Thread: problem with realloc()

Hi...
Been working on some project and ran into trouble with dynamic memory allocation... I wrote a short sample code to understand the matter, but I really can't get it to work... So I need help...
Here's the code:

Code:

#include <stdio.h>
#include <stdlib.h>

int main()
{
	char **pointers;
	int i, strings = 2;

	pointers = malloc(strings++ * sizeof(char));
	for (i = 0; i < 5; i++)
	{
		pointers[i] = malloc(256 * sizeof(char));
		scanf("%s", pointers[i]);
		pointers = realloc(pointers, strings++ * sizeof(char));
	}
	printf("\n--------------\n");
	for (i = 0; i < 5; i++)
		printf("%s\n", pointers[i]);
	return 0;
}

When I'm trying to run the compiled program, after inputting all 5 strings I get the following message:

Code:

*** glibc detected *** ./realloc: realloc(): invalid next size: 0x0804a008 ***
======= Backtrace: =========
/lib/libc.so.6[0xb7ee4105]
/lib/libc.so.6[0xb7ee7d19]
/lib/libc.so.6(realloc+0x106)[0xb7ee8a66]
./realloc[0x80484a6]
/lib/libc.so.6(__libc_start_main+0xe5)[0xb7e8b6a5]
./realloc[0x8048391]
======= Memory map: ========
08048000-08049000 r-xp 00000000 03:01 335765     /home/dark/code/realloc
08049000-0804a000 rw-p 00000000 03:01 335765     /home/dark/code/realloc
0804a000-0806b000 rw-p 0804a000 00:00 0          [heap]
b7d00000-b7d21000 rw-p b7d00000 00:00 0
b7d21000-b7e00000 ---p b7d21000 00:00 0
b7e74000-b7e75000 rw-p b7e74000 00:00 0
b7e75000-b7fcf000 r-xp 00000000 03:01 14788      /lib/libc-2.9.so
b7fcf000-b7fd1000 r--p 0015a000 03:01 14788      /lib/libc-2.9.so
b7fd1000-b7fd2000 rw-p 0015c000 03:01 14788      /lib/libc-2.9.so
b7fd2000-b7fd6000 rw-p b7fd2000 00:00 0
b7fea000-b7ff6000 r-xp 00000000 03:01 206        /usr/lib/libgcc_s.so.1
b7ff6000-b7ff7000 rw-p 0000b000 03:01 206        /usr/lib/libgcc_s.so.1
b7ff7000-b7ff8000 rw-p b7ff7000 00:00 0
b7ff8000-b8016000 r-xp 00000000 03:01 14830      /lib/ld-2.9.so
b8016000-b8017000 r--p 0001e000 03:01 14830      /lib/ld-2.9.so
b8017000-b8018000 rw-p 0001f000 03:01 14830      /lib/ld-2.9.so
bfa02000-bfa17000 rw-p bffeb000 00:00 0          [stack]
ffffe000-fffff000 r-xp 00000000 00:00 0          [vdso]
Aborted

So... what's wrong?

Code:

		pointers = realloc(pointers, strings++ * sizeof(char));

Here you allocate "strings" bytes. No "strings" pointers. So this results in a buffer overflow.

Use sizeof(char *) if you want to avoid this. A char ** points to many char *s each of them pointing to chars. So you're basically allocating an array of strings, not chars.

EDIT: Sorry for the double-post. I thought I was editing it.

Originally Posted by Brafil

Use sizeof(char *) if you want to avoid this. A char ** points to many char *s each of them pointing to chars. So you're basically allocating an array of strings, not chars.

EDIT: Sorry for the double-post. I thought I was editing it.

Great! Thank you very much, now it's working perfectly