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The C Standard, 7.1.4 paragraph 1, [ISO/IEC 9899:2024] states

Any function declared in a header may be additionally implemented as a function-like macro defined in the header, so if a library function is declared explicitly when its header is included, one of the techniques shown later in the next subclause can be used to ensure the declaration is not affected by such a macro. Any macro definition of a function can be suppressed locally by enclosing the name of the function in parentheses, because the name is then not followed by the left parenthesis that indicates expansion of a macro function name. For the same syntactic reason, it is permitted to take the address of a library function even if it is also defined as a macro. 220) The use of #undef to remove any macro definition will also ensure that an actual function is referred to.

220)This means that an implementation is required to provide an actual function for each library function, even if it also provides a macro for that function.

 

However, the C Standard enumerates specific exceptions enumerates several instances in which the behavior of accessing an object or function expanded to be a standard library macro definition is undefined. The macros are assert, errno, math_errhandling, setjmp, va_startarg, va_argcopy, va_copyend, and va_endstart. These cases are recorded in Annex J, subclause J.2, items described by undefined behaviors 110, 114, 122, 124, and 138. Programmers should never attempt must not suppress these macros to access anything underlying any of these macrosthe underlying object or function.

Noncompliant Code Example (assert)

In this noncompliant code example, the standard standard assert() macro is suppressed in favor of calling a user-defined assertan attempt to pass it as a function pointer to the  execute_handler() function.Suppose the custom <myassert.h> declares a function assert() that does nonstandard verification, and the standard <assert.h> defines an assert macro as required by the standard: Attempting to suppress the assert() macro is undefined behavior.

#include <assert.h>
#include "myassert.h" 
typedef void (*handler_type)(int);
 
void fullAssert(execute_handler(handler_type handler, int evalue) {
   assert(e > 0); /* Invoke standard library assert() */
  (assert)(e > 0); /*
                    * assert() macro suppressed; calling
                    * function assert().
                    */
}

...

handler(value);
}
 
void func(int e) {
  execute_handler(&(assert), e < 0);
} 

Compliant Solution (assert)

The programmer should place nonstandard verification in a function that does not conflict with the standard library macro assert—for example, myassert()In this compliant solution, the assert() macro is wrapped in a helper function, removing the undefined behavior:

#include <assert.h>
 
#include "myassert.h"typedef void (*handler_type)(int);
 
void fullAssert(execute_handler(handler_type handler, int evalue) {
  handler(value);
}
 
static void assert(e > 0); /* Standard library assert() */
  myassert(e > 0); /* Well-defined custom assertion function */
}
_handler(int value) {
  assert(value);
}
 
void func(int e) {
  execute_handler(&assert_handler, e < 0);
}

Noncompliant Code Example (Redefining errno)

Legacy code is apt to include an incorrect declaration, such as the following in this noncompliant code example:

extern int errno;

Compliant Solution (

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Declaring errno)

The This compliant solution demonstrates the correct way to declare errno is to include by including the header <errno.h>:

#include <errno.h>

C-conforming implementations are required to declare errno in <errno.h>, although some historic implementations failed to do so.

Risk Assessment

Accessing objects or functions underlying these macros does not produce defined behavior, which may lead to incorrect or unexpected program the specific macros enumerated in this rule is undefined behavior.

Automated Detection

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

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Bibliography

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