fwrite() Returns 0 When the Item Size Is 0

MS-DOS                      | OS/2            | WINDOWS

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, and 6.0ax
      - Microsoft C for OS/2, versions 5.1, 6.0, and 6.0a
      - Microsoft C/C++ for MS-DOS, version 7.0
      - Microsoft Visual C++ for Windows, versions 1.0 and 1.5
  

SUMMARY

The "Microsoft C Optimizing Compiler: Run-Time Library Reference" for version 5.1, the Microsoft C versions 6.0, 6.0a, 6.0ax Advisor, and Microsoft C/C++ 7.0 and Visual C++ "Run-Time Library Reference" document the syntax for fwrite() as:

   size_t fwrite( const void *buffer,
                         size_t size,
                        size_t count,
                        FILE *stream)

   The fwrite() function returns the number of full items actually
   written, which may be less than <count> if an error occurs.

MORE INFORMATION

Section 4.9.8.2 of the ANSI standard documents the syntax for fwrite() as:

   size_t fwrite( const void *ptr,
                      size_t size,
                     size_t nmemb,
                     FILE *stream)

   The fwrite() function returns the number of elements successfully
   written, which will be less than <nmemb> only if a write error is
   encountered.

For example, if an item of size 0 is sent to test whether a file is write enabled, without actually writing anything to the file, the Microsoft implementation of the function will return a value of 0. This leaves a return value of 0 ambiguous; in this case it can be mistakenly interpreted as an error return.

The sample code below illustrates the ambiguity. The program output is:

   Result is 0.

Sample Code

/* Compile options needed: none
*/

#include<stdio.h>

#define SIZE 0
#define NUMBER_TO_WRITE 10

void main( void );

void main()

   int result;
   FILE *fp;

   fp = fopen( "c:\\test.tmp", "w+t" );
   result = fwrite( &result, SIZE, NUMBER_TO_WRITE, fp );

   printf("Result is %d.\n", result);

   fclose( fp );

NOTE: In Visual C++ for Windows NT, versions 1.0, 2.0 and 2.1, fwrite() returns the number elements written regardless of the size. The sample program in this article will return a 10.