SEI CERT C Coding Standard

Preprocessor (PRE) - PRE31-C. Avoid side effects in arguments to unsafe macros
statement expression, makes it possible for the block statement to appear where an expression is
expected:
#define ABS(x) __extension__ ({ __typeof (x) tmp = x; \
tmp < 0 ? -tmp : tmp; })
Relying on such extensions makes code nonportable and violates MSC14-C. Do not introduce unnecessary
platform dependencies.
2.2.6 Noncompliant Code Example (assert())
The assert() macro is a convenient mechanism for incorporating diagnostic tests in code. (See
MSC11-C. Incorporate diagnostic tests using assertions.) Expressions used as arguments to the
standard assert() macro should not have side effects. The behavior of the assert() macro depends
on the definition of the object-like macro NDEBUG. If the macro NDEBUG is undefined, the
assert() macro is defined to evaluate its expression argument and, if the result of the expression
compares equal to 0, call the abort() function. If NDEBUG is defined, assert is defined to
expand to ((void)0). Consequently, the expression in the assertion is not evaluated, and no side
effects it may have had otherwise take place in non-debugging executions of the code.
This noncompliant code example includes an assert() macro containing an expression (index++)
that has a side effect:
#include
#include
void process(size_t index) {
assert(index++ > 0); /* Side effect */
/* ... */
}
2.2.7 Compliant Solution (assert())
This compliant solution avoids the possibility of side effects in assertions by moving the expression
containing the side effect outside of the assert() macro.
#include
#include
void process(size_t index) {
assert(index > 0); /* No side effect */
++index;
SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems 28
Software Engineering Institute | Carnegie Mellon University
[DISTRIBUTION STATEMENT A] Approved for public release and unlimited distribution.