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Wiki MarkupThe arguments to a macro should must not include preprocessor directives, such as {{\#define}}, {{\#ifdef}}, and {{\#include}}. Doing so is [undefined behavior|BB. Definitions#undefined behavior], according to Section so results in undefined behavior, according to the C Standard, 6.10.35, Paragraph 11 of the C99 Standard \[paragraph 11 [ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\]. 2024]:

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.

(See also undefined behavior 87 of Annex J92.)

This rule also applies to the use of The scope of this rule includes using preprocessor directives in arguments to a any function where it is unknown whether or not the function is implemented using a macro. For example, This includes all standard library functions, such as memcpy(), printf(), and assert(), because any standard library function may be implemented as macrosa macro. (C24, 7.1.4, paragraph 1).

Noncompliant Code Example

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In this noncompliant code example \[ [GCC Bugs|http://gcc.gnu.org/bugs.html#nonbugs_c]\], the author programmer uses preprocessor directives to specify platform-specific arguments to {{memcpy()}}. However, if {{memcpy()}} is implemented using a macro, the code results in undefined behavior.

Code Block
bgColor #FFCCCC lang c
#include <string.h>   void func(const char *src) { /* Validate the source string; calculate size */ char *dest; /* malloc() destination string */ memcpy(dest, src, #ifdef PLATFORM1 12 #else 24 #endif );   /* ... */ }

Compliant

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Solution

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.:

#include <string.h>

void func(const char *src) {
  /* Validate the source string; calculate size */
  char *dest;
  /* malloc() destination string */ 
  #ifdef PLATFORM1
    memcpy(dest, src, 12);
  #else
    memcpy(dest, src, 24);
  #endif
  /* ... */
}

Risk Assessment

Improper use of macros may result in Including preprocessor directives in macro arguments is undefined behavior.

Bibliography

\[[GCC Bugs|http://gcc.gnu.org/bugs.html#nonbugs_c]\] "Non-bugs"
\[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] Section 6.10.3.1, "Argument substitution," paragraph 11

Automated Detection

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Bibliography

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Image Added Image Added Image AddedPRE31-C. Avoid side-effects in arguments to unsafe macros      01. Preprocessor (PRE)      02. Declarations and Initialization (DCL)