INFO: sizeof() Function Returns unsigned Instead of int

 Last reviewed: September 4, 1997
Article ID: Q60332

The information in this article applies to:
  • Microsoft C for MS-DOS, versions 6.0, 6.0a, 6.0ax
  • Microsoft C for OS/2, versions 6.0, 6.0a
  • Microsoft C/C++ for MS-DOS and OS/2, versions 7.0
  • Microsoft Visual C++ for Windows, versions 1.0, 1.5
  • Microsoft Visual C++ 32-bit Edition, versions 1.0, 2.0, 4.0, 5.0

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)

Keywords          : CLngIss kbfasttip
Version           : MS-DOS:6.0,6.00a,6.00ax,7.0; OS/2:6.0,6.00a,7.0;  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

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Last reviewed: September 4, 1997
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