HOWTO: Redirect stdout to and from a File Using C

• The C Run-time (CRT) included with: - Microsoft C for MS-DOS, versions 5.1, 6.0, 6.0a, 6.0ax - Microsoft C for OS/2, versions 5.1, 6.0, 6.0a - Microsoft C/C++ for MS-DOS, version 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

This article explains how to redirect stdout to a file from a C program, then restore the original stdout at a later point in the same program.

MORE INFORMATION

The C function typically used to redirect stdout or stdin is freopen(). To redirect stdout to a file called FILE.TXT, use the following call:

   freopen( "file.txt", "w", stdout );

To return stdout to the display (the default stdout), use the following call:

   freopen( "CON", "w", stdout );

Below is a short program to demonstrate the redirection of stdout:

Sample Code

   // Compile options needed: none

   #include <stdio.h>
   #include <stdlib.h>

   void main(void)
   {
      FILE *stream ;
      if((stream = freopen("file.txt", "w", stdout)) == NULL)
         exit(-1);

      printf("this is stdout output\n");

      stream = freopen("CON", "w", stdout);

      printf("And now back to the console once again\n");
   }

Keywords          : CRTIss kbfasttip
Version           : MS-DOS:5.1,6.0,6.00a,6.00ax,7.0; OS/2:5.1,6.0,6.00a;  WINDOWS:1.0,1.5; WINDOWS NT:1.0,2.0,4.0,5.0
Platform          : MS-DOS NT OS/2 WINDOWS
Issue type        : kbhowto