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
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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
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SUMMARY
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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
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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:
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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
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