problem with realloc()

This is a discussion on problem with realloc() within the C Programming forums, part of the General Programming Boards category; Hi... Been working on some project and ran into trouble with dynamic memory allocation... I wrote a short sample code ...

  1. #1
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    Red face 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?

  2. #2
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    Code:
    		pointers = realloc(pointers, strings++ * sizeof(char));
    Here you allocate "strings" bytes. No "strings" pointers. So this results in a buffer overflow.

  3. #3
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    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.

  4. #4
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    Wink

    Quote Originally Posted by Brafil View Post
    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

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