INFO: Casting a Pointer to Type Equivalent to Multidimensional

ID: Q43703

The information in this article applies to:

Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax
Microsoft C/C++ for MS-DOS, version 7.0
Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5, 1.51, 1.52
Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 2.1, 4.0, 5.0, 6.0

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' ;

Additional query words:

Keywords : kbLangC kbVC100 kbVC150 kbVC151 kbVC152 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Version : MS-DOS:6.0,6.00a,6.00ax,7.0; WINDOWS:1.0,1.5,1.51,1.52; WINDOWS NT:1.0,2.0,2.1,4.0,5.0
Platform : MS-DOS NT WINDOWS
Issue type : kbinfo