INFO: sizeof() Function Returns unsigned Instead of int (60332)


The information in this article applies to:

  • Microsoft C for MS-DOS
  • Microsoft C for OS/2
  • Microsoft Visual C++ 1.0
  • Microsoft Visual C++ 1.5
  • Microsoft Visual C++ 2.0
  • Microsoft Visual C++ 4.0
  • Microsoft Visual C++, 32-bit Enterprise Edition 5.0
  • Microsoft Visual C++, 32-bit Enterprise Edition 6.0
  • Microsoft Visual C++, 32-bit Professional Edition 5.0
  • Microsoft Visual C++, 32-bit Professional Edition 6.0
  • Microsoft Visual C++, 32-bit Learning Edition 6.0
This article was previously published under Q60332

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)
Modification Type:MajorLast Reviewed:12/11/2003
Keywords:kbinfo kbLangC KB60332
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