public inbox for gcc-bugs@sourceware.org
help / color / mirror / Atom feed
* [Bug c/45779] New: pointer difference error/ptrdiff_t representability
@ 2010-09-24 16:06 aklauer at rumms dot uni-mannheim.de
2010-09-27 18:27 ` [Bug c/45779] " aklauer at rumms dot uni-mannheim.de
2015-02-20 8:29 ` pinskia at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: aklauer at rumms dot uni-mannheim.de @ 2010-09-24 16:06 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=45779
Summary: pointer difference error/ptrdiff_t representability
Product: gcc
Version: 4.6.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
AssignedTo: unassigned@gcc.gnu.org
ReportedBy: aklauer@rumms.uni-mannheim.de
Note: initially found on gcc 4.3.2, confirmed on 4.6.0 20100924 from svn.
Consider the following program:
<code>
/* test.c */
#include<assert.h>
#include<inttypes.h>
#include<stddef.h>
#include<stdio.h>
#include<stdlib.h>
int main(int argc, char ** argv) {
printf("ptrdiff_t max: %ju, size_t max: %ju\n", (uintmax_t)
PTRDIFF_MAX, (uintmax_t) SIZE_MAX);
assert (argc > 1);
size_t size = atoll(argv[1]);
printf("requested array size: %zu\n", size);
assert (size > 0);
uint16_t * array = malloc(size * sizeof(*array));
assert (array != NULL);
printf("array one-past-end/start difference: %td\n",
&array[size] - &array[0]);
}
</code>
$ gcc -std=c99 -pedantic -Wall -Wextra test.c
$ ./a.out 1200000000
ptrdiff_t max: 2147483647, size_t max: 4294967295
requested array size: 1200000000
array one-past-end/start difference: -947483648
The output "-947483648" violates the C99 standard, it should be "1200000000".
This program was compiled and run on an IA-32 host with 2.5 GiB memory.
The pointer returned by the successful call to malloc() points to an array of
1200000000 uint16_t's. In the present case, the number 1200000000 is smaller
than PTRDIFF_MAX and thus representable by the ptrdiff_t type. Hence, by the
C99 standard, 6.5.6p9, the expression &array[size] - &array[0] above is defined
to have type ptrdiff_t and value 1200000000.
Note that if one replaced uint16_t with char in the above code and called the
program with argument 2400000000 (a number larger than PTRDIFF_MAX), the
behaviour would be undefined. Therefore I suspect that, internally, gcc first
calculates the value of &array[size] - &array[0] as if array had type
pointer-to-char and then erroneously interprets the result as a negative 32-bit
2's complement signed integer, which it then divides by 2 (that is,
sizeof(uint16_t)) with a signed integer division.
Best regards,
Alexander
--
Configure bugmail: http://gcc.gnu.org/bugzilla/userprefs.cgi?tab=email
------- You are receiving this mail because: -------
You are on the CC list for the bug.
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2015-02-20 8:29 UTC | newest]
Thread overview: 3+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2010-09-24 16:06 [Bug c/45779] New: pointer difference error/ptrdiff_t representability aklauer at rumms dot uni-mannheim.de
2010-09-27 18:27 ` [Bug c/45779] " aklauer at rumms dot uni-mannheim.de
2015-02-20 8:29 ` pinskia at gcc dot gnu.org
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).