Basic C Memory Allocation

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

void allocateMem(int *a)
{
    a = (int*)malloc(5 * sizeof(int));
}

int main()
{
    int *ptr;
    allocateMem(ptr);
    ptr[0] = 5454;
    ptr[1] = 54;
    printf("Hi %d\n", ptr[1]);
    free(ptr);
    return 0;
}

I didn't get any output and error with the code. But if I allocate memory in main function, it actually works.

Here's the fix:

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

void allocateMem(int **a)
{
    *a = malloc(5 * sizeof(int));
}

int main()
{
    int *ptr;
    allocateMem(&ptr);
    ptr[0] = 5454;
    ptr[1] = 54;
    printf("Hi %d\n", ptr[1]);
    free(ptr);
    return 0;
}

To write to a variable in another function, you need to pass a pointer to it. Since the intention here is to write to a pointer, you need to pass the address of the pointer - &ptr.

As an address was passed, the allocateMem() function dereferences a once to hop into the memory location which is to be updated and lays down the address returned by malloc(). When this function returns, main() finds that ptr is pointing to a valid address and writes data inside the allocated memory.