Knowledge Base

Unexpected output in your console application when the standard output and error streams are redirected or piped

Article ID: 122442

Article Last Modified on 8/5/2005

The C Run-Time (CRT), when used with:
- Microsoft Visual C++ 1.5 Professional Edition
- Microsoft Visual C++ 1.51
- Microsoft Visual C++ 2.0 Professional Edition
- Microsoft Visual C++ 2.1
- Microsoft Visual C++ 4.0 Standard Edition
- Microsoft Visual C++ 5.0 Enterprise Edition
- Microsoft Visual C++ 6.0 Enterprise Edition
- Microsoft Visual C++ 5.0 Professional Edition
- Microsoft Visual C++ 6.0 Professional Edition
- Microsoft Visual C++ 6.0 Standard Edition

This article was previously published under Q122442

SYMPTOMS

When the standard output and error streams are redirected or piped, console application output may not be serialized. This can result in unexpected output.

CAUSE

Although devices as serialized by default, the current Microsoft CRT implementation does not treat pipes as if they were devices (due to the potentially high overhead--added code--of doing so.) Thus, if stderr and stdout refer to the same file or device, and output is redirected with a pipe, the output will not be serialized.

STATUS

This behavior is by design. The potential for slow execution currently outweighs the effort necessary to implement pipes such that they are treated as devices.

MORE INFORMATION

To demonstrate this behavior, build and run the sample code (TEST.C) below. If it is executed from an MS-DOS or Windows NT command prompt, the following output is generated:

  c:\>test
  hello world (stdout)
  hello world (stderr)
  hello world (stdout)
  hello world (stderr)

However, if the output is piped with the MORE command, it will not be serialized, as follows:

  c:\>test | more
  hello world (stderr)
  hello world (stderr)
  hello world (stdout)
  hello world (stdout)

Sample code

   /* Compile options needed: none
   /*

   /* TEST.C */ 

   #include <stdio.h>

   int main(void)
   {
     printf ( "Hello world (stdout)\n" );
     fprintf ( stderr, "Hello world (stderr)\n" );
     printf ( "Hello world (stdout)\n" );
     fprintf ( stderr, "Hello world (stderr)\n" );
   }

Keywords: kbtshoot kbprb KB122442