Assign Memory to 3D array using triple pointer

Question

I have to assign memory to a 3D array using a triple pointer.

#include 
int main()
{
    int m=10,n=20,p=30;
    char ***z;
    z = (char***)          


        

Answer 1

If you don't need the memory to be allocated in a single, contiguous chunk (which IME is the usual case), you would do something like this:

char ***z;
z = malloc(sizeof *z * m); // allocate m elements of char **
if (z)
{
  int i;
  for (i = 0; i < m; i++)
  {
    z[i] = malloc(sizeof *z[i] * n); // for each z[i], 
    if (z[i])                        // allocate n elements char *
    {
      int j;
      for (j = 0; j < n;j++)
      {
        z[i][j] = malloc(sizeof *z[i][j] * p); // for each z[i][j], 
        if (z[i][j])                           // allocate p elements of char
        {
           // initialize each of z[i][j][k]
        }
      }
    }
  }
}

Note that you will need to free this memory in reverse order:

for (i = 0; i < m; i++)
{
  for (j = 0; j < n; j++)
    free(z[i][j];
  free(z[i]);
}
free(z);

If you really need the memory to be allocated in a contiguous chunk, you have a couple of choices. You could allocate a single block and compute your offsets manually:

char *z = malloc(sizeof *z * m * n * p); // note type of z!
...
z[i * m + j * n + k] = some_value();

When you're done, you just need to do a single free:

free(z);

If you have a C99 compiler or a C11 compiler that supports variable-length arrays, you could do something like this:

int m=..., n=..., p=...;
char (*z)[n][p] = malloc(sizeof *z * m);

This declares z as a pointer to an nxp array of char, and we allocate m such elements. The memory is allocated contiguously and you can use normal 3-d array indexing syntax (z[i][j][k]). Like the above method, you only need a single free call:

free(z);

If you don't have a C99 compiler or a C11 compiler that supports VLAs, you would need to make n, and p compile-time constants, such as

#define n 20
#define p 30

otherwise that last method won't work.

Edit

m doesn't need to be a compile-time constant in this case, just n and p.