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Opening and closing braces for
if
,
for
, and
while
statements should always be used even if the statement's body contains only a single statement.
If an
if
,
while
, or
for
statement is used in a macro, the macro definition should not conclude with a semicolon. (See
PRE11-C. Do not conclude macro definitions with a semicolon
.)
Braces improve the uniformity and readability of code. More important, when inserting an additional statement into a body containing only a single statement, it is easy to forget to add braces because the indentation gives strong (but misleading) guidance to the structure.
Braces also help ensure that macros with multiple statements are properly expanded. Such a macro should be wrapped in a
do-while
loop. (See
PRE10-C. Wrap multistatement macros in a do-while loop
.) However, when the
do-while
loop is not present, braces can still ensure that the macro expands as intended.
Noncompliant Code Example
This noncompliant code example uses an
if
statement without braces to authenticate a user:
int login;
if (invalid_login())
login = 0;
login = 1;
A developer might add a debugging statement to determine when the login is valid but forget to add opening and closing braces:
int login;
if (invalid_login())
login = 0;
printf("Login is valid\n"); /* Debugging line added here */
login = 1; /* This line always gets executed
/* regardless of a valid login! */
Because of the indentation of the code, it is difficult to tell that the code will not function as intended by the programmer, potentially leading to a security breach.
Compliant Solution
In the compliant solution, opening and closing braces are used even when the body is a single statement:
int login;
if (invalid_login()) {
login = 0;
} else {
login = 1;
Noncompliant Code Example
This noncompliant code example has an
if
statement nested in another
if
statement without braces around the
if
and
else
bodies:
int privileges;
if (invalid_login())
if (allow_guests())
privileges = GUEST;
privileges = ADMINISTRATOR;
The indentation could lead the programmer to believe that a user is given administrator privileges only when the user's login is valid. However, the
else
statement actually attaches to the inner
if
statement:
int privileges;
if (invalid_login())
if (allow_guests())
privileges = GUEST;
privileges = ADMINISTRATOR;
This is a security loophole: users with invalid logins can still obtain administrator privileges.
Compliant Solution
In the compliant solution, adding braces removes the ambiguity and ensures that privileges are correctly assigned:
int privileges;
if (invalid_login()) {
if (allow_guests()) {
privileges = GUEST;
} else {
privileges = ADMINISTRATOR;
Noncompliant Code Example (empty block)
This noncompliant code example has a
while
statement with no block:
Note that if
invalid_login()
has no side effects (such as warning the user if their login failed), this code also violates
MSC12-C. Detect and remove code that has no effect or is never executed
.
Compliant Solution (empty block)
This compliant solution features an explicit empty block, which clarifies the developer's intent:
while (invalid_login()) {}
Risk Assessment
|
Recommendation
|
Severity
|
Likelihood
|
Remediation Cost
|
Priority
|
Level
|
|
EXP19-C
|
Medium
|
Probable
|
Medium
|
P8
|
L2
|
Automated Detection
Related Vulnerabilities
CVE-2014-1266
was due, in large part, to failing to follow this recommendation. There is a spurious "goto fail" statement on line 631 of
sslKeyExchange.c
. This "goto" gets executed unconditionally, even though it is indented as if it were part of the preceding "if" statement. As a result, the call to sslRawVerify (which performs the actual signature verification) is rendered dead code. [
ImperialViolet 2014
]. If the body of the "if" statement had been enclosed in braces, then this defect likely would not have happened.
Related Guidelines
Bibliography
In that case adding a debugging statement would throw an error when compiling, because of a dangling
else
statement not paired with a corresponding
if
statement:
if (valid_login())
printf("Login Successful\n"); /* debugging statement added here */
login = 1;
else /* unpaired else statement */
login = 0;
I recognize my code for the 3rd NCCE is similar to that of PRE10-C. Wrap multi-statement macros in a do-while loop. However, PRE10-C does not specify using braces. I was trying to show that even if one did not follow PRE10-C's recommendation to wrap multi-statement macros in a do-while statement, the situation can still be salvaged if braces are used in the code itself. How can I best do this?
The definition of the SWAP() macro in PRE10-C is suboptimal since it tacitly assumes that the tmp variable is declared and of the same type as the arguments but for the purposes of this exercise it could be defined as a single expression (note that this definition violates PRE12-C. Do not define unsafe macros):
#define SWAP(x, y) ((tmp = a), ((a) = (b)), ((b) = tmp))
In my opinion, using a swap function would be better (although not entirely without problems either):
#define SWAP(x, y) swap(&x, &y, sizeof x)
inline void swap(void *x, void *y, size_t nbytes) {
unsigned char tmp [nbytes];
memcpy(tmp, x, nbytes);
memcpy(x, y, nbytes);
memcpy(y, tmp, nbytes);
Another possible solution is to rely on a language extension such as gcc's typeof operator and Statements in Expressions (although this one skirts MSC14-C. Do not introduce unnecessary platform dependencies in addition to violating PRE33-C):
#define SWAP(x, y) ({ typeof x tmp = (x); (x) = (y); (y) = tmp; })
A better term than a single body line is a single statement: there could be more than one statement on a line:
if (a == 1)
b = 2; c = 3; // two statements on one line
This guideline is especially relevant when PRE11-C. Do not conclude macro definitions with a semicolon isn't being followed. It would be nice to tie the two together on both ends (and do the same for PRE10-C. Wrap multi-statement macros in a do-while loop that you already mentioned here).
Can I also provide hyperlinks to non-authoritative, personal web pages that discuss using braces in if statements?
Also, does my recommendation fulfil your earlier comments?
Can I also provide hyperlinks to non-authoritative, personal web pages that discuss using braces in if statements?
Yes, but only if you can't find any 'authoritative' pages. Books discussing C would prob be best, followed by official standards (eg MISRA) followed by well-known webpages (eg famous bloggers or stackoverflow.com).
Your third NCCE is already covered by PRE10-C. Wrap multi-statement macros in a do-while loop. You should probably just drop it as it is redundant.
Where do you get the information in "Automated detection" table from? Is it from the tool vendors? Or are you building this list on your own?
I ask this because I just found that this precise rule can be scanned using Coverity (tested with v8.5.0). You simply need to run a MISRA C 2012 scan using rule 15.6 checker. Could this information be added in the "Automated detection" table?
In the newly added exception, the keywords should be in code font. Also, was do left off that list purposefully?
FWIW, I'm not a fan of the new exception though I understand why it was added. I think empty compound statements should be encouraged because it calls out the oddity of the code.
if (something()); // Hard to spot
if (something()) {} // Easier to spot
if (something()) { /* Explanation */ } // Even better
I excluded do statements b/c the rec did. Since they are typically do...while statements, I assumed that they are a subset of while statements.
I'm now leaning against the exception...that is, empty statements should require an explicit {} block for clarity. I'll make this change tomorrow if I don't hear otherwise.
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