Re: [PATCH 05/11] A pointer to a pointer is nonnull.

[Peter Foley <pefoley2 at pefoley dot com>]

On Sun, Mar 20, 2016 at 7:15 AM, Corinna Vinschen
<corinna-cygwin@cygwin.com> wrote:
> Eh, what?!?  How on earth can gcc assert memptr is always non-NULL?
> An application can call posix_memalign(NULL, 4096, 4096) just fine,
> can't it?  If so, *memptr = res crashes.
>

So, it looks like what's happening is that gcc special-cases
posix_memalign as a builtin function.

See https://github.com/gcc-mirror/gcc/blob/master/gcc/builtins.def#L831

This causes the below warning to be outputted for my testcase:

a.cc:9:25: warning: null argument where non-null required (argument 1)
[-Wnonnull]
     posix_memalign(0,1,1);
                         ^
a.cc: In function âint posix_memalign(void**, long unsigned int, long
unsigned int)â:
a.cc:3:3: warning: nonnull argument âmemptrâ compared to NULL
[-Wnonnull-compare]
   if (memptr)
   ^~

Testcase:

extern "C" posix_memalign(void **memptr, unsigned long, unsigned long) {
  void *a = 0;
  if (memptr)
    *memptr = a;
  return 0;
}

int main() {
    posix_memalign(0,1,1);
}

(Note that passing -fno-builtin causes the warning to go away, as
posix_memalign is no-longer special-cased)

In addition, both newlib and glibc appear to assume the memptr arg is nonnull.
https://sourceware.org/git/?p=glibc.git;a=blob;f=malloc/malloc.c#l5008
https://sourceware.org/git/?p=newlib-cygwin.git;a=blob;f=newlib/libc/sys/linux/malloc.c#l4938

Hope that makes this more understandable.

Thanks,

Peter