< c‎ | io
Revision as of 04:51, 1 May 2014 by Newatthis (Talk | contribs)

File input/output

File access
Direct input/output
Unformatted input/output
Formatted input
Formatted output
File positioning
Error handling
Operations on files

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.



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.
When a file is opened with update mode ('+' as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end- of-file. Opening (or creating) a text file with update mode may instead open (or create) a binary stream in some implementations.
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)
When using fopen_s or freopen_s, file access permissions for any file created with "w" or "a" prevents other users from accessing it. File access mode flag "u" can optionally be prepended to any specifier that begins with "w" or "a", to enable the default fopen permissions. (C11)
stream - the file stream to modify

Return value

stream on success, NULL on failure


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)
       fprintf(stderr,"freopen() failed in file %s at line # %d\n", __FILE__,__LINE__-3);
    printf("stdout is redirected to a file");
    return EXIT_SUCCESS;


stdout is printed to console

See also

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