INFO: qsort(): Parameters to the Compare Function

ID: Q43971

The information in this article applies to:

The C Run-Time (CRT), included with:
- Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax
- Microsoft C for OS/2, versions 5.1, 6.0, 6.0a
- Microsoft C/C++ for MS-DOS, version 7.0
- Microsoft Visual C++ for Windows, 16-bit edition, versions 1.0, 1.5
- Microsoft Visual C++, 32-bit Editions, versions 1.0, 2.0, 4.0, 5.0, 6.0

SUMMARY

When you define or prototype a comparison function for the run-time library function qsort(), the formal parameters should be of type "const void *" because qsort() doesn't know the type of data that it is dealing with. Internally, qsort() is dealing with void pointers only. The const directive is there because the compare routine shouldn't modify the values that are being compared. Inside of your compare routine, you must cast the void pointers back to pointers of the correct type.

If the declaration of the array to be sorted is:


   ELEMENT_TYPE arr[10] ;

where ELEMENT_TYPE is the type of the array elements, then the function for comparison should be prototyped as follows:


   int comp( const void *, const void * ) ;

Additional query words:

Keywords : kbCRT kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Version : MS-DOS:5.1,6.0,6.00a,6.00ax,7.0; OS/2:5.1,6.0,6.00a; WINDOWS:1.0,1.5; WINDOWS NT:1.0,2.0,4.0,5.0
Platform : MS-DOS NT OS/2 WINDOWS
Issue type : kbinfo