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DoNotTreatAPredefinedIdentifierAsObject.md

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MSC38-C: Do not treat a predefined identifier as an object if it might only be implemented as a macro

This query implements the CERT-C rule MSC38-C:

Do not treat a predefined identifier as an object if it might only be implemented as a macro

Description

The C Standard, 7.1.4 paragraph 1, [ISO/IEC 9899:2011] 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 below 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.185

  1. This means that an implementation shall 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_arg, va_copy, va_end, 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);
}

Compliant Solution (assert)

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);
}

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 (Declaring errno)

This compliant solution demonstrates the correct way to declare errno by including the header <errno.h>:

#include <errno.h>

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

Risk Assessment

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

Rule Severity Likelihood Remediation Cost Priority Level
MSC38-C Low Unlikely Medium P2 L3

Automated Detection

Tool Version Checker Description
Astrée 22.04 Supported, but no explicit checker
CodeSonar 7.0p0 BADMACRO.STDARG_H Use of <stdarg.h> Feature
Helix QAC 2022.2 C3437, C3475 C++3127, C++5039
Parasoft C/C++test 2022.1 CERT_C-MSC38-a A function-like macro shall not be invoked without all of its arguments
Polyspace Bug Finder R2022a CERT C: Rule MSC38-C Checks for predefined macro used as an object (rule fully covered)
PRQA QA-C 9.7 3437, 3475
RuleChecker 22.04 Supported, but no explicit checker

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)

Taxonomy Taxonomy item Relationship
CERT C DCL37-C. Do not declare or define a reserved identifier Prior to 2018-01-12: CERT: Unspecified Relationship

Bibliography

ISO/IEC 9899:2011 7.1.4, "Use of Library Functions"

Implementation notes

This query reports locations corresponding to both redefinitions of those standard library macros as well as locations where the identifiers used for accesses.

References