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\[[ISO/IEC 9899-1999|AA. References#ISO/IEC 9899-1999]\] 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 ... */
}

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.