INFO: Dynamic Memory Allocation for Two-Dimensional Arrays

MORE INFORMATION

For the compiler to generate code for two-dimensional array element dereferencing, the number of columns of the array must be known at compile time. Therefore, it is possible to dynamically allocate a two-dimensional array if pointer declaration includes the "width" of the array. Below is a code fragment that illustrates this:

char (*array)[columns];

array = (char (*)[columns]) malloc(sizeof(char) * rows * columns);
if (array == NULL)
{
    printf("Not enough memory!\n");
    return;
}
				

However, to give both dimensions of a two-dimensional array at run time the array must be dynamically allocated. In this case, a dynamically allocated two-dimensional array should be thought of as an array of one-dimensional arrays. There is some memory overhead, however, which requires allocation of an array of array pointers that is not necessary for statically defined two-dimensional arrays. Despite the overhead, each element of the two- dimensional arrays can be referenced with the bracket notation just as for a statically defined two-dimensional array.

These are the steps for dynamically allocating a two-dimensional array:

1. Declare a double dereferenced pointer of a desired type.
2. Allocate the number of rows times the size of a pointer and assign the first dereference to the beginning of this allocation.
3. Loop through the rows and allocate the number of columns times the size of the element.

The structure conceptually resembles the following:

   Array of                    Arrays of
   Pointers                    Elements

   ------     -------------------------                      -------
   |    | -->|   |   |   |   |   |   |     . . . . . . .       |   |
   ------     -------------------------                      -------
   |    | -->|   |   |   |   |   |   |     . . . . . . .       |   |
   ------     -------------------------                      -------
   |    | -->|   |   |   |   |   |   |     . . . . . . .       |   |
   ------     -------------------------                      -------
   |    | -->|   |   |   |   |   |   |     . . . . . . .       |   |
   ------     -------------------------                      -------
         .
         .
         .
         .
         .    -------------------------                      -------
   |    | -->|   |   |   |   |   |   |     . . . . . . .       |   |
   ------     -------------------------                      -------
   |    | -->|   |   |   |   |   |   |     . . . . . . .       |   |
   ------     -------------------------                      -------
				

Use _fmalloc() for MS-DOS and 16-Bit Windows to use the far/global heap. Note for Windows: _fmalloc() is available to be used when compiling with Microsoft C/C++ compiler versions 7.0, 8.0, and 8.0c. Do not use GlobalAlloc and GlobalLock because this technique may use up too many selectors for relatively small allocations.

Unlike a statically declared two-dimensional array, the rows are not contiguous with one another. But because this is an array of arrays it is possible to have a total structure larger than 64K without using huge pointers. If either the number of rows or the size of the rows is greater than 64K, then huge pointers are necessary.

Sample Code

/* Semi-pseudo code for a two-dimensional array of char's
* Note that this sample uses the far heap and uses far pointers.
* For 32-Bit Windows applications on Windows NT or Win32S or for
* OS/2, use malloc() and remove the _far keywords.
*/ 

   char _far * _far * array;
   unsigned int i;
   unsigned int rows, columns;  /* let's keep it within 64k */ 

   /*  Set the rows and columns to desired dimensions. */ 
   rows    = 8;
   columns = 12;

   array = (char _far * _far *) _fmalloc(sizeof(char _far *) * rows);
   if (array == NULL)
   {
       printf("Not enough memory\n");
       return;
   }
   for (i = 0; i < rows; i++)
   {
       array[i] = (char _far *) _fmalloc(sizeof(char) * columns);
       if (array[i] == NULL)
       {
           printf("Not enough memory\n");
           /* handle error, _free() the previously allocated rows */ 
           return;
       }
   }

   /* to use the array: array[row][column] */ 
   array[0][1] = 'x';

   /* etc.   */ 

* To Free the memory used by the array
**
*/ 
   for (i = 0; i< rows; i++)
       _ffree(array[i]);
   _ffree(array);