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freopen

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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.

Contents

[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

#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

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