25.2.1 Using the
getopt
function
Here are the details about how to call the
getopt
function. To
use this facility, your program must include the header file
unistd.h
.
Variable:
int
opterr
¶
If the value of this variable is nonzero, then
getopt
prints an
error message to the standard error stream if it encounters an unknown
option character or an option with a missing required argument. This is
the default behavior. If you set this variable to zero,
getopt
does not print any messages, but it still returns the character
?
to indicate an error.
Variable:
int
optopt
¶
When
getopt
encounters an unknown option character or an option
with a missing required argument, it stores that option character in
this variable. You can use this for providing your own diagnostic
messages.
Variable:
int
optind
¶
This variable is set by
getopt
to the index of the next element
of the
argv
array to be processed. Once
getopt
has found
all of the option arguments, you can use this variable to determine
where the remaining non-option arguments begin. The initial value of
this variable is
1
.
Variable:
char *
optarg
¶
This variable is set by
getopt
to point at the value of the
option argument, for those options that accept arguments.
Function:
int
getopt
(int
argc
, char *const *
argv
, const char *
options
)
¶
Preliminary:
| MT-Unsafe race:getopt env
| AS-Unsafe heap i18n lock corrupt
| AC-Unsafe mem lock corrupt
| See
POSIX Safety Concepts
.
The
getopt
function gets the next option argument from the
argument list specified by the
argv
and
argc
arguments.
Normally these values come directly from the arguments received by
main
.
The
options
argument is a string that specifies the option
characters that are valid for this program. An option character in this
string can be followed by a colon (‘
:
’) to indicate that it takes a
required argument. If an option character is followed by two colons
(‘
::
’), its argument is optional; this is a GNU extension.
getopt
has three ways to deal with options that follow
non-options
argv
elements. The special argument ‘
--
’ forces
in all cases the end of option scanning.
The default is to permute the contents of
argv
while scanning it
so that eventually all the non-options are at the end. This allows
options to be given in any order, even with programs that were not
written to expect this.
If the
options
argument string begins with a hyphen (‘
-
’), this
is treated specially. It permits arguments that are not options to be
returned as if they were associated with option character ‘
\1
’.
POSIX demands the following behavior: the first non-option stops option
processing. This mode is selected by either setting the environment
variable
POSIXLY_CORRECT
or beginning the
options
argument
string with a plus sign (‘
+
’).
The
getopt
function returns the option character for the next
command line option. When no more option arguments are available, it
returns
-1
. There may still be more non-option arguments; you
must compare the external variable
optind
against the
argc
parameter to check this.
If the option has an argument,
getopt
returns the argument by
storing it in the variable
optarg
. You don’t ordinarily need to
copy the
optarg
string, since it is a pointer into the original
argv
array, not into a static area that might be overwritten.
If
getopt
finds an option character in
argv
that was not
included in
options
, or a missing option argument, it returns
‘
?
’ and sets the external variable
optopt
to the actual
option character. If the first character of
options
is a colon
(‘
:
’), then
getopt
returns ‘
:
’ instead of ‘
?
’ to
indicate a missing option argument. In addition, if the external
variable
opterr
is nonzero (which is the default),
getopt
prints an error message.
