Pointers and Dynamic Allocation of Memory

           One of those things beginners in C find difficult is the concept of pointers. The purpose of this tutorial is to provide an introduction to pointers and their use to these beginners.

There are times when it is convenient to allocate memory at run time using malloc(), calloc(), or other allocation functions. Using this approach permits postponing the decision on the size of the memory block need to store an array, for example, until run time. Or it permits using a section of memory for the storage of an array of integers at one point in time, and then when that memory is no longer needed it can be freed up for other uses, such as the storage of an array of structures.
When memory is allocated, the allocating function (such as malloc(), calloc(), etc.) returns a pointer. The type of this pointer depends on whether you are using an older K&R compiler or the newer ANSI type compiler. With the older compiler the type of the returned pointer is char, with the ANSI compiler it is void.
If you are using an older compiler, and you want to allocate memory for an array of integers you will have to cast the char pointer returned to an integer pointer. For example, to allocate space for 10 integers we might write:

      int *iptr;
      iptr = (int *)malloc(10 * sizeof(int));
      if (iptr == NULL)
      { .. ERROR ROUTINE GOES HERE .. }

If you are using an ANSI compliant compiler, malloc() returns a void pointer and since a void pointer can be assigned to a pointer variable of any object type, the (int *) cast shown above is not needed. The array dimension can be determined at run time and is not needed at compile time. That is, the 10 above could be a variable read in from a data file or keyboard, or calculated based on some need, at run time.
Because of the equivalence between array and pointer notation, once iptr has been assigned as above, one can use the array notation. For example, one could write:

      int k;
      for (k = 0; k < 10; k++)
          iptr[k] = 2;

to set the values of all elements to 2.

Even with a reasonably good understanding of pointers and arrays, one place the newcomer to C is likely to stumble at first is in the dynamic allocation of multidimensional arrays. In general, we would like to be able to access elements of such arrays using array notation, not pointer notation, wherever possible. Depending on the application we may or may not know both dimensions at compile time. This leads to a variety of ways to go about our task.
As we have seen, when dynamically allocating a one dimensional array its dimension can be determined at run time. Now, when using dynamic allocation of higher order arrays, we never need to know the first dimension at compile time. Whether we need to know the higher dimensions depends on how we go about writing the code. Here I will discuss various methods of dynamically allocating room for 2 dimensional arrays of integers. First we will consider cases where the 2nd dimension is known at compile time.