Variadic functions access their variable arguments by using va_start() to initialize an object of type va_list, iteratively invoking the va_arg() macro, and finally calling va_end(). The va_list may be passed as an argument to another function, but calling va_arg() within that function causes the va_list to have an indeterminate value in the calling function. As a result, attempting to read variable arguments without reinitializing the va_list can have unexpected behavior. According to the C Standard, 7.16, paragraph 3 [ISO/IEC 9899:2024],

If access to the varying arguments is desired, the called function shall declare an object (generally referred to as ap in this subclause) having type va_list. The object ap may be passed as an argument to another function; if that function invokes the va_arg macro with parameter ap, the representation of ap in the calling function is indeterminate and shall be passed to the va_end macro prior to any further reference to ap.

295) A pointer to a va_list can be created and passed to another function, in which case the original function can make further use of the original list after the other function returns.

Noncompliant Code Example

This noncompliant code example attempts to check that none of its variable arguments are zero by passing a va_list to helper function contains_zero(). After the call to contains_zero(), the value of ap is indeterminate.

#include <stdarg.h>
#include <stdio.h>
 
int contains_zero(size_t count, va_list ap) {
  for (size_t i = 1; i < count; ++i) {
    if (va_arg(ap, double) == 0.0) {
      return 1;
    }
  }
  return 0;
}

int print_reciprocals(size_t count, ...) {
  va_list ap;  
  va_start(ap, count);

  if (contains_zero(count, ap)) {
    va_end(ap);
    return 1;
  }

  for (size_t i = 0; i < count; ++i) {
    printf("%f ", 1.0 / va_arg(ap, double));
  }

  va_end(ap);
  return 0;
}

Compliant Solution

The compliant solution modifies contains_zero() to take a pointer to a va_list. It then uses the va_copy macro to make a copy of the list, traverses the copy, and cleans it up. Consequently, the print_reciprocals() function is free to traverse the original va_list.

#include <stdarg.h>
#include <stdio.h>
 
int contains_zero(size_t count, va_list *ap) {
  va_list ap1;
  va_copy(ap1, *ap);
  for (size_t i = 1; i < count; ++i) {
    if (va_arg(ap1, double) == 0.0) {
      return 1;
    }
  }
  va_end(ap1);
  return 0;
}
 
int print_reciprocals(size_t count, ...) {
  int status;
  va_list ap;
  va_start(ap, count);
 
  if (contains_zero(count, &ap)) {
    printf("0 in arguments!\n");
    status = 1;
  } else {
    for (size_t i = 0; i < count; i++) {
      printf("%f ", 1.0 / va_arg(ap, double));
    }
    printf("\n");
    status = 0;
  }
 
  va_end(ap);
  return status;
}

Risk Assessment

Reading variable arguments using a va_list that has an indeterminate value can have unexpected results.

Automated Detection

Tool	Version	Checker	Description
CodeSonar		BADMACRO.STDARG_H	Use of <stdarg.h> Feature
Cppcheck Premium		premium-cert-msc39-c
Helix QAC		C3497 C++3146, C++3147, C++3148, C++3149, C++3167
Klocwork		VA.LIST.INDETERMINATE
Parasoft C/C++test		CERT_C-MSC39-a	Use macros for variable arguments correctly
Polyspace Bug Finder		CERT C: Rule MSC39-C	Checks for: Invalid va_list argument Too many va_arg calls for current argument list Rule partially covered.
Security Reviewer - Static Reviewer		RTOS_13	Fully implemented
TrustInSoft Analyzer		variadic	Exhaustively verified.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Bibliography