Question

I know how to allocate memory to 2-D array but if the ptr is defined as int *ptr[][100];

Then how can I allocate memory for ptr ?

Answer 1

Quoting from section 6.2.5 of the C standard:

An array type of unknown size is an incomplete type. It is completed,
for an identifier of that type, by specifying the size in a later
declaration (with internal or external linkage).

thus, to use int *ptr[][100]; you have to link against a definition of it. If you specify that length then you have a 2D array of pointers to int. At this point you have to allocate a chunk of memory large enough for the 3D array, and assign correctly each int * in the 2D array.

A more straightforward approach could be the following:

int nx = 100;
int ny = 200;
int nz = 300;

int (*parray)[ny][nz] = malloc(nx*ny*nz*sizeof(int));

The last line allocate a pointer to an array of an array of int, that can be effectively used like a 3D array. For instance:

for(int ii = 0; ii < nx; ii++)
  for (int jj = 0; jj < ny; jj++)
    for (int kk = 0; kk < nz; kk++)
      parray[ii][jj][kk] = ii*ny*nz + jj*nz + kk;

The accepted answer to this question gives a really nice and precise overview of the use of pointers to array of types.