Reassigns a file pointer.
#include <stdio.h>
FILE *freopen( const char *filename, const char *mode, FILE *stream );
| filename | Path name of new file | |
| mode | Type of access permitted | |
| stream | Pointer to FILE structure |
The freopen function closes the file currently associated with stream and reassigns stream to the file specified by filename. The freopen function is typically used to redirect the pre-opened files stdin, stdout, and stderr to files specified by the user. The new file associated with stream is opened with mode, which is a character string specifying the type of access requested for the file, as follows:
| Type | Description |
| "r" | Opens for reading. If the file does not exist or cannot be found, the freopen call fails. |
| "w" | Opens an empty file for writing. If the given file exists, its contents are destroyed. |
| "a" | Opens for writing at the end of the file (appending); creates the file first if it does not exist. |
| "r+" | Opens for both reading and writing. (The file must exist.) |
| "w+" | Opens an empty file for both reading and writing. If the given file exists, its contents are destroyed. |
| "a+" | Opens for reading and appending; creates the file first if it does not exist. |
Use the "w" and "w+" types with care, as they can destroy existing files.
When a file is opened with the "a" or "a+" access type, all write operations take place at the end of the file. Although the file pointer can be repositioned using fseek or rewind, the file pointer is always moved back to the end of the file before any write operation is carried out. Thus, existing data cannot be overwritten.
When the "r+", "w+", or "a+" access type is specified, both reading and writing are allowed (the file is said to be open for “update”). However, when you switch between reading and writing, there must be an intervening fsetpos, fseek, or rewind operation. The current position can be specified for the fsetpos or fseek operation, if desired.
In addition to the values listed above, one of the following characters may be included in the mode string to specify the translation mode for new lines.
| Mode | Meaning |
| t | Open in text (translated) mode; carriage-return–line-feed (CR-LF) combinations are translated into single line-feed (LF) characters on input; LF characters are translated to CR-LF combinations on output. Also, CTRL+Z is interpreted as an end-of-file character on input. In files opened for reading, or writing and reading, the run-time library checks for a CTRL+Z at the end of the file and removes it, if possible. This is done because using the fseek and ftell functions to move within a file may cause fseek to behave improperly near the end of the file. |
| b | Open in binary (untranslated) mode; the above translations are suppressed. |
If t or b is not given in the mode string, the translation mode is defined by the default mode variable _fmode.
See “Input and Output” for a discussion of text and binary modes.
The freopen function returns a pointer to the newly opened file. If an error occurs, the original file is closed and the function returns a NULL pointer value.
Standards:ANSI, UNIX
16-Bit:DOS, QWIN, WIN, WIN DLL
32-Bit:DOS32X
The t option is not part of the ANSI standard for freopen; it is a Microsoft extension that should not be used where ANSI portability is desired.
fclose, _fcloseall, _fdopen, _fileno, fopen, _open, _setmode
/* FREOPEN.C: This program reassigns stdaux to the file
* named FREOPEN.OUT and writes a line to that file.
*/
#include <stdio.h>
#include <stdlib.h>
FILE *stream;
void main( void )
{
/* Reassign "stdaux" to "freopen.out": */
stream = freopen( "freopen.out", "w", stdaux );
if( stream == NULL )
fprintf( stdout, "error on freopen\n" );
else
{
fprintf( stream, "This will go to the file 'freopen.out'\n" );
fprintf( stdout, "successfully reassigned\n" );
fclose( stream );
}
system( "type freopen.out" );
}
successfully reassigned
This will go to the file 'freopen.out'