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From: Eric Blake <eblake AT redhat DOT com>
Subject: SSIZE_MAX bug on 32-bit
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Message-ID: <578658A1.2030201@redhat.com>
Date: Wed, 13 Jul 2016 09:05:05 -0600
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From: Eric Blake <eblake AT redhat DOT com>
To: The Cygwin Mailing List <cygwin AT cygwin DOT com>
Message-ID: <578658A1 DOT 2030201 AT redhat DOT com>
Subject: SSIZE_MAX bug on 32-bit


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I recently stumbled on a glibc bug:
https://sourceware.org/bugzilla/show_bug.cgi?id=3D13575

and note that cygwin has the same bug in 32 bit.

test case 1:

$ cat <<\EOF | gcc -E -P - | grep size_t
#include <stdint.h>
#include <sys/types.h>
#include <limits.h>
size_t a =3D SIZE_MAX; ssize_t b =3D SSIZE_MAX;
EOF

On 32-bit, we see that both types are based on int:
typedef unsigned int size_t;
typedef signed int _ssize_t;
typedef _ssize_t ssize_t;

but that the constant for SSIZE_MAX is based on long
size_t a =3D (0xffffffffU); ssize_t b =3D (0x7fffffffL);

On 64-bit, we see that both types are based on long, as are the constants:
typedef long unsigned int size_t;
typedef long signed int _ssize_t;
typedef _ssize_t ssize_t;
size_t a =3D (0xffffffffffffffffUL); ssize_t b =3D (0x7fffffffffffffffL);

which is correct.

test case 2:

$ cat foo.c
#include <stdio.h>
#include <stdint.h>
#include <sys/types.h>
#include <limits.h>
int main(void) {
  printf("%zu %zd\n", SIZE_MAX, SSIZE_MAX);
  return 0;
}
$ gcc -o foo -Wall -Wformat=3D2 foo.c

Technically, there is NO portable way to pass ssize_t to printf without
a cast (the standards do not guarantee that %zd is ssize_t, because
ssize_t can be a different rank type than size_t) - but in practice,
most platforms use the same rank for the two types (both glibc and
Cygwin fall into this camp).  Therefore, this should compile without
warning; 64-bit is fine, but 32-bit gripes:

foo.c: In function 'main':
foo.c:6:10: warning: format '%zd' expects argument of type 'signed
size_t', but argument 3 has type 'long int' [-Wformat=3D]

Also, recall that both SIZE_MAX and SSIZE_MAX must be usable in #if
expressions, so we can't use a cast to size_t in the macro, but instead
have to expand directly to an expression with the right type.

I'll probably try my hand at a patch later today.

--=20
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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