SAS/C Library Reference, Vol. 1

freopen -- Reopen a File

SYNOPSIS

 #include <stdio.h>

 FILE *freopen(const char *name, const char *mode, FILE *oldf);

DESCRIPTION

The freopen function closes the stream associated with the FILE object addressed by oldf and then reopens it, using the filename and open mode specified by name and mode.

name is the external name of the file to be opened. The form of name is system dependent. Note that the name to be opened may be different from the filename currently associated with oldf. The mode string defines how the file is used. For more information about open-mode values, see Open modes .

Portable use of freopen requires that oldf identify an open file. This implementation permits oldf to reference a closed file, which permits you to reuse a FILE pointer by calling freopen after fclose.

RETURN VALUE

If freopen is successful, the value of oldf is returned. The FILE object addressed by oldf is now associated with the file specified by name.

If freopen is unsuccessful, a NULL FILE pointer is returned. Further use of oldf after an unsuccessful freopen is not permitted.

EXAMPLE

This example uses freopen to change the stderr file, thereby enabling library diagnostic messages to be redirected:

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

  main()
  {
     FILE *f;
     char answer;

     puts("Do you wish to save library messages on disk? (Y or N)?");
     answer = toupper(getchar());
     if (answer == 'Y'){
           f = freopen("tso:saved.messages", "w", stderr);
           if (!f){
                 puts("Failed to reopen stderr.");
                 exit(EXIT_FAILURE);
           }
     }

        /* The following fopen() is deliberately invalid and causes */
        /*  a library diagnostic to be written to stderr.           */
     f = fopen("tso:impossible.file.name", "w");
     exit(EXIT_SUCCESS);
  }

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