Re: test-memchr failure on rawhide

[Bruno Haible]

Ben Pfaff wrote:
> Yes.  C99 7.2.21p2 says:
[In my copy of C99 this is 7.21.1p2.]
> 
>      Where an argument declared as size_t n specifies the length
>      of the array for a function, n can have the value zero on a
>      call to that function.  Unless explicitly stated otherwise
>      in the description of a particular function in this
>      subclause, pointer arguments on such a call shall still have
>      valid values, as described in 7.1.4.
> 
> 7.1.4p1 says:
> 
>      If an argument to a function has an invalid value (such as a
>      value outside the domain of the function, or a pointer
>      outside the address space of the program, or a null pointer,
>      or a pointer to non-modifiable storage when the
>      corresponding parameter is not const-qualified) or a type
>      (after promotion) not expected by a function with variable
>      number of arguments, the behavior is undefined.
> 
> In other words, 7.2.21 says that pointer arguments to these
> functions must have valid values, and 7.1.4 says that null is an
> invalid value.

Thanks a lot, Ben. This makes it clear that in the case ptr = NULL, n = 0,
glibc's behaviour is standards compliant and "only" a deviation from
traditional behaviour. We'll need to change the gnulib unit tests.

Now about the case ptr != NULL, n = 0. Jim, what is the result of these two
programs on the particular Fedora Rawhide platform for which you reported
the original bug?

==================================== foo1.c =================================
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main ()
{
  int i;
  for (i = 1; i <= 16384; i++)
    {
      void *p = malloc (i);
      if (p == NULL)
        {
          fprintf (stderr, "malloc failed\n");
          exit (1);
        }
      if (memchr (p + i, '*', 0) != NULL)
        {
          fprintf (stderr, "memchr returned non-NULL\n");
          exit (1);
        }
      free (p);
    }
  return 0;
}
=================================== foo2.c ================================
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/mman.h>

int main ()
{
  int pagesize = getpagesize ();
  char *two_pages = (char *) mmap (NULL, 2 * pagesize, PROT_READ | PROT_WRITE, 
MAP_PRIVATE | MAP_ANONYMOUS, -1, 0);
  if (two_pages == (char *)-1)
    {
      fprintf (stderr, "mmap failed\n");
      exit (1); 
    }
  if (mprotect (two_pages + pagesize, pagesize, PROT_NONE) != 0)
    {
      fprintf (stderr, "mprotect failed\n");
      exit (1);
    }
  if (memchr (two_pages + pagesize, '*', 0) != NULL)
    {
      fprintf (stderr, "memchr returned non-NULL\n");
      exit (1);
    }
  return 0;
}
=========================================================================

Bruno