free 2D array allocated using calloc

I am using a 2D array, and am required to allocate it as shown:

char ** buf; //global var

void allocate()
{
    buf = (char **) malloc (10 * sizeof (char*));
    char * data = (char *) calloc (1, 1000);
    int i;
    for(i=0; i<10; i++开发者_JS百科)
        buf[i] = &(data[i*100]);
}

int main()
{
    allocate();

    //something goes here

    free(buf[0]);
    free(buf);
    return 0;
}

Now to free the arrays, since I cannot access the variable 'data' in main(), I cannot do free(data), hence I do free(buf[0]), assuming that I am freeing the entire array of 1000 elements. Is this the right way to do this? Does free(buf[0]) free the entire 'data' array?

(It would have been convenient to malloc every element of buf as buf[i] = malloc (100), but I cant do that since I HAVE to calloc a big block first).

Thanks in advance.

This code is correct. The value of buf[0] is &(data[0]) which is data by definition. This will free all the allocated memory.

Note that if you really just want one buffer to free, you can set:

offset = 10*sizeof(char*);
buf = calloc(10*100+offset);

and in the loop do:

buf[i] = buf + offset + i*100;

It is slightly unusual, but it is correct. You made two allocations; you make two frees. And you release the pointers that were allocated. All should be clean.

Did you run valgrind on your program?

Valgrind agrees with you. It is correct because buf[0] holds the pointer to the HEAD of the calloc'd memory block.

valgrind ./temp
==15404== Memcheck, a memory error detector
==15404== Copyright (C) 2002-2009, and GNU GPL'd, by Julian Seward et al.
==15404== Using Valgrind-3.6.0.SVN-Debian and LibVEX; rerun with -h for copyright info
==15404== Command: ./temp
==15404== 
==15404== 
==15404== HEAP SUMMARY:
==15404==     in use at exit: 0 bytes in 0 blocks
==15404==   total heap usage: 2 allocs, 2 frees, 1,040 bytes allocated
==15404== 
==15404== All heap blocks were freed -- no leaks are possible
==15404== 
==15404== For counts of detected and suppressed errors, rerun with: -v
==15404== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 12 from 7)