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Q43971: INFO: qsort(): Parameters to the Compare Function

Article: Q43971
Product(s): Microsoft C Compiler
Version(s): 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.
Operating System(s): 
Keyword(s): kbCRT kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Last Modified: 22-JUL-2001

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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 
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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:

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Keywords          : kbCRT kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbAudDeveloper kbCRT
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
Issue type        : kbinfo

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