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
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