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The C Standard, 7.1.4 paragraph 1, [ISO/IEC

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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 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_

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arg, va_

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copy, va_

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end, and va_

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start.

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These cases are

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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, a programmer attempts to access their own verification functionality by suppressing the assert macro and instead sending control to an 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 standardthe 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 <myassert.h>
#include <assert.h>
 
typedef void fullAssert(*handler_type)(int e) {
  assert(0 < e); // invoke standard library assert()
  (assert)(0 < e);   // assert() macro suppressed, calling function assert()
}

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

Compliant Solution (assert)

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

Compliant Solution (assert)

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

#include <myassert<assert.h>
#include <assert.h>
 
typedef void (*handler_type)(int);
 
void fullAssert(execute_handler(handler_type handler, int evalue) {
  handler(value);
}
 
static void assert_handler(0int < evalue); // standard library {
  assert(value);
}
 
void func(int myAssert(e); // well defined custom assertion function
}
{
  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>

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

Risk Assessment

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

References

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)

Bibliography

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