[Bug c/96832] New: Wrong assumption for -Werror=nonnull check

[jernej.skrabec at siol dot net]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96832

            Bug ID: 96832
           Summary: Wrong assumption for -Werror=nonnull check
           Product: gcc
           Version: 10.2.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jernej.skrabec at siol dot net
  Target Milestone: ---

Following code:

#include <stdlib.h>
#include <unistd.h>
int main()
{
        char *path = getcwd(0, 4096);
        free(path);
        return 0;
}

Fails to compile with -Wall -Werror with:

main.c: In function ‘main’:
main.c:5:15: error: argument 1 is null but the corresponding size argument 2
value is 4096 [-Werror=nonnull]
    5 |  char *path = getcwd(0, 4096);
      |               ^~~~~~~~~~~~~~~
In file included from main.c:2:
/usr/include/unistd.h:520:14: note: in a call to function ‘getcwd’ declared
with attribute ‘write_only (1, 2)’
  520 | extern char *getcwd (char *__buf, size_t __size) __THROW __wur
      |              ^~~~~~
cc1: all warnings being treated as errors

even though it's correct according to
https://man7.org/linux/man-pages//man2/getcwd.2.html for glibc, which have
extension for exactly this case.

This was tested with gcc 10.2.0 and glibc 2.32 on Arch Linux. Code in question
comes from nss library.