vxWorks network driver pool exhaustion have also been reported (this can also result in ENOBUF diagnostic messages).

Related Diagnostics

The EPICS command "casr [interest level]" displays information about the CA server and how many clients are connected.

The vxWorks command "inetstatShow" indicates how many bytes are pending in mbufs and indirectly (based on the number of circuits listed) how many mbuf based protocol control blocks have been consumed. The vxWorks commands (availability depending on vxWorks version) mbufShow, netStackSysPoolShow, and netStackDataPoolShow indicate how much space remains in the network stack pool.

The RTEMS command "netstat [interest level]" displays network information including mbuf consumption statistics.

Server Subscription Update Queuing

If the subscription update producer in the server produces subscription updates faster than the subscription update consumer in the client consumes them, then events have to be discarded if the buffering in the server isn’t allowed to grow to an infinite size. This is a law of nature – based on queuing theory of course.

What is done depends on the version of the CA server. All server versions place quotas on the maximum number of subscription updates allowed on the subscription update queue at any given time. If this limit is reached, an intervening update is discarded in favor of a more recent update. Depending on the version of the server, rapidly updating subscriptions are or are not allowed to cannibalize the quotas of slow updating subscriptions in limited ways. Nevertheless, there is always room on the queue for at least one update for each subscription. This guarantees that the most recent update is always sent.

Adding further complication, the CA client library also implements a primitive type of flow control. If the client library sees that it is reading a large number of messages one after another w/o intervening delay it knows that it is not consuming events as fast as they are produced. In that situation it sends a message telling the server to temporarily stop sending subscription update messages. When the client catches up it sends another message asking the server to resume with subscription updates. This prevents slow clients from getting time warped, but also guarantees that intervening events are discarded until the slow client catches up.

There is currently no message on the IOC’s console when a particular client is slow on the uptake. A message of this type used to exist many years ago, but it was a source of confusion (and what we will call message noise) so it was removed.

There is unfortunately no field in the protocol allowing the server to indicate that an intervening subscription update was discarded. We should probably add that capability in a future version. Such a feature would, for example, be beneficial when tuning an archiver installation.

Function Call Interface General Guidelines

Flushing and Blocking

Significant performance gains can be realized when the CA client library doesn't wait for a response to return from the server after each request. All requests which require interaction with a CA server are accumulated (buffered) and not forwarded to the IOC until one of ca_flush_io, ca_pend_io, ca_pend_event, or ca_sg_pend are called allowing several operations to be efficiently sent over the network together. Any process variable values written into your program's variables by ca_get() should not be referenced by your program until ECA_NORMAL has been received from ca_pend_io().

Status Codes

If successful, the routines described here return the status code ECA_NORMAL. Unsuccessful status codes returned from the client library are listed with each routine in this manual. Operations that appear to be valid to the client can still fail in the server. Writing the string "off" to a floating point field is an example of this type of error. If the server for a channel is located in a different address space than the client then the ca_xxx() operations that communicate with the server return status indicating the validity of the request and whether it was successfully enqueued to the server, but communication of completion status is deferred until a user callback is called, or lacking that an exception handler is called. An error number and the error's severity are embedded in CA status (error) constants. Applications shouldn't test the success of a CA function call by checking to see if the returned value is zero as is the UNIX convention. Below are several methods to test CA function returns. See ca_signal() and SEVCHK for more information on this topic.

status = ca_XXXX(); 
SEVCHK( status, "ca_XXXX() returned failure status"); 
if ( status & CA_M_SUCCESS ) { 
        printf ( "The requested ca_XXXX() operation didn't complete successfully"); 
if ( status != ECA_NORMAL ) { 
        printf("The requested ca_XXXX() operation didn't complete successfully because \"%s\"\n",
                ca_message ( status ) ); 
Channel Access Data Types
CA channels form a virtual circuit between a process variable (PV) and a
client side application program. It is possible to connect a wide variety of
data sources into EPICS using the CA server library. When a CA channel
communicates with an EPICS Input Output Controller (IOC) then a field is a
specialization of a PV, and an EPICS record is a plug compatible function
block that contains fields, and the meta data below frequently are mapped
onto specific fields within the EPICS records by the EPICS record support
(see the EPICS Application Developer Guide).
Arguments of type chtype specifying the data type you wish to transfer.
They expect one of the set of DBR_XXXX data type codes defined in
db_access.h. There are data types for all of the C primitive types, and there
are also compound (C structure) types that include various process variable
properties such as units, limits, time stamp, or alarm status. The primitive
C types follow a naming convention where the C typedef dbr_xxxx_t corresponds
to the DBR_XXXX data type code. The compound (C structure) types follow a
naming convention where the C structure tag dbr_xxxx corresponds to the
DBR_XXXX data type code. The following tables provides more details on the
structure of the CA data type space. Since data addresses are passed to the
CA client library as typeless "void *" pointers then care should be taken to
ensure that you have passed the correct C data type corresponding to the
DBR_XXXX type that you have specified. Architecture independent types are
provided in db_access.h to assist programmers in writing portable code. For
example "dbr_short_t" should be used to send or receive type DBR_SHORT. Be
aware that type name DBR_INT has been deprecated in favor of the less
confusing type name DBR_SHORT. In practice, both the DBR_INT type code and
the DBR_SHORT type code refer to a 16 bit integer type, and are functionally
equivalent.
  Channel Access Primitive Data Types
      CA Type Code
      Primitive C Data Type
      Data Size
      DBR_CHAR
      dbr_char_t
      8 bit character
      DBR_SHORT
      dbr_short_t
      16 bit integer
      DBR_ENUM
      dbr_enum_t
      16 bit unsigned integer
      DBR_LONG
      dbr_long_t
      32 bit signed integer
      DBR_FLOAT
      dbr_float_t
      32 bit IEEE floating point
      DBR_DOUBLE
      dbr_double_t
      64 bit IEEE floating point
      DBR_STRING
      dbr_string_t
      40 character string
      struct dbr_gr_<primitive type>
      value, alarm status, alarm severity, units, display precision, and
        graphic limits
      DBR_CTRL_<PRIMITIVE TYPE>
      struct dbr_ctrl_<primitive type>
      value, alarm status, alarm severity, units, display precision,
        graphic limits, and control limits
      DBR_PUT_ACKT
      dbr_put_ackt_t
      Used for global alarm acknowledgement. Do transient alarms have to
        be acknowledged? (0,1) means (no, yes).
      DBR_PUT_ACKS
      dbr_put_acks_t
      Used for global alarm acknowledgement. The highest alarm severity
        to acknowledge. If the current alarm severity is less then or equal
        to this value the alarm is acknowledged.
      DBR_STSACK_STRING
      struct dbr_stsack_string
      value, alarm status, alarm severity, ackt, ackv
      DBR_CLASS_NAME
      dbr_class_name_t
      name of enclosing interface (name of the record if channel is
        attached to EPICS run time database)
Channel value arrays can also be included within the structured CA data
types. If more than one element is requested, then the individual elements
can be accessed in an application program by indexing a pointer to the value
field in the DBR_XXX structure. For example, the following code computes the
sum of the elements in a array process variable and prints its time stamp.
The dbr_size_n function can be used to determine the
correct number of bytes to reserve when there are more than one value field
elements in a structured CA data type.
#include <stdio.h>
#include <stdlib.h>
#include "cadef.h"
int main ( int argc, char ** argv )
    struct dbr_time_double * pTD;
    const dbr_double_t * pValue;
    unsigned nBytes;
    unsigned elementCount;
    char timeString[32];
    unsigned i;    
    chid chan;
    double sum;
    int status;
    if ( argc != 2 ) {
        fprintf ( stderr, "usage: %s <channel name>", argv[0] );
        return -1;
    status = ca_create_channel ( argv[1], 0, 0, 0, & chan );
    SEVCHK ( status, "ca_create_channel()" );
    status = ca_pend_io ( 15.0 );
    if ( status != ECA_NORMAL ) {
        fprintf ( stderr, "\"%s\" not found.\n", argv[1] );
        return -1;
    elementCount = ca_element_count ( chan );
    nBytes = dbr_size_n ( DBR_TIME_DOUBLE, elementCount );
    pTD = ( struct dbr_time_double * ) malloc ( nBytes );
    if ( ! pTD ) {
        fprintf ( stderr, "insufficient memory to complete request\n" );
        return -1;
    status = ca_array_get ( DBR_TIME_DOUBLE, elementCount, chan, pTD );
    SEVCHK ( status, "ca_array_get()" );
    status = ca_pend_io ( 15.0 );
    if ( status != ECA_NORMAL ) {
        fprintf ( stderr, "\"%s\" didnt return a value.\n", argv[1] );
        return -1;
    pValue = & pTD->value;     
    sum = 0.0;      
    for ( i = 0; i < elementCount; i++ ) {         
        sum += pValue[i];     
    epicsTimeToStrftime ( timeString, sizeof ( timeString ),
        "%a %b %d %Y %H:%M:%S.%f", & pTD->stamp );
    printf ( "The sum of elements in %s at %s was %f\n", 
        argv[1], timeString, sum );
    ca_clear_channel ( chan );
    ca_task_exit ();
    free ( pTD );
    return 0;
User Supplied Callback Functions
Certain CA client initiated requests asynchronously execute an application
supplied call back in the client process when a response arrives. The
functions ca_put_callback, ca_get_callback, and ca_add_event all request
notification of asynchronous completion via this mechanism. The
event_handler_args structure is passed by value to the
application supplied callback. In this structure the dbr field
is a void pointer to any data that might be returned. The
status field will be set to one of the CA error
codes in caerr.h and will indicate the status of the operation performed in
the IOC. If the status field isn't set to ECA_NORMAL or data isn't normally
returned from the operation (i.e. put call back) then you should expect that
the dbr field  will be set to a nill pointer (zero). The fields
usr, chid, and type are set to the
values specified when the request was made by the application. The "dbr"
pointer, and any data that it points to, are valid only when executing within
the user's callback function.
typedef struct event_handler_args {
    void            *usr;   /* user argument supplied with request */
    chanId          chid;   /* channel id */
    long            type;   /* the type of the item returned */ 
    long            count;  /* the element count of the item returned */
    const void      *dbr;   /* a pointer to the item returned */
    int             status; /* ECA_XXX status of the requested op from the server */
} evargs;
void myCallback ( struct event_handler_args args )
    if ( args.status != ECA_NORMAL ) {
    if ( args.type == DBR_TIME_DOUBLE ) {
         const struct dbr_time_double * pTD =
              ( const struct dbr_time_double * ) args.dbr;
Channel Access Exceptions
When the server detects a failure, and there is no client call back
function attached to the request, then an exception handler is executed in
the client. The default exception handler prints a message on the console and
exits if the exception condition is severe. Certain internal exceptions
within the CA client library, and failures detected by the SEVCHK macro may
also cause the exception handler to be invoked. To modify this behavior see
ca_add_exception_event().
Server and Client Share the Same Address Space on The
Same Host
If the Process Variable's server and it's client are colocated within the
same memory address space and the same host then the ca_xxx() operations
bypass the server and directly interact with the server tool component
(commonly the IOC's function block database). In this situation the ca_xxx()
routines frequently return the completion status of the requested operation
directly to the caller with no opportunity for asynchronous notification of
failure via an exception handler. Likewise, callbacks may be directly invoked
by the CA library functions that request them.
Arrays
For routines that require an argument specifying the number of array
elements, no more than the process variable's maximum native element count
may be requested. The process variable's maximum native element count is
available from ca_element_count() when the channel is connected. If less
elements than the process variable's native element count are requested the
requested values will be fetched beginning at element zero. By default CA
limits the number of elements in an array to be no more than approximately
16k divided by the size of one element in the array. Starting with EPICS
R3.14 the maximum array size may be configured in the client and in the
server.
Connection Management
Application programs should assume that CA servers may be restarted, and
that network connectivity is transient. When you create a CA channel its
initial connection state will most commonly be disconnected. If the Process
Variable's server is available the library will immediately initiate the
necessary actions to make a connection with it. Otherwise, the client library
will monitor the state of servers on the network and connect or reconnect
with the process variable's server as it becomes available. After the channel
connects the application program can freely perform IO operations through the
channel, but should expect that the channel might disconnect at any time due
to network connectivity disruptions or server restarts.
Three methods can be used to determine if a channel is connected: the
application program might call ca_state
to obtain the current connection state, block in ca_pend_io until the channel connects, or
install a connection callback handler when it calls ca_create_channel. The ca_pend_io approach is best suited to simple
command line programs with short runtime duration, and the connection
callback method is best suited to toolkit components with long runtime
duration. Use of ca_state is appropriate
only in programs that prefer to poll for connection state changes instead of
opting for asynchronous notification. The ca_pend_io function
blocks only for channels created specifying a nill connection handler
callback function. The user's connection state change function will be run
immediately from within ca_create_channel if the CA client and
CA server are both hosted within the same address space (within the same
process).
Thread Safety and Preemptive Callback to User
Starting with EPICS R3.14 the CA client libraries are fully thread safe on
all OS (in past releases the library was thread safe only on vxWorks). When
the client library is initialized the programmer may specify if preemptive
call back is enabled. Preemptive call back is disabled by default. If
preemptive call back is enabled then the user's call back functions might be
called by CA's auxiliary threads when the main initiating channel access
thread is not inside of a function in the channel access client library.
Otherwise, the user's call back functions will be called only when the main
initiating channel access thread is executing inside of the CA client
library. When the CA client library invokes a user's call back function it
will always wait for the current callback to complete prior to executing
another call back function. Programmers enabling preemptive callback should
be familiar with using mutex locks to create a reliable multi-threaded
program.
To set up a traditional single threaded client you will need code like
this (see ca_context_create and CA Client Contexts and Application Specific Auxiliary
Threads) .
SEVCHK ( ca_context_create(ca_disable_preemptive_callback ),
"application pdq calling ca_context_create" );
To set up a preemptive callback enabled CA client context you will need
code like this (see ca_context_create and CA Client Contexts and Application Specific Auxiliary
Threads).
SEVCHK ( ca_context_create(ca_enable_preemptive_callback ),
"application pdq calling ca_context_create" );
CA Client Contexts and Application Specific Auxiliary
Threads
It is often necessary for several CA client side tools running in the same
address space (process) to be independent of each other. For example, the
database CA links and the sequencer are designed to not use the same CA
client library threads, network circuits, and data structures. Each thread
that calls ca_context_create() for the first
time either directly, or implicitly when calling any CA library function for
the first time, creates a CA client library context. A CA client library
context contains all of the threads, network circuits, and data structures
required to connect and communicate with the channels that a CA client
application has created. The priority of auxiliary threads spawned by the CA
client library are at fixed offsets from the priority of the thread that
called ca_context_create(). An application
specific auxiliary thread can join a CA context by calling ca_attach_context() using the CA context
identifier that was returned from ca_current_context() when it is called by the
thread that created the context which needs to be joined. A context which is
to be joined must be created using ca_context_create(ca_enable_preemptive_callback).
It is not possible to attach a thread to a CA context created explicitly
or implicitly with
ca_create_context(ca_disable_preemptive_callback). Once a thread has joined
with a CA context it need only make ordinary ca_xxxx() library calls to use
the context. There is no need to specify the context identifier when invoking
the CA y calls because the context identifier is stored in a thread
privatelibrary calls because the context identifier is stored in a thread
private variable by ca_attach_context().
A CA client library context can be shut down and cleaned up, after
destroying any channels or application specific threads that are attached to
it, by calling ca_context_destroy(). The
context may be created and destroyed by different threads as long as they are
both part of the same context.
Polling the CA Client Library From Single Threaded
Applications
If preemptive call back is not enabled, then for proper operation CA must
periodically be polled to take care of background activity. This requires
that your application must either wait in one of ca_pend_event(),
ca_pend_io(), or ca_sg_block() or alternatively it must call ca_poll() at
least every 100 milli-seconds. In single threaded applications a file
descriptor manager like Xt or the interface described in fdManager.h can be
used to monitor both mouse clicks and also CA's file descriptors so that
ca_poll() can be called immediately when CA server messages arrives over the
network.
Avoid Emulating Bad Practices that May Still be
Common
With the embryonic releases of EPICS it was a common practice to examine a
channel's connection state, its native type, and its native element count by
directly accessing fields in a structure using a pointer stored in type
chid. Likewise, a user private pointer in the per channel
structure was also commonly set by directly accessing fields in the channel
structure. A number of difficulties arise from this practice, which has long
since been deprecated. For example, prior to release 3.13 it was recognized
that transient changes in certain private fields in the per channel structure
would make it difficult to reliably test the channels connection state using
these private fields directly. Therefore, in release 3.13 the names of
certain fields were changed to discourage this practice. Starting with
release 3.14 codes written this way will not compile. Codes intending to
maintain the highest degree of portability over a wide range of EPICS
versions should be especially careful. For example you should replace all
instances off channel_id->count with
ca_element_count(channel_id). This approach should be reliable
on all versions of EPICS in use today. The construct ca_puser(chid) =
xxxx is particularly problematic. The best mechanisms for setting the
per channel private pointer will be to pass the user private pointer in when
creating the channel. This approach is implemented on all versions.
Otherwise, you can also use ca_set_puser(CHID,PUSER), but this
function is available only after the first official (post beta) release of
EPICS 3.13.
Calling CA Functions from the vxWorks Shell
Thread
Calling CA functions from the vxWorks shell thread is a somewhat
questionable practice for the following reasons.




    

  The vxWorks shell thread runs at the very highest priority in the
    system and therefore socket calls are made at a priority that is above
    the priority of tNetTask − a practice that has caused the WRS IP
    kernel to get sick in the past. That symptom was observed some time ago,
    but we don’t know if WRS has fixed the problem.
  The vxWorks shell thread runs at the very highest priority in the
    system and therefore certain CA auxiliary threads will not get the
    priorities that are requested for them. This might cause problems only
    when in a CPU saturation situations.
  If the code does not call ca_context_destroy (ca_task_exit in past
    releases) then resources are left dangling.
  In EPICS R3.13 the CA client library installed vxWorks task exit
    handlers behaved strangely if CA functions were called from the vxWorks
    shell, ca_task_exit() wasn’t called, and the vxWorks shell
    restarted. In EPICS R3.14 vxWorks task exit handlers are not installed
    and therefore cleanup is solely the responsibility of the user. With
    EPICS R3.14 the user must call ca_context_destroy or ca_task_exit to
    clean up on vxWorks. This is the same behavior as on all other OS.
Calling CA Functions from POSIX signal
handlers
As you might expect, it isnt safe to call the CA client library from a
POSIX signal handler. Likewise, it isnt safe to call the CA client library
from interrupt context.
Function Call Reference

ca_context_create()

#include <cadef.h>
enum ca_preemptive_callback_select  
    { ca_disable_preemptive_callback, ca_enable_preemptive_callback }; 
int ca_context_create ( enum ca_preemptive_callback_select SELECT );
Description
This function, or ca_attach_context(),
should be called once from each thread prior to making any of the other
Channel Access calls. If one of the above is not called before making other
CA calls then a non-preemptive context is created by default, and future
attempts to create a preemptive context for the current threads will fail.
If ca_disable_preemptive_callback is specified then
additional threads are not allowed to join the CA context using
ca_context_attach() because allowing other threads to join implies that CA
callbacks will be called preemptively from more than one thread.
Arguments
  SELECT
    Specifies if preemptive callback is allowed. If it is allowed your
      callbacks might be called when the thread that calls this routine is
      not executing in the CA client library. Programmers who are unfamiliar
      with mutual exclusion locking in a multi-threaded environment should
      specify ca_disable_preemptive_callback. If
      ca_enable_preemptive_callback is specified then CA client background
      activities, such as connection management, will proceed even if the
      thread that calls this routine is not executing in the CA client
      library.
Returns
ECA_NORMAL - Normal successful completion
ECA_ALLOCMEM - Failed, unable to allocate space in pool
ECA_NOTTHREADED - Current thread is already a member of a non-preemptive
callback CA context (possibly created implicitly)
See Also
ca_context_destroy()
ca_context_destroy()
#include <cadef.h>
void ca_context_destroy();
Description
Shut down the calling thread's channel access client context and free any
resources allocated. Detach the calling thread from any CA client context.
Be advised that any user created threads that might have attached
themselves to the CA context must of course stop using it prior to its being
destroyed.
On many OS that execute programs in a process based environment the
resources used by the client library such as sockets and allocated memory are
automatically released by the system when the process exits and
ca_context_destroy() hasn't been called, but on light weight systems such as
vxWorks or RTEMS no cleanup occurs unless the application call
ca_context_destroy().
Returns
ECA_NORMAL - Normal successful completion
See Also
ca_context_create()
ca_create_channel()
#include <cadef.h>
typedef void ( *pCallBack ) (
         struct connection_handler_args );
int ca_create_channel
        const char     *PROCESS_VARIABLE_NAME, 
        caCh           *USERFUNC, 
        void           *PUSER,
        capri          priority,
        chid           *PCHID
Description
This function creates a CA channel. The CA client library will attempt to
establish and maintain a virtual circuit between the caller's application and
a named process variable in a CA server. Each call to ca_create_channel
allocates resources in the CA client library and potentially also a CA
server. The function ca_clear_channel() is used to release these resources.
If successful, the routine writes a channel identifier into the user's
variable of type "chid". This identifier can be used with any channel access
call that operates on a channel.
The circuit may be initially connected or disconnected depending on the
state of the network and the location of the channel. A channel will
only enter a connected state after server's address is determined, and only
if channel access successfully establishes a virtual circuit through the
network to the server. Channel access routines that send a request to a
server will return ECA_DISCONNCHID if the channel is currently
disconnected.
There are two ways to obtain asynchronous notification when a channel
enters a connected state.
  The first and simplest method requires that you call ca_pend_io(), and
    wait for successful completion, prior to using a channel that was created
    specifying a nil connection call back function pointer.
  The second method requires that you register a connection handler by
    supplying a valid connection callback function pointer. This connection
    handler is called whenever the connection state of the channel changes.
    If you have installed a connection handler then ca_pend_io() will
    not block waiting for the channel to enter a connected
  state.
The function ca_state(CHID) can be used to test the connection state of a
channel. Valid connections may be isolated from invalid ones with this
function if ca_pend_io() times out.
Due to the inherently transient nature of network connections the order of
connection call backs relative to the order that ca_create_channel() calls
are made by the application can't be guaranteed, and application programs may
need to be prepared for a connected channel to enter a disconnected state at
any time.
Example
See caExample.c in the example application created by makeBaseApp.pl.
Arguments
  PROCESS_VARIABLE_NAME
    A nil terminated process variable name string. EPICS process control
      function block database variable names are of the form "<record
      name>.<field name>". If the field name and the period
      separator are omitted then the "VAL" field is implicit. For example
      "RFHV01" and "RFHV01.VAL" reference the same EPICS process control
      function block database variable.
  USERFUNC
    Optional address of the user's call back function to be run when the
      connection state changes. Casual users of channel access may decide to
      set this field to nil or 0 if they do not need to have a call back
      function run in response to each connection state change event.
      The following structure is passed by value to the user's
      connection connection callback function. The op field will
      be set by the CA client library to CA_OP_CONN_UP when the
      channel connects, and to CA_OP_CONN_DOWN when the channel
      disconnects. See ca_puser if the
      PUSER argument is required in your callback
      handler.
      struct  ca_connection_handler_args { 
    chanId  chid;  /* channel id */   
    long    op;    /* one of CA_OP_CONN_UP or CA_OP_CONN_DOWN */ 
  PUSER
    The value of this void pointer argument is retained in
      storage associated with the specified channel. See the MACROS manual
      page for reading and writing this field. Casual users of channel access
      may wish to set this field to nil or 0.
  PRIORITY
    The priority level for dispatch within the server or network with 0
      specifying the lowest dispatch priority and 99 the highest. This
      parameter currently does not impact dispatch priorities within the
      client, but this might change in the future. The abstract priority
      range specified is mapped into an operating system specific range of
      priorities within the server. This parameter is ignored if the server
      is running on a network or operating system that does not have native
      support for prioritized delivery or execution respectively. Specifying
      many different priorities within the same program can increase resource
      consumption in the client and the server because an independent virtual
      circuit, and associated data structures, is created for each priority
      that is used on a particular server.
  PCHID
    The user supplied channel identifier storage is overwritten with a
      channel identifier if this routine is successful.
Returns
ECA_NORMAL - Normal successful completion
ECA_BADTYPE - Invalid DBR_XXXX type
ECA_STRTOBIG - Unusually large string
ECA_ALLOCMEM - Unable to allocate memory
ca_clear_channel()

#include <cadef.h>
int ca_clear_channel (chid CHID);
Description
Shutdown and reclaim resources associated with a channel created by
ca_create_channel().
All remote operation requests such as the above are accumulated (buffered)
and not forwarded to the IOC until one of ca_flush_io, ca_pend_io,
ca_pend_event, or ca_sg_pend are called. This allows several requests to be
efficiently sent over the network in one message.
Clearing a channel does not cause its disconnect handler to be called, but
clearing a channel does shutdown and reclaim any channel state change event
subscriptions (monitors) registered with the channel.
Arguments
    Identifies the channel to delete.
Returns
ECA_NORMAL - Normal successful completion
ECA_BADCHID - Corrupted CHID
ca_put()
#include <cadef.h>
int ca_put ( chtype TYPE, 
        chid CHID, void *PVALUE ); 
int ca_array_put ( chtype TYPE, 
        unsigned long COUNT, 
        chid CHID, const void *PVALUE);
typedef void ( *pCallBack ) (struct event_handler_args );
int ca_put_callback ( chtype TYPE, 
        chid CHID, const void *PVALUE, 
        pCallBack PFUNC, void *USERARG ); 
int ca_array_put_callback ( chtype TYPE, 
        unsigned long COUNT, 
        chid CHID, const void *PVALUE, 
        pCallBack PFUNC, void *USERARG );
Description
Write a scalar or array value to a process variable.
When ca_array_put  or ca_put are invoked the client will receive no
response unless the request can not be fulfilled in the server. If
unsuccessful an exception handler is run on the client side. If a connection
is lost and then resumed outstanding ca_array_put  or ca_put  requests are
not automatically reissued following reconnect, and no additional
notification are provided to the user for each put request.
When ca_array_put_callback are invoked the user supplied asynchronous call
back is called only after the initiated write operation and all actions
resulting from the initiating write operation complete. If unsuccessful the
call back function is invoked indicating bad status. If the channel
disconnects before a put callback request can be completed, then the client's
call back function is called with bad status, but this does not guarantee
that the server did not receive and process the request before the
disconnect.
All of these functions return ECA_DISCONN if the channel is currently
disconnected.
All put requests are accumulated (buffered) and not forwarded to the IOC
until one of ca_flush_io, ca_pend_io, ca_pend_event, or ca_sg_pend are
called. This allows several requests to be efficiently combined into one
message.
Arguments
    The external type of the supplied value to be written. Conversion
      will occur if this does not match the native type. Specify one from the
      set of DBR_XXXX in db_access.h
  COUNT
    Element count to be written to the specified channel. This must match
      the array pointed to by PVALUE.
    Channel identifier
  PVALUE
    Pointer to a value or array of values provided by the application to
      be written to the channel.
  PFUNC
    address of user supplied callback function to be
      run when the requested operation completes
  USERARG
    pointer sized variable retained and then passed back to user supplied
      function above
Returns
ECA_NORMAL - Normal successful completion
ECA_BADCHID - Corrupted CHID
ECA_BADTYPE - Invalid DBR_XXXX type
ECA_BADCOUNT - Requested count larger than native element count
ECA_STRTOBIG - Unusually large string supplied
ECA_NOWTACCESS - Write access denied
ECA_ALLOCMEM - Unable to allocate memory
ECA_DISCONN - Channel is disconnected
See Also
ca_flush_io()
ca_pend_event()
ca_get()

#include <cadef.h>
int ca_get ( chtype TYPE, 
        chid CHID, void *PVALUE ); 
int ca_array_get ( chtype TYPE, unsigned long COUNT, 
        chid CHID, void *PVALUE ); 
typedef void ( *pCallBack ) (struct event_handler_args );
int ca_get_callback ( chtype TYPE, 
        chid CHID, pCallBack USERFUNC, void *USERARG);
int ca_array_get_callback ( chtype TYPE, unsigned long COUNT, 
        chid CHID, 
        pCallBack USERFUNC, void *USERARG );
Description
Read a scalar or array value from a process variable.
When ca_get or ca_array_get are invoked the returned channel value cant be
assumed to be stable in the application supplied buffer until after
ECA_NORMAL is returned from ca_pend_io. If a connection is lost outstanding
get requests are not automatically reissued following reconnect.
When ca_get_callback or ca_array_get_callback are invoked a value is read
from the channel and then the user's callback is invoked with a pointer to
the retrieved value. Note that ca_pend_io will not block for the delivery of
values requested by ca_get_callback. If the channel disconnects before a get
callback request can be completed, then the clients call back function is
called with bad status. 
All of these functions return ECA_DISCONN if the channel is currently
disconnected.
All get requests are accumulated (buffered) and not forwarded to the IOC
until one of ca_flush_io, ca_pend_io, ca_pend_event, or ca_sg_pend are
called. This allows several requests to be efficiently sent over the network
in one message.
Example
See caExample.c in the example application created by makeBaseApp.pl.
Arguments
    The external type of the user variable to return the value into.
      Conversion will occur if this does not match the native type. Specify
      one from the set of DBR_XXXX in db_access.h
  COUNT
    Element count to be read from the specified channel. Must match the
      array pointed to by PVALUE.
    Channel identifier
  PVALUE
    Pointer to an application supplied buffer where the current value of
      the channel is to be written.
  USERFUNC
    Address of user supplied callback function to be
      run when the requested operation completes.
  USERARG
    Pointer sized variable retained and then passed back to user supplied
      call back function above.
Returns
ECA_NORMAL - Normal successful completion
ECA_BADTYPE - Invalid DBR_XXXX type
ECA_BADCHID - Corrupted CHID
ECA_BADCOUNT - Requested count larger than native element count
ECA_GETFAIL - A local database get failed
ECA_NORDACCESS - Read access denied
ECA_ALLOCMEM - Unable to allocate memory
ECA_DISCONN - Channel is disconnected
See Also
ca_pend_io()
ca_pend_event()
ca_create_subscription()

#include <cadef.h>
typedef void ( *pCallBack ) (
        struct event_handler_args );
int ca_create_subscription ( chtype TYPE, 
        unsigned long COUNT, chid CHID, 
        unsigned long MASK, pCallBack USERFUNC, void *USERARG, 
        evid *PEVID );
Description
Register a state change subscription and specify a call back function to
be invoked whenever the process variable undergoes significant state changes.
A significant change can be a change in the process variable's value, alarm
status, or alarm severity. In the process control function block database the
deadband field determines the magnitude of a significant change for for the
process variable's value. Each call to this function consumes resources in
the client library and potentially a CA server until one of ca_clear_channel
or ca_clear_event is called.
Subscriptions may be installed or canceled against both connected and
disconnected channels. The specified USERFUNC is called once immediately
after the subscription is installed with the process variable's current state
if the process variable is connected. Otherwise, the specified USERFUNC is
called immediately after establishing a connection (or reconnection) with the
process variable. The specified USERFUNC is called immediately with the
process variable's current state from within ca_add_event() if the client and
the process variable share the same address space.




    

If a subscription is installed on a channel in a disconnected state then
the requested count will be set to the native maximum element count of the
channel if the requested count is larger.
All subscription requests such as the above are accumulated (buffered) and
not forwarded to the IOC until one of ca_flush_io, ca_pend_io, ca_pend_event,
or ca_sg_pend are called. This allows several requests to be efficiently sent
over the network in one message.
If at any time after subscribing, read access to the specified process
variable is lost, then the call back will be invoked immediately indicating
that read access was lost via the status argument. When read access is
restored normal event processing will resume starting always with at
least one update indicating the current state of the channel.
A better name for this function might have been ca_subscribe.
Example
See caMonitor.c in the example application created by makeBaseApp.pl.
Arguments
    The type of value presented to the call back funstion. Conversion
      will occur if it does not match native type. Specify one from the set
      of DBR_XXXX in db_access.h
  COUNT
    The element count to be read from the specified channel. A count of
      zero specifies the native elemnt count.
    channel identifier
  USRERFUNC
    The address of user supplied callback function to
      be invoked with each subscription update.
  USERARG
    pointer sized variable retained and passed back to user callback
      function
  RESERVED
    Reserved for future use. Specify 0.0 to remain upwardly
    compatible.
  PEVID
    This is a pointer to user supplied event id which is overwritten if
      successful. This event id can later be used to clear a specific
      event. This option may may be omitted by passing a nil pointer.
    A mask with bits set for each of the event trigger types requested.
      The event trigger mask must be a bitwise or of one or more of
      the following constants.
        DBE_VALUE - Trigger events when the channel value exceeds the
          monitor dead band
        DBE_LOG - Trigger events when the channel value exceeds the
          archival dead band
        DBE_ALARM - Trigger events when the channel alarm state
        changes.
      For functions above that do not include a trigger specification,
      events will be triggered when there are significant changes in the
      channel's value or when there are changes in the channel's alarm state.
      This is the same as "DBE_VALUE | DBE_ALARM."
Returns
ECA_NORMAL - Normal successful completion
ECA_BADCHID - Corrupted CHID
ECA_BADTYPE - Invalid DBR_XXXX type
ECA_ALLOCMEM - Unable to allocate memory
ECA_ADDFAIL - A local database event add failed
See Also
ca_pend_event()
ca_flush_io()
ca_clear_subscription()

#include <cadef.h>
int ca_clear_subscription ( evid EVID );
Description
Cancel a subscription.
All ca_clear_event() requests such as the above are accumulated (buffered)
and not forwarded to the server until one of ca_flush_io, ca_pend_io,
ca_pend_event, or ca_sg_pend are called. This allows several requests to be
efficiently sent together in one message.
Arguments
    
event id returned by ca_add_event()

Returns
ECA_NORMAL - Normal successful completion
ECA_BADCHID - Corrupted CHID SEE ALSO ca_add_event()
ca_pend_io()
#include <cadef.h>
int ca_pend_io ( double TIMEOUT );
Description
This function flushes the send buffer and then blocks until outstanding ca_get requests complete, and until channels created
specifying nill connection handler function pointers connect for the first
time.
  If ECA_NORMAL is returned then it can be safely assumed that all
    outstanding ca_get requests have completed
    successfully and  channels created specifying nill connection handler
    function pointers have connected for the first time.
  If ECA_TIMEOUT is returned then it must be assumed for all previous ca_get requests and properly qualified first time
    channel connects have failed.
If ECA_TIMEOUT is returned then get requests may be reissued followed by a
subsequent call to ca_pend_io(). Specifically, the function will block only
for outstanding ca_get requests issued, and also any
channels created specifying a nill connection handler function pointer, after
the last call to ca_pend_io() or ca client context creation whichever is
later. Note that ca_create_channel requests
generally should not be reissued for the same process variable unless ca_clear_channel is called first.
If no ca_get or connection state change events are
outstanding then ca_pend_io() will flush the send buffer and return
immediately without processing any outstanding channel access background
activities.
The delay specified to ca_pend_io() should take into account worst case
network delays such as Ethernet collision exponential back off until
retransmission delays which can be quite long on overloaded networks.
Unlike ca_pend_event, this routine will
not process CA's background activities if none of the selected IO requests
are pending.
Arguments
  TIMEOUT
    Specifies the time out interval. A TIMEOUT interval of
      zero specifies forever.
Returns
ECA_NORMAL - Normal successful completion
ECA_TIMEOUT - Selected IO requests didnt complete before specified
timeout
ECA_EVDISALLOW - Function inappropriate for use within an event handler
See Also
ca_get()
ca_create_channel()
ca_test_io()
ca_test_io()
#include <cadef.h>
int ca_test_io();
Description
This function tests to see if all ca_get requests
are complete and channels created specifying a nill connection callback
function pointer are connected. It will report the status of outstanding ca_get requests issued, and channels created specifying a
nill connection callback function pointer, after the last call to
ca_pend_io() or CA context initialization whichever is later.
Returns
ECA_IODONE - All IO operations completed
ECA_IOINPROGRESS - IO operations still in progress
See Also
ca_pend_io()
ca_pend_event()
#include <cadef.h>
int ca_pend_event ( double TIMEOUT );
int ca_poll ();
Description
When ca_pend_event is invoked the send buffer is flushed and CA background
activity is processed for TIMEOUT seconds.
When ca_poll is invoked the send buffer is flushed and any outstanding CA
background activity is processed.
The ca_pend_event function will not return before the specified
time-out expires and all unfinished channel access labor has been processed,
and unlike ca_pend_io returning from
the function does not indicate anything about the status of pending
IO requests.
Both ca_pend_event and ca_poll return
ECA_TIMEOUT when successful. This behavior probably isn't intuitive, but it
is preserved to insure backwards compatibility.
See also Thread Safety and Preemptive Callback to User
Code.
Arguments
  TIMEOUT
    The duration to block in this routine in seconds. A timeout of zero
      seconds blocks forever.
Returns
ECA_TIMEOUT - The operation timed out
ECA_EVDISALLOW - Function inappropriate for use within a call back
handler
ca_flush_io()
#include <cadef.h>
int ca_flush_io();
Description
Flush outstanding IO requests to the server. This routine might be useful
to users who need to flush requests prior to performing client side labor in
parallel with labor performed in the server.
Outstanding requests are also sent whenever the buffer which holds them
becomes full.
Returns
ECA_NORMAL - Normal successful completion
ca_signal()
#include <cadef.h>
int ca_signal ( long CA_STATUS, const char * CONTEXT_STRING ); 
void SEVCHK( CA_STATUS, CONTEXT_STRING );
Description
Provide the error message character string associated with the supplied
channel access error code and the supplied error context to diagnostics. If
the error code indicates an unsuccessful operation a stack dump is printed,
if this capability is available on the local operating system, and execution
is terminated.
SEVCHK is a macro envelope around ca_signal which only calls ca_signal()
if the supplied error code indicates an unsuccessful operation. SEVCHK is the
recommended error handler for simple applications which do not wish to write
code testing the status returned from each channel access call.
Examples
status = ca_context_create (...); 
SEVCHK ( status, "Unable to create a CA client context" );
If the application only wishes to print the message associated with an
error code or test the severity of an error there are also functions provided
for this purpose.
Arguments
  CA_STATUS
    The status (error code) returned from a channel access function.
  CONTEXT_STRING
    A null terminated character string to supply as error context to
      diagnostics.
Returns
ECA_NORMAL - Normal successful completion
ca_add_exception_event()
#include <cadef.h> 
typedef void (*pCallback) ( struct exception_handler_args HANDLERARGS );
int ca_add_exception_event ( pCallback  USERFUNC, void *USERARG );
Description
Replace the currently installed CA context global exception handler call
back.
When an error occurs in the server asynchronous to the clients thread then
information about this type of error is passed from the server to the client
in an exception message. When the client receives this exception message an
exception handler callback is called.The default exception handler prints a
diagnostic message on the client's standard out and terminates execution if
the error condition is severe.
Note that certain fields in "struct exception_handler_args" are not
applicable in the context of some error messages. For instance, a failed get
will supply the address in the client task where the returned value was
requested to be written. For other failed operations the value of the addr
field should not be used.
Arguments
  USERFUNC
    Address of user callback function to be executed when an exceptions
      occur. Passing a nil value causes the default exception handler to be
      reinstalled. The following structure is passed by value to the user's
      callback function. Currently, the op field can be one of
      CA_OP_GET, CA_OP_PUT, CA_OP_CREATE_CHANNEL, CA_OP_ADD_EVENT,
      CA_OP_CLEAR_EVENT, or CA_OP_OTHER.
      struct exception_handler_args {
    void            *usr;   /* user argument supplied when installed */
    chanId          chid;   /* channel id (may be nill) */
    long            type;   /* type requested */
    long            count;  /* count requested */
    void            *addr;  /* user's address to write results of CA_OP_GET */
    long            stat;   /* channel access ECA_XXXX status code */
    long            op;     /* CA_OP_GET, CA_OP_PUT, ..., CA_OP_OTHER */
    const char      *ctx;   /* a character string containing context info */
    sonst char      *pFile; /* source file name (may be NULL) */
    unsigned        lineNo; /* source file line number (may be zero) */
  USERARG
    pointer sized variable retained and passed back to user function
    above
Example
void ca_exception_handler (
        struct exception_handler_args args)
        char buf[512];
        char *pName;
        if ( args.chid ) {
                pName = ca_name ( args.chid );
        else{
                pName = "?";
        sprintf ( buf,
                "%s - with request chan=%s op=%d data type=%s count=%d", 
                args.ctx, pName, args.op, dbr_type_to_text ( args.type ), args.count ); 
        ca_signal ( args.stat, buf ); 
ca_add_exception_event ( ca_exception_handler , 0 );
Returns
ECA_NORMAL - Normal successful completion
ca_replace_printf_handler
#include <cadef.h>
typedef int caPrintfFunc ( const char *pFromat, va_list args );
int ca_replace_printf_handler ( caPrintfFunc *PFUNC );
Description
Replace the default handler for formatted diagnostic message output. The
default handler uses fprintf to send messages to 'stderr'.
Arguments
  PFUNC
    The address of a user supplied call back handler to be invoked when
      CA prints diagnostic messages. Installing a nil pointer will cause the
      default call back handler to be reinstalled.
Examples
int my_printf ( char *pformat, va_list args ) {
        int status;
        status = vfprintf( stderr, pformat, args);
        return status;
status = ca_replace_printf_handler ( my_printf );
SEVCHK ( status, "failed to install my printf handler" );
Returns
ECA_NORMAL - Normal successful completion
ca_replace_access_rights_event()

#include <cadef.h>
typedef void ( *pCallBack )( struct access_rights_handler_args );
int ca_replace ( chid CHAN, pCallBack PFUNC );
Description
Install or replace the access rights state change callback handler for the
specified channel.
The callback handler is called in the following situations.
  whenever CA connects the channel immediately before the channel's
    connection handler is called
  whenever CA disconnects the channel immediately after the channel's
    disconnect call back is called
  once immediately after installation if the channel is connected.
  whenever the access rights state of a connected channel changes
When a channel is created no access rights handler is installed.
Arguments
    The channel identifier.
  PFUNC
    Address of user supplied call back function. A nil pointer uninstalls
      the current handler. The following arguments are passed by
      value to the supplied callback handler.
      typedef struct ca_access_rights {
    unsigned    read_access:1;
    unsigned    write_access:1;
} caar;
/* arguments passed to user access rights handlers */
struct  access_rights_handler_args {
    chanId  chid;   /* channel id */
    caar    ar;     /* new access rights state */
Returns
ECA_NORMAL - Normal successful completion
See Also
ca_modify_user_name()
ca_modify_host_name()
ca_field_type()
#include <cadef.h>
chtype ca_field_type ( CHID );
Description
Return the native type in the server of the process variable.
Arguments
    channel identifier
Returns
    The data type code will be a member of the set of DBF_XXXX in
      db_access.h. The constant TYPENOTCONN is returned if the channel is
      disconnected.

ca_element_count()
#include <cadef.h>
unsigned ca_element_count ( CHID );
Description
Return the maximum array element count in  the server for the specified IO
channel.
Arguments
    channel identifier
Returns
  COUNT
    The maximum array element count in  the server. An element count of
      zero is returned if the channel is disconnected.
ca_name()

#include <cadef.h>
char * ca_name ( CHID );
Description
Return the name provided when the supplied channel id was created.
Arguments
    channel identifier
Returns
  PNAME
    The channel name. The string returned is valid as long as the channel
      specified exists.
ca_set_puser()
#include <cadef.h>
void ca_set_puser ( chid CHID, void *PUSER );
Description
Set a user private void pointer variable retained with each channel for
use at the users discretion.
Arguments
    channel identifier
  PUSER
    user private void pointer
ca_puser()
#include <cadef.h>
void * ca_puser ( CHID );
Description
Return a user private void pointer variable retained with each channel for
use at the users discretion.
Arguments
    channel identifier
Returns
  PUSER
    user private pointer
ca_state()
#include <cadef.h>
enum channel_state {
        cs_never_conn, /* valid chid, server not found or unavailable */
        cs_prev_conn,  /* valid chid, previously connected to server */
        cs_conn,       /* valid chid, connected to server */
        cs_closed };   /* channel deleted by user */
enum channel_state ca_state ( CHID );
Description
Returns an enumerated type indicating the current state of the specified
IO channel.




    

Arguments
    channel identifier
Returns
  STATE
    the connection state
ca_message()
#include <cadef.h>
const char * ca_message ( STATUS );
Description
return a message character string corresponding to a user specified CA
status code.
Arguments
  STATUS
    a CA status code
Returns
  STRING

    the corresponding error message string
ca_host_name()
#include <cadef.h>
char * ca_host_name ( CHID );
Description
Return a character string which contains the name of the host to which a
channel is currently connected.
Arguments
    the channel identifier
Returns
  STRING
    The process variable server's host name. If the channel is
      disconnected the string "<disconnected>" is returned.
ca_read_access()
#include <cadef.h>
int ca_read_access ( CHID );
Description
Returns boolean true if the client currently has read access to the
specified channel and boolean false otherwise.
Arguments
    the channel identifier
Returns
  STRING
    boolean true if the client currently has read access to the specified
      channel and boolean false otherwise
ca_write_access()
#include <cadef.h>
int ca_write_access ( CHID );
Description
Returns boolean true if the client currently has write access to the
specified channel and boolean false otherwise.
Arguments
    the channel identifier
Returns
  STRING
    boolean true if the client currently has write access to the
      specified channel and boolean false otherwise
dbr_size[]
#include <db_access.h>
extern unsigned dbr_size[/*TYPE*/];
Description
An array that returns the size in bytes for a DBR_XXXX type.
Arguments
    The data type code. A member of the set of DBF_XXXX in
    db_access.h.
Returns
    the size in bytes of the specified type
dbr_size_n()
#include <db_access.h>
unsigned dbr_size_n  (  TYPE, COUNT  );
Description
Returns the size in bytes for a DBR_XXXX type with COUNT elements. If the
DBR type is a structure then the value field is the last field in the
structure. If COUNT is greater than one then COUNT-1 elements are appended to
the end of the structure so that they can be addressed as an array through a
pointer to the value field.
Arguments
    The data type
  COUNT
    The element count
Returns
    the size in bytes of the specified type with the specified number of
      elements
dbr_value_size[]
#include <db_access.h>
extern unsigned dbr_value_size[/* TYPE */];
Description
The array dbr_value_size[TYPE] returns the size in bytes for the value
stored in a DBR_XXXX type. If the type is a structure the size of the value
field is returned otherwise the size of the type is returned.
Arguments
    The data type code. A member of the set of DBF_XXXX in
    db_access.h.
Returns
    the size in bytes of the value field if the type is a structure and
      otherwise the size in bytes of the type
dbr_type_to_text()
#include <db_access.h>
const char * dbr_type_text ( chtype TYPE );
Description
Returns a constant null terminated string corresponding to the specified
dbr type.
Arguments
    The data type code. A member of the set of DBR_XXXX in
    db_access.h.
Returns
  STRING
    The const string corresponding to the DBR_XXX type.
ca_test_event()
#include <cadef.h>
Description
void ca_test_event ( struct event_handler_args );
A built-in subscription update call back handler for debugging purposes
that prints diagnostics to standard out.
Examples
void ca_test_event (); 
status = ca_add_event ( type, chid, ca_test_event, NULL, NULL ); 
SEVCHK ( status, .... );
See Also
ca_add_event()
ca_sg_create()
#include <cadef.h>
int ca_sg_create ( CA_SYNC_GID *PGID );
Description
Create a synchronous group and return an identifier for it.
A synchronous group can be used to guarantee that a set of channel access
requests have completed. Once a synchronous group has been created then
channel access get and put requests may be issued within it using ca_sg_get()
and ca_sg_put() respectively. The routines ca_sg_block() and ca_sg_test() can
be used to block for and test for completion respectively. The routine
ca_sg_reset() is used to discard knowledge of old requests which have timed
out and in all likelihood will never be satisfied.
Any number of asynchronous groups can have application requested
operations outstanding within them at any given time.
Arguments
    Pointer to a user supplied CA_SYNC_GID.
Examples
CA_SYNC_GID gid; 
status = ca_sg_create ( &gid ); 
SEVCHK ( status, Sync group create failed );
Returns
ECA_NORMAL - Normal successful completion
ECA_ALLOCMEM - Failed, unable to allocate memory
See Also
ca_sg_delete()
ca_sg_block()
ca_sg_test()
ca_sg_reset()
ca_sg_put()
ca_sg_get()
ca_sg_delete()
#include <cadef.h>
int ca_sg_delete ( CA_SYNC_GID GID );
Description
Deletes a synchronous group.
Arguments
    Identifier of the synchronous group to be deleted.
Examples
CA_SYNC_GID gid; 
status = ca_sg_delete ( gid ); 
SEVCHK ( status, Sync group delete failed );
Returns
ECA_NORMAL - Normal successful completion
ECA_BADSYNCGRP - Invalid synchronous group
See Also
ca_sg_create()
ca_sg_block()
#include <cadef.h>
int ca_sg_block ( CA_SYNC_GID GID, double timeout );
Description
Flushes the send buffer and then waits until outstanding requests complete
or the specified time out expires. At this time outstanding requests include
calls to ca_sg_array_get() and calls to ca_sg_array_put(). If ECA_TIMEOUT is
returned then failure must be assumed for all outstanding queries. Operations
can be reissued followed by another ca_sg_block(). This routine will only
block on outstanding queries issued after the last call to ca_sg_block(),
ca_sg_reset(), or ca_sg_create() whichever occurs later in time. If no
queries are outstanding then ca_sg_block() will return immediately without
processing any pending channel access activities.
Values written into your program's variables by a channel access
synchronous group request should not be referenced by your program until
ECA_NORMAL has been received from ca_sg_block(). This routine will process
pending channel access background activity while it is waiting.
Arguments
    Identifier of the synchronous group.
Examples
CA_SYNC_GID gid; 
status = ca_sg_block(gid); 
SEVCHK(status, Sync group block failed);
Returns
ECA_NORMAL - Normal successful completion
ECA_TIMEOUT - The operation timed out
ECA_EVDISALLOW - Function inappropriate for use within an event handler
ECA_BADSYNCGRP - Invalid synchronous group
See Also
ca_sg_test()
ca_sg_reset()
ca_sg_test()
#include <cadef.h>
int ca_sg_test  ( CA_SYNC_GID GID )
Description
Test to see if all requests made within a synchronous group have
completed.
Arguments
    Identifier of the synchronous group.
Description
Test to see if all requests made within a synchronous group have
completed.
Examples
CA_SYNC_GID gid;
status = ca_sg_test ( gid );
Returns
ECA_IODONE - IO operations completed
ECA_IOINPROGRESS - Some IO operations still in progress
ca_sg_reset()
#include <cadef.h>
int ca_sg_reset ( CA_SYNC_GID GID )
Description
Reset the number of outstanding requests within the specified synchronous
group to zero so that ca_sg_test() will return ECA_IODONE and ca_sg_block()
will not block unless additional subsequent requests are made.
Arguments
    Identifier of the synchronous group.
Examples
CA_SYNC_GID gid; 
status = ca_sg_reset(gid);
Returns
ECA_NORMAL - Normal successful completion
ECA_BADSYNCGRP - Invalid synchronous group
ca_sg_put()
#include <cadef.h>
int ca_sg_array_put ( CA_SYNC_GID GID, chtype TYPE, 
        unsigned long COUNT, chid CHID, void *PVALUE );
Write a value, or array of values, to a channel and increment the
outstanding request count of a synchronous group.
All remote operation requests such as the above are accumulated (buffered)
and not forwarded to the server until one of ca_flush_io(), ca_pend_io(),
ca_pend_event(), or ca_sg_pend() are called. This allows several requests to
be efficiently sent in one message.
If a connection is lost and then resumed outstanding puts are not
reissued.
Arguments
    synchronous group identifier
    The type of supplied value. Conversion will occur if it does not
      match the native type. Specify one from the set of DBR_XXXX in
      db_access.h.
  COUNT
    element count to be written to the specified channel - must match the
      array pointed to by PVALUE
    channel identifier
  PVALUE
    A pointer to an application supplied buffer containing the value or
      array of values returned
Returns
ECA_NORMAL - Normal successful completion
ECA_BADSYNCGRP - Invalid synchronous group
ECA_BADCHID - Corrupted CHID
ECA_BADTYPE - Invalid DBR_XXXX type
ECA_BADCOUNT - Requested count larger than native element count
ECA_STRTOBIG - Unusually large string supplied
ECA_PUTFAIL - A local database put failed
See Also
ca_flush_io()
ca_sg_get()
#include <cadef.h>
int ca_sg_array_get ( CA_SYNC_GID GID, 
        chtype TYPE, unsigned long COUNT, 
        chid CHID, void *PVALUE );
Description
Read a value from a channel and increment the outstanding request count of
a synchronous group.
The values written into your program's variables by ca_sg_get should not
be referenced by your program until ECA_NORMAL has been received from
ca_sg_block , or until ca_sg_test returns ECA_IODONE.
All remote operation requests such as the above are accumulated (buffered)
and not forwarded to the server until one of ca_flush_io, ca_pend_io,
ca_pend_event, or ca_sg_pend are called. This allows several requests to be
efficiently sent in one message.
If a connection is lost and then resumed outstanding gets are not
reissued.
Arguments
    Identifier of the synchronous group.
    External type of returned value. Conversion will occur if this does
      not match native type. Specify one from the set of DBR_XXXX in
      db_access.h
  COUNT
    Element count to be read from the specified channel. It must match
      the array pointed to by PVALUE.
    channel identifier
  PVALUE
    Pointer to application supplied buffer that is to contain the value
      or array of values to be returned
Returns
ECA_NORMAL - Normal successful completion 
ECA_BADSYNCGRP - Invalid synchronous group 
ECA_BADCHID - Corrupted CHID
ECA_BADCOUNT - Requested count larger than native element count
ECA_BADTYPE - Invalid DBR_XXXX type
ECA_GETFAIL - A local database get failed
See Also
ca_pend_io()
ca_flush_io()
ca_get_callback()
ca_client_status()
int ca_client_status ( unsigned level );
int ca_context_status ( struct ca_client_context *, 
       unsigned level );
Description
Prints information about the client context including, at higher interest
levels, status for each channel. Lacking a CA context pointer,
ca_client_status() prints information about the calling threads CA
context.
Arguments
  CONTEXT
    A pointer to the CA context to join with.
  LEVEL
    The interest level. Increasing level produces increasing detail.
ca_current_context()
struct ca_client_context * ca_current_context ();
Description
Returns a pointer to the current thread's CA context. If none then nil is
returned.
See Also
ca_attach_context()
ca_detach_context()
ca_context_create()
ca_context_destroy()
ca_attach_context()
int ca_attach_context (struct ca_client_context *CONTEXT);
Description
The calling thread becomes a member of the specified CA context. If
ca_disable_preemptive_callback is specified when
ca_context_create() is called (or if ca_task_initialize() is called) then
additional threads are not allowed to join the CA context because
allowing other threads to join implies that CA callbacks will be called
preemptively from more than one thread.
Arguments
  CONTEXT
    A pointer to the CA context to join with.
Returns
ECA_ISATTACHED - already attached to a CA context
ECA_NOTTHREADED - the specified context is non-preemptive and therefore
does not allow other threads to join
ECA_ISATTACHED - the current thread is already attached to a CA context
See Also
ca_current_context()
ca_detach_context()
ca_context_create()
ca_context_destroy()
ca_detach_context()
void ca_detach_context();
Description
Detach from any CA context currently attached to the calling thread. This
does not cleanup or shutdown any currently attached CA context (for
that use ca_context_destroy).
See Also
ca_current_context()
ca_attach_context()
ca_context_create()
ca_context_destroy()
ca_dump_dbr()
void ca_dump_dbr ( chtype TYPE, unsigned COUNT, const
void * PDBR );
Description
Dumps the specified dbr data type to standard out.
Arguments
    The data type (from the DBR_XXX set described in db_access.h).
  COUNT
    The array element count
    A pointer to data of the specified count and number.
Return Codes
  ECA_NORMAL
    Normal successful completion
  ECA_ALLOCMEM
    Unable to allocate additional dynamic memory
  ECA_TOLARGE
    The requested data transfer is greater than available memory or
      EPICS_CA_MAX_ARRAY_BYTES
  ECA_BADTYPE
    The data type specified is invalid
  ECA_BADSTR
    Invalid string
  ECA_BADCHID
    Invalid channel identifier
  ECA_BADCOUNT
    Invalid element count requested
  ECA_PUTFAIL
    Channel write request failed
  ECA_GETFAIL
    Channel read request failed
  ECA_ADDFAIL
    unable to install subscription request
  ECA_TIMEOUT
    User specified timeout on IO operation expired
  ECA_EVDISALLOW
    function called was inappropriate for use within a callback
    function
  ECA_IODONE
    IO operations have completed
  ECA_IOINPROGRESS
    IO operations are in progress
  ECA_BADSYNCGRP
    Invalid synchronous group identifier
  ECA_NORDACCESS
    Read access denied
  ECA_NOWTACCESS
    Write access denied
  ECA_DISCONN
    Virtual circuit disconnect"
  ECA_DBLCHNL
    Identical process variable name on multiple servers
  ECA_EVDISALLOW
    Request inappropriate within subscription (monitor) update
    callback
  ECA_BADMONID
    Bad event subscription (monitor) identifier
  ECA_BADMASK
    Invalid event selection mask
  ECA_PUTCBINPROG
    Put callback timed out
  ECA_PUTCBINPROG
    Put callback timed out
  ECA_ANACHRONISM
    Requested feature is no longer supported
  ECA_NOSEARCHADDR
    Empty PV search address list
  ECA_NOCONVERT
    No reasonable data conversion between client and server types
  ECA_BADFUNCPTR
    Invalid function pointer
  ECA_ISATTACHED
    Thread is already attached to a client context
  ECA_UNAVAILINSERV
    Not supported by attached service
  ECA_CHANDESTROY
    User destroyed channel
  ECA_BADPRIORITY
    Invalid channel priority
  ECA_NOTTHREADED
    Preemptive callback not enabled - additional threads may not join
      context
  ECA_16KARRAYCLIENT
    Client's protocol revision does not support transfers exceeding 16k
      bytes

Table of Contents

Functionality Index

Deprecated Function Call Interface Function Index

Why Reconfigure Channel Access

Server Beacons

Binding a Server to a Limited Set of Network Interfaces

Ignoring Process Variable Name Resolution Requests From Certain Hosts

Client Configuration that also Applies to Servers

Typical Linux Build Options

Typical Solaris Build Options

Typical Windows Build Options

Typical vxWorks Build Options

Other Systems and Compilers

Description

Description

Description

Description

Description

Description

Contributing Circumstances

Related Diagnostics

Server Subscription Update Queuing

Description

Arguments

Returns

See Also

Description

Returns

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Description

Example

Arguments

Returns

Description

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Description

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Example

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Example

Arguments

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Description

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Description

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Description

Arguments

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Description

Returns

ca_signal()

Description

Examples

Arguments

Returns

Description

Arguments

Example

Returns

Description

Arguments

Examples

Returns

Description

Arguments

`ca_signal()`