PRB: stdout/stderr May Not Be In Sync When Using Redirection

• The C Run-Time (CRT), included with:
  • Microsoft Visual C++ for Windows, 16-bit edition, versions 1.5, 1.51
  • Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 4.0, 5.0, 6.0

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) 

  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" );
   } 

Additional query words:

Keywords : kbCRT kbVC kbVC150 kbVC151 kbVC200 kbVC210 kbVC400 kbVC500 kbVC600
Version : 1.5 1.51 2.0 2.1 4.0 5.0
Platform : NT WINDOWS
Issue type : kbprb