Unable to allocate memory inside a function and return it in C

Question

I want to allocate some memory to a pointer using a function. Following is the code I have written:

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

int test( unsigned char **q)
{
    *q = (unsigned char *) malloc(250);
    if(*q == NULL)
    {
        printf("\n Error: failed to allocate memory");
        return -1;
    }   
//  *q[0] = 10;
//  *q[1] = 10;
    *q[2] = 10;

    return 0;   /* Returns 0 on success, non-zero value on failure */
}

int main(void)
{
    unsigned char *p;

//  printf("\n &p = %u", &p);
    int result = test(&p);  

//  printf("\n p[2] = %d", p[2]);
    return 0;
}

If I write something in *q[0] or *q[1], there is no error. But when I try to write something to *q[2], it gives "Segmentation fault (core dumped)".

Also I'm getting all data as 0 using p[ ] except p[0].

What is wrong with this code?

Answer 1

Instead of this wrong assignment

*q[2] = 10;

that is due to the operator precedence equivalent to

*( q[2] ) = 10;

and as result the dereferenced pointer q[2] is uninitialized and has indeterminate value. You should write either like

( *q )[2] = 10;

or even just like

q[0][2] = 10;

Another way is to introduce an intermediate variable and use it to initialize elements of the array. For example

char *p = *q;

p[2] = 10;

This allows to escape that kind of errors with pointers.