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The C Standard, subclause 7.1.4 paragraph 1, states [ISO/IEC 9899:2011]:

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However, the C Standard enumerates specific exceptions in which the behavior of accessing an object or function expanded to be a standard library macro definition is undefined. The macros are :assert

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, errno

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, math_errhandling

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, setjmp, va_arg

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, va_copy

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, va_end

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 These , and va_start. These cases are described by undefined behaviors 110, 114, 122, 124, and 138. Programmers must not suppress these macros to access the underlying object or function.

Noncompliant Code Example (assert)

In this noncompliant code example, the standard assert() macro is suppressed in an attempt to pass it as a function pointer to the  execute_handler() function. Attempting to suppress the assert() macro is undefined behavior.

#include <assert.h>
 
typedef void (*handler_type)(int);
 
void execute_handler(handler_type handler, int value) {
  handler(value);
}
 
void func(int e) {
  execute_handler(&(assert), e < 0);
} 

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In this compliant solution, the assert() macro is wrapped in a helper function, removing the undefined behavior:

#include <assert.h>
 
typedef void (*handler_type)(int);
 
void execute_handler(handler_type handler, int value) {
  handler(value);
}
 
static void assert_handler(int value) {
  assert(value);
}
 
void func(int e) {
  execute_handler(&assert_handler, e < 0);
}

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

extern int errno;

Compliant Solution (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>

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Accessing objects or functions underlying these the specific macros results enumerated in this rule is undefined behavior.

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Bibliography

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