INFO: Casting a Pointer to Type Equivalent to Multidimensional (43703)

SUMMARY

In some situations you may need to cast a pointer or an address to a type that is equivalent to a multidimensional array. The following example demonstrates such a situation:

typedef char Arr2Dim [][20] ;
void myfunc (Arr2Dim) ;
char * ptr ;
...
void main (void)
{
    ...
    myfunc ( (Arr2Dim) ptr ) ;  /* illegal */ 
    ...
}

Casting the variable "ptr" to the array type "Arr2Dim" is not correct. The Microsoft C Compiler displays the following error message:

error C2067: cast to array type is illegal

The correct procedure is to cast the pointer "ptr" to a pointer type equivalent to the array type Arr2Dim. This pointer type can be defined as follows:

   typedef char (*Ptr2Dim) [20] ;

Casting "ptr" to the type of "Ptr2Dim", as follows, is correct and produces no warning messages when compiled at warning level 3:

   myfunc ( (Ptr2Dim) ptr ) ;

The address (or pointer) passed to the function is used correctly.

MORE INFORMATION

A similar solution may be applied to the problem of dynamically allocating a multidimensional array. For example, the following code fragment allocates a memory block, which can be used as a 10 x 20 x 30 three- dimensional array:

#include <malloc.h>

typedef char (*Ptr3Dim) [20][30] ;
Ptr3Dim ptr3arr ;
...
void main (void)
{
    ...
    ptr3arr = (Ptr3Dim) malloc (10 * sizeof(char) * 20 * 30) ;
    ...
}

After the allocation, "ptr3arr" can be used as a three-dimensional array, as follows, provided i, j, and k are integers within the proper range:

   ptr3arr [i][j][k] = 'a' ;