freopen

#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
    FILE *freopen( const char *fname, const char *mode, FILE *stream );

closes file associated with stream, then opens file filename with specified mode and associates it with stream. Returns stream or NULL on error.

/* 
 * freopen example code
 * http://code-reference.com/c/stdio.h/freopen 
 */
 
 
#include <stdio.h> /* including standard library */
//#include <windows.h> /* uncomment this for Windows */
 
 
int main ( void )
{
  freopen ("test.txt","w",stdout);
  printf ("This content is written to a file");
  fclose (stdout);
  return 0;
}

  user@host:~$ ./freopen 
  user@host:~$ cat test.txt 
  This content is written to a file