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)