realloc() : invalid pointer ?

[creeping death]

i m trying to understand dynamic memory allocation.

this is what i tried...and failed at

#include<stdio.h>
#include<stdlib.h>
int main()
{
    int *start,*values,i;

/*hmm...i dunno lets allocate 20 integer size block, to be on the safe side*/

    values=(int *) malloc(20 * sizeof(int));
    start=values;

    printf(" add(start)=&#37;u, add(value)=%u\n",start,values);
    printf("Enter the values(0 to stop)\n");
    for(i=0 ; i<10 ; i++)
    {
        scanf("%d",values); 
        printf("\tvalue(value)=%d\tadd(value)=%u\t",*values,values);  
        values++;
    }
/* well, only 10 integers were stored! looks like i allocated more earlier, lets reduce the size :) */

    values =(int *) realloc(values,10 * sizeof(int));

    values=start;
    for(i=0 ; i<20 ; i++)
    {
        printf("%d\t%u\n",*values,values);
        values++;
    }
     return 0;
}

there are no errors or warnings in compilation
when i run the program, the first half of program(taking in values) executes. after i enter the 10th number.

i get an error message

*** glibc detected *** ./a.out: realloc(): invalid pointer: 0x081df030 ***
======= Backtrace: =========
/lib/libc.so.6[0x48b3a4]
/lib/libc.so.6(realloc+0x242)[0x48fdc2]
/lib/libc.so.6(realloc+0x42)[0x48fbc2]
./a.out[0x80484f9]
/lib/libc.so.6(__libc_start_main+0xe5)[0x4326e5]
./a.out[0x80483c1]
======= Memory map: ========
00110000-00111000 r-xp 00110000 00:00 0          [vdso]
003f7000-00417000 r-xp 00000000 08:02 805385327  /lib/ld-2.9.so
00418000-00419000 r--p 00020000 08:02 805385327  /lib/ld-2.9.so
00419000-0041a000 rw-p 00021000 08:02 805385327  /lib/ld-2.9.so
0041c000-0058a000 r-xp 00000000 08:02 808903508  /lib/libc-2.9.so
0058a000-0058c000 r--p 0016e000 08:02 808903508  /lib/libc-2.9.so
0058c000-0058d000 rw-p 00170000 08:02 808903508  /lib/libc-2.9.so
0058d000-00590000 rw-p 0058d000 00:00 0 
00a6c000-00a79000 r-xp 00000000 08:02 808904130  /lib/libgcc_s-4.3.2-20081105.so.1
00a79000-00a7a000 rw-p 0000c000 08:02 808904130  /lib/libgcc_s-4.3.2-20081105.so.1
08048000-08049000 r-xp 00000000 08:02 809327092  /home/c_d/workspace/Info Security/RSA/a.out
08049000-0804a000 rw-p 00000000 08:02 809327092  /home/c_d/workspace/Info Security/RSA/a.out
081df000-08200000 rw-p 081df000 00:00 0          [heap]
b7f8a000-b7f8b000 rw-p b7f8a000 00:00 0 
b7fae000-b7fb1000 rw-p b7fae000 00:00 0 
bfb9c000-bfbb1000 rw-p bffeb000 00:00 0          [stack]
	1	136179756	Aborted

isnt this how one is supposed to use realloc() ?
what am i doing wrong here?

[Lindley]

What you are doing wrong is

values++;

in the loop. After that, values no longer points to the array returned by malloc(), so it is improper to pass it to realloc.

However, your start pointer should still be pointed at the proper location. Try passing that instead.

Also, no program may be considered correct if it does not free() all allocated memory before program termination.

[Paul McKenzie]

    values =(int *) realloc(values,10 * sizeof(int));

    values=start;
    for(i=0 ; i<20 ; i++)

See the problem in red?

Regards,

Paul McKenzie

[creeping death]

What you are doing wrong is

values++;

in the loop. After that, values no longer points to the array returned by malloc(), so it is improper to pass it to realloc.

However, your start pointer should still be pointed at the proper location. Try passing that instead.

thanks. i forgot about that basic point.

Also, no program may be considered correct if it does not free() all allocated memory before program termination.

that point taken too! thanks.

[creeping death]

    values =(int *) realloc(values,10 * sizeof(int));

    values=start;
    for(i=0 ; i<20 ; i++)

See the problem in red?

Regards,

Paul McKenzie

i willingly set i till 20 because i wanted to see if i'd get any runtime errors because i have deallocated those memory locations...

i dont get any errors.

[Paul McKenzie]

i willingly set i till 20 because i wanted to see if i'd get any runtime errors because i have deallocated those memory locations...

i dont get any errors.

That is because accessing memory that has been deallocated is undefined behaviour. This means that anything can happen -- work all the time, crash all the time, work some of the time and crash other times, work at 12:00 on Tuesday and fail at 5:00 PM on a Wednesday, etc...

Regards,

Paul McKenzie

[Lindley]

Paul is correct. However, some practical knowledge which may help:

Since you're only reading from invalid locations, odds are nothing will go wrong until you try to do a calculation with the retrieved values. Since all you're doing is outputting them, the worst that will (probably) happen is you might get wonky output.

If you were to *write* to an invalid location, that is far more likely to cause a runtime error. Either an immediate segmentation fault or, under some circumstances, you may get a "heap corruption detected" message the next time you try to allocate or free memory. The fact that this message may not correspond to the location where the corruption occurred is a particularly troublesome issue.

One of the many reasons to prefer std::vector over bare arrays in C++ is because it will (under Windows at least) throw an exception immediately if you try to access an invalid array index. It is *much* better to catch these things as they happen (or even at compile time if possible) than to let them pass unremarked and try to figure out what went wrong when things start breaking further down the line.

[creeping death]

thanks for the useful insights!!