Q60332: INFO: sizeof() Function Returns unsigned Instead of int

Article: Q60332
Product(s): Microsoft C Compiler
Version(s): 1.0,1.5,2.0,4.0,5.0,6.0
Operating System(s): 
Keyword(s): kbLangC kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600
Last Modified: 14-NOV-2001

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft C for MS-DOS 
- Microsoft C for OS/2 
- Microsoft Visual C++, versions 1.0, 1.5, 2.0, 4.0 
- Microsoft Visual C++, 32-bit Enterprise Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Professional Edition, versions 5.0, 6.0 
- Microsoft Visual C++, 32-bit Learning Edition, version 6.0 
-------------------------------------------------------------------------------

SUMMARY
=======

To comply with new ANSI specifications, the sizeof() operator in Microsoft C
versions 6.0 and later and QuickC versions 2.5 and later returns an unsigned
integer. In C version 5.1 and QuickC version 2.0, this function returns a signed
int.

MORE INFORMATION
================

The change to an unsigned return value can cause problems in code that uses the
idiom "-sizeof(type)." The following code example attempts to position the file
pointer one record from the end of a file. The method the code demonstrates
works with C version 5.1, but fails with C version 6.0 and later.

     fseek(file, (long)(-sizeof(record)), SEEK_END);

This fseek call positions the file pointer far beyond the end of the file because
the compiler does not sign extend the generated value. Because the unary minus
(-) operator does not change the "signedness" of a variable and the sizeof()
operator returns an unsigned int, the compiler performs a zero extension to
convert the unsigned int to a long.

In other words, if the record variable was of type char, the following code
returns the long value -1 when compiled with C 5.1 and 32,767 when compiled with
C 6.0:

     (long)(-sizeof(record))

One method to work around this problem involves casting the result of the sizeof
operator to a signed int before negating the value and casting it to a long. For
example:

     (long)-(int)sizeof(record)

Additional query words:

======================================================================
Keywords          : kbLangC kbVC100 kbVC150 kbVC200 kbVC400 kbVC500 kbVC600 
Technology        : kbVCsearch kbVC400 kbAudDeveloper kbvc150 kbvc100 kbCCompSearch kbZNotKeyword3 kbVC500 kbVC600 kbVC200 kbVC32bitSearch kbVC500Search
Version           : :1.0,1.5,2.0,4.0,5.0,6.0
Issue type        : kbinfo

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.