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

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Noncompliant Code Example (assert)

In this example, a programmer attempts to access their own verification functionality by suppressing the assert macro and instead sending control to an assert a user-defined assert() function.

Suppose the custom <myassert.h> declares a function assert() which does nonstandard verification, and the standard <assert.h> defines an assert macro as required by the standard.

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Having this function and attempting to access it does not produce defined produces undefined behavior.

Compliant Solution (assert)

The programmer should place nonstandard verification in a function that does not conflict with the standard library macro assert, e.g. myassert().

#include <myassert.h>
#include <assert.h>

void fullAssert(int e) {
  assert(0 < e); // standard library assert()
  myAssertmyassert(e); // well defined custom assertion function
}

Noncompliant Code Example (

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redefining errno)

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

extern int errno;

Compliant Solution (

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redefining errno)

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

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