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# freopen

From cppreference.com
< c‎ | io

Defined in header `<stdio.h>` |  |

FILE *freopen( const char *filename, const char *mode, FILE *stream ); |  | (until C99)
FILE *freopen( const char *restrict filename, const char *restrict mode, FILE *restrict stream ); |  | (since C99)

Reassigns an existing file stream `stream` to a different file identified by `filename` using specified mode. `mode` is used to determine the new file access mode.

### [edit] Parameters

filename | - | file name to associate the file stream to
mode | - | null-terminated character string determining new file access mode
File access mode string | Meaning | Explanation | Action if file already exists | Action if file does not exist
--- | --- | --- | --- | ---
`"r"` | read | Open a file for reading | read from start | failure to open
`"w"` | write | Create a file for writing | destroy contents | create new
`"a"` | append | Append to a file | write to end | create new
`"r+"` | read extended | Open a file for read/write | read from start | error
`"w+"` | write extended | Create a file for read/write | destroy contents | create new
`"a+"` | append extended | Open a file for read/write | write to end | create new
File access mode flag `"b"` can optionally be specified to open a file in binary mode. This flag has effect only on Windows systems. On the append file access modes, data is written to the end of the file regardless of the current position of the file position indicator.
File access mode flag `"x"` can optionally be appended to "w" or "w+" specifiers. This flag forces the function to fail if the file exists, instead of overwriting it. (C11)
stream | - | the file stream to modify

### [edit] Return value

`stream` on success, NULL on failure

### [edit] Example

freopen with error checking. The following code redirects `stdout` to a file

Run this code

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

    int main(void)
    {
        printf("stdout is printed to console");
        if (freopen("redir.txt", "w", stdout) == NULL)
        {
           perror("freopen()");
           fprintf(stderr,"freopen() failed in file %s at line # %d\n", __FILE__,__LINE__-3);
           exit(EXIT_FAILURE);
        }
        printf("stdout is redirected to a file");
        fclose(stdout);
        return EXIT_SUCCESS;
    }

Output:

    stdout is printed to console

### [edit] See also

fopen | opens a file (function) [edit]
fclose | closes a file (function) [edit]
C++ documentation for freopen

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