\[[ISO/IEC 9899-1999|AA. References#ISO/IEC 9899-1999]\] enumerates several instances where the behavior of accessing the object or function expanded to be a standard library macro definition is [undefined |BB. Definitions#undefined behavior].

The macros are assert, errno, math_errhandling, setjmp, va_start, va_arg, va_copy, and va_end.

These cases are recorded in the list of undefined behavior , items 104 , 108 , 116 , 118 and 130 .

Programmers should never attempt to access anything underlying any of these macros.

Noncompliant Code Example (assert)

In this example, a programmer working with a known <assert.h> is attempting to subvert the standard assert functionality so that assertions are always made, regardless of whether ndebug is set.

#include <assert.h>

#undef assert

void myassert(int e) {
  __assert13(__FILE__, __LINE__, __assert_function__, e);
}

This call results in undefined behavior, so the programmer can now never rely on assertions, even in debug builds.

Compliant Solution (assert)

In this solution, the programmer does not #undef a standard macro, and explicitly handles any release-build assertion checks.

#include <assert.h>

void myassert(int e) {
  assert(e);
  /* other verification ... */
}

Noncompliant Code Example (Redefining errno)

Legacy code is apt to include an incorrect declaration such as the following.

extern int errno;

Compliant Solution (Redefining errno)

The correct way to declare errno is to include the header <errno.h>.

#include <errno.h>

Implementations conforming to C99 are required to declare errno in <errno.h>, although some historic implementations failed to do so.

Risk Assessment

Accessing objects or function underlying these macros does not produce defined behavior, which may lead to incorrect or unexpected program behavior.

References

\[[ISO/IEC 9899:1999|AA. References#ISO/IEC 9899-1999]\], all sections indicated by the [undefined behavior items |CC. Undefined Behavior] noted above.

koders.com, <assert.h> source.