Pointers and Dynamic Allocation of Memory method 1 in C programming language

Pointers and Dynamic Allocation of Memory

METHOD 1:

One way of dealing with the problem is through the use of the typedef keyword. To allocate a 2 dimensional array of integers recall that the following two notations result in the same object code being generated: 


    multi[row][col] = 1;     *(*(multi + row) + col) = 1;

It is also true that the following two notations generate the same code: 

    multi[row]            *(multi + row)

Since the one on the right must evaluate to a pointer, the array notation on the left must also evaluate to a pointer. In fact multi[0] will return a pointer to the first integer in the first row, multi[1] a pointer to the first integer of the second row, etc. Actually, multi[n] evaluates to a pointer to that array of integers that make up the n-th row of our 2 dimensional array. That is, multi can be thought of as an array of arrays and multi[n] as a pointer to the n-th array of this array of arrays. Here the word pointer is being used to represent an address value. While such usage is common in the literature, when reading such statements one must be careful to distinguish between the constant address of an array and a variable pointer which is a data object in itself. 
Consider now: 

--------------- Program 9.1 --------------------------------

/* Program 9.1 from PTRTUT10.HTM  6/13/97 */

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

#define COLS 5

typedef int RowArray[COLS];
RowArray *rptr;

int main(void)
{
    int nrows = 10;
    int row, col;
    rptr = malloc(nrows * COLS * sizeof(int));
    for (row = 0; row < nrows; row++)
 {
        for (col = 0; col < COLS; col++)
        {
            rptr[row][col] = 17;
        }
    }

    return 0;
}
------------- End of Prog. 9.1 --------------------------------

Here I have assumed an ANSI compiler so a cast on the void pointer returned by malloc() is not required. If you are using an older K&R compiler you will have to cast using: 

    rptr = (RowArray *)malloc(.... etc.

Using this approach, rptr has all the characteristics of an array name name, (except that rptr is modifiable), and array notation may be used throughout the rest of the program. That also means that if you intend to write a function to modify the array contents, you must use COLS as a part of the formal parameter in that function, just as we did when
discussing the passing of two dimensional arrays to a function.