man pages section 3: Basic Library Functions

Exit Print View

Updated: July 2014



freopen - open a stream


#include <stdio.h>

FILE *freopen(const char *filename, const char *mode, FILE *stream);


The freopen() function first attempts to flush the stream and close any file descriptor associated with stream. Failure to flush or close the file successfully is ignored. The error and end-of-file indicators for the stream are cleared.

The freopen() function opens the file whose pathname is the string pointed to by filename and associates the stream pointed to by stream with it. The mode argument is used just as in fopen(3C).

If filename is a null pointer and the application conforms to SUSv3 (see standards(5)), the freopen() function attempts to change the mode of the stream to that specified by mode, as though the name of the file currently associated with the stream had been used. The following changes of mode are permitted, depending upon the access mode of the file descriptor underlying the stream:

  • When + is specified, the file descriptor mode must be O_RDWR.

  • When r is specified, the file descriptor mode must be O_RDONLY or O_RDWR.

  • When a or w is specified, the file descriptor mode must be O_WRONLY or O_RDWR.

If the filename is a null pointer and the application does not conform to SUSv3, freopen() returns a null pointer.

The original stream is closed regardless of whether the subsequent open succeeds.

After a successful call to the freopen() function, the orientation of the stream is cleared, the encoding rule is cleared, and the associated mbstate_t object is set to describe an initial conversion state.

The largest value that can be represented correctly in an object of type off_t will be established as the offset maximum in the open file description.

Return Values

Upon successful completion, freopen() returns the value of stream. Otherwise, a null pointer is returned and errno is set to indicate the error.


The freopen() function will fail if:


Search permission is denied on a component of the path prefix, or the file exists and the permissions specified by mode are denied, or the file does not exist and write permission is denied for the parent directory of the file to be created.


The application conforms to SUSv3, the filename argument is a null pointer, and either the underlying file descriptor is not valid or the mode specified when the underlying file descriptor was opened does not support the file access modes requested by the mode argument.


The application does not conform to SUSv3 and the filename argument is a null pointer.


A signal was caught during freopen().


The named file is a directory and mode requires write access.


Too many symbolic links were encountered in resolving path.


There are {OPEN_MAX} file descriptors currently open in the calling process.


The length of the filename exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}.


The maximum allowable number of files is currently open in the system.


A component of filename does not name an existing file or filename is an empty string.


The directory or file system that would contain the new file cannot be expanded, the file does not exist, and it was to be created.


A component of the path prefix is not a directory.


The named file is a character special or block special file, and the device associated with this special file does not exist.


The current value of the file position cannot be represented correctly in an object of type off_t.


The named file resides on a read-only file system and mode requires write access.

The freopen() function may fail if:


The value of the mode argument is not valid.


Pathname resolution of a symbolic link produced an intermediate result whose length exceeds {PATH_MAX}.


Insufficient storage space is available.


A request was made of a non-existent device, or the request was outside the capabilities of the device.


The file is a pure procedure (shared text) file that is being executed and mode requires write access.


The freopen() function is typically used to attach the preopened streams associated with stdin, stdout and stderr to other files. By default stderr is unbuffered, but the use of freopen() will cause it to become buffered or line-buffered.

The freopen() function has a transitional interface for 64-bit file offsets. See lf64(5).


See attributes(5) for descriptions of the following attributes:

Interface Stability

See also

fclose(3C), fdopen(3C), fopen(3C), stdio(3C), attributes (5), lf64(5), standards (5)