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The arguments to a macro should not include preprocessor directives such as {{\#define}}, {{\#ifdef}}, and {{\#include}}. Doing so is [undefined behavior|BB. Definitions#undefined behavior] according to section 6.10.3.11 of the C99 standard \[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\]. This includes using preprocessor directives in arguments to a function where it is unknown whether or not the function is implemented using a macro. Examples include standard library functions such as {{memcpy()}}, {{printf()}}, and {{assert()}}: |
The sequence of preprocessing tokens bounded by the outside-most matching parentheses forms the list of arguments for the function-like macro. The individual arguments within the list are separated by comma preprocessing tokens, but comma preprocessing tokens between matching inner parentheses do not separate arguments. If there are sequences of preprocessing tokens within the list of arguments that would otherwise act as preprocessing directives, the behavior is undefined.
The scope of this rule includes using preprocessor directives in arguments to a function where it is unknown whether or not the function is implemented using a macro. For example, standard library functions such as memcpy(), printf(), and assert() may be implemented as macros.
Noncompliant Code Example
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In this noncompliant code example \[[GCC Bugs|http://gcc.gnu.org/bugs.html#nonbugs_c]\], the author isuses preprocessor attemptingdirectives to specify anplatform-specific argumentarguments to {{memcpy()}} depending on the current platform by using preprocessor directives within the function call. However, if {{memcpy()}} is implemented using a macro, the code will result in undefined behavior. For example, this code will compile using GCC version 3.3 and later, but will not compile using GCC versions prior to 3.3 if {{memcpy()}} is a macro. |
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memcpy(dest, src, #ifdef PLATFORM1 12 #else 24 #endif ); |
Compliant Code Example
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In this compliant solution \[[GCC Bugs|http://gcc.gnu.org/bugs.html#nonbugs_c]\], the appropriate call to {{memcpy()}} is determined outside the function call. |
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#ifdef PLATFORM1 memcpy(dest, src, 12); #else memcpy(dest, src, 24); #endif |
Risk Assessment
Improper use of macros may result in undefined behavior.
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
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PRE13 PRE32-C | low | unlikely | medium | P2 | L3 |
References
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\[[GCC Bugs|http://gcc.gnu.org/bugs.html#nonbugs_c]\] "Non-bugs"
\[[ISO/IEC 9899:1999|AA. C References#ISO/IEC 9899-1999]\] Section 6.10.3.11, "Macro replacement" |