Synopsis

Do not call va_arg with an argument of the incorrect type

Enabled by default

Yes

Severity/Certainty

Medium/High

Full description

Ensure that an invocation of the va_arg() macro does not attempt to access an argument that was not passed to the variadic function. Further, the type passed to the va_arg() macro must match the type passed to the variadic function after default argument promotions have been applied.

Coding standards

Code examples

The following code example fails the check and will give a warning:

#include <stdarg.h>
#include <stddef.h>

void func(size_t num_vargs, ...) {
  va_list ap;
  va_start(ap, num_vargs);
  if (num_vargs > 0) {
    unsigned char c = va_arg(ap, unsigned char);
    // ...
  }
  va_end(ap);
}

void f(void) {
  unsigned char c = 0x12;
  func(1, c);
}

The following code example passes the check and will not give a warning about this issue:

#include <stdarg.h>
#include <stddef.h>

void func(size_t num_vargs, ...) {
  va_list ap;
  va_start(ap, num_vargs);
  if (num_vargs > 0) {
    unsigned char c = (unsigned char) va_arg(ap, int);
    // ...
  }
  va_end(ap);
}

void f(void) {
  unsigned char c = 0x12;
  func(1, c);
}