[Bug c/88354] New: Please warn on the use of a va_list argument in *printf functions instead of v*printf

[vincent-gcc at vinc17 dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88354

            Bug ID: 88354
           Summary: Please warn on the use of a va_list argument in
                    *printf functions instead of v*printf
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: vincent-gcc at vinc17 dot net
  Target Milestone: ---

A {,f,d,s,sn}printf function can mistakenly be used instead of
v{,f,d,s,sn}printf. for instance, this is what happened in atop:

 
https://github.com/Atoptool/atop/pull/43/commits/0e43049493e65b412e1c0def62a09ceac2bb3110

Type checking should warn when this occurs. Implementing the warning is
possible because the {,f,d,s,sn}printf functions cannot take a va_list
argument, so that this is necessarily an error.

An example of buggy code for which one should get a warning:

#include <stdio.h>
#include <stdarg.h>

void f (int i, const char *s, ...)
{
  if (i)
    {
      va_list args;
      va_start (args, s);
      fprintf (stderr, s, args);
      va_end (args);
    }
}

int main (void)
{
  unsigned long d = 17;
  f (1, "n = %lu\n", d);
  return 0;
}

(The correct code is obtained by replacing fprintf with vfprintf.)