ID Number: Q43703
5.10 6.00 6.00a 6.00ax 7.00 | 5.10 6.00 6.00a
MS-DOS | OS/2
Summary:
In Microsoft C versions 5.1, 6.0, 6.0a, 6.0ax, and C/C++ version 7.0,
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 foo (Arr2Dim) ;
char * ptr ;
...
void main (void)
{
...
foo ( (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:
foo ( (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:
typedef char (*Ptr3Dim) [20][30] ;
Ptr3Dim ptr3arr ;
...
void main (void)
{
...
ptr3arr = (Ptr3Dim) malloc (10 * sizeof(char) * 20 * 30) ;
...
}
After the allocation, "ptr2arr" can be used as a three-dimensional
array, as follows, provided i, j, and k are integers within the proper
range:
ptr2arr [i][j][k] = 'a' ;
Additional reference words: 5.10 6.00 6.00a 6.00ax 7.00