The Open Group Base Specifications Issue 6
IEEE Std 1003.1-2001
Copyright © 2001 The IEEE and The Open Group, All Rights reserved.


freopen - open a stream


#include <stdio.h>

FILE *freopen(const char *restrict
filename, const char *restrict mode,
       FILE *restrict


[CX] [Option Start] The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of IEEE Std 1003.1-2001 defers to the ISO C standard. [Option End]

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

The freopen() function shall open the file whose pathname is the string pointed to by filename and associate the stream pointed to by stream with it. The mode argument shall be used just as in fopen().

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

If filename is a null pointer, the freopen() function shall attempt to change the mode of the stream to that specified by mode, as if the name of the file currently associated with the stream had been used. It is implementation-defined which changes of mode are permitted (if any), and under what circumstances.

After a successful call to the freopen() function, the orientation of the stream shall be cleared, [XSI] [Option Start]  the encoding rule shall be cleared, [Option End] and the associated mbstate_t object shall be set to describe an initial conversion state.

[CX] [Option Start] The largest value that can be represented correctly in an object of type off_t shall be established as the offset maximum in the open file description. [Option End]


Upon successful completion, freopen() shall return the value of stream. Otherwise, a null pointer shall be returned, [CX] [Option Start]  and errno shall be set to indicate the error. [Option End]


The freopen() function shall fail if:

[CX] [Option Start] 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. [Option End]
[CX] [Option Start] A signal was caught during freopen(). [Option End]
[CX] [Option Start] The named file is a directory and mode requires write access. [Option End]
[CX] [Option Start] A loop exists in symbolic links encountered during resolution of the path argument. [Option End]
[CX] [Option Start] {OPEN_MAX} file descriptors are currently open in the calling process. [Option End]
[CX] [Option Start]
The length of the filename argument exceeds {PATH_MAX} or a pathname component is longer than {NAME_MAX}. [Option End]
[CX] [Option Start] The maximum allowable number of files is currently open in the system. [Option End]
[CX] [Option Start] A component of filename does not name an existing file or filename is an empty string. [Option End]
[CX] [Option Start] 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. [Option End]
[CX] [Option Start] A component of the path prefix is not a directory. [Option End]
[CX] [Option Start] The named file is a character special or block special file, and the device associated with this special file does not exist. [Option End]
[CX] [Option Start] The named file is a regular file and the size of the file cannot be represented correctly in an object of type off_t. [Option End]
[CX] [Option Start] The named file resides on a read-only file system and mode requires write access. [Option End]

The freopen() function may fail if:

[CX] [Option Start] The value of the mode argument is not valid. [Option End]
[CX] [Option Start] More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the path argument. [Option End]
[CX] [Option Start]
Pathname resolution of a symbolic link produced an intermediate result whose length exceeds {PATH_MAX}. [Option End]
[CX] [Option Start] Insufficient storage space is available. [Option End]
[CX] [Option Start] A request was made of a nonexistent device, or the request was outside the capabilities of the device. [Option End]
[CX] [Option Start] The file is a pure procedure (shared text) file that is being executed and mode requires write access. [Option End]

The following sections are informative.


Directing Standard Output to a File

The following example logs all standard output to the /tmp/logfile file.

#include <stdio.h>
FILE *fp;
fp = freopen ("/tmp/logfile", "a+", stdout);


The freopen() function is typically used to attach the preopened streams associated with stdin, stdout, and stderr to other files.






fclose() , fopen() , fdopen() , mbsinit() , the Base Definitions volume of IEEE Std 1003.1-2001, <stdio.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 5

The DESCRIPTION is updated to indicate that the orientation of the stream is cleared and the conversion state of the stream is set to an initial conversion state by a successful call to the freopen() function.

Large File Summit extensions are added.

Issue 6

Extensions beyond the ISO C standard are marked.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

The following changes are made for alignment with the ISO/IEC 9899:1999 standard:

The wording of the mandatory [ELOOP] error condition is updated, and a second optional [ELOOP] error condition is added.

The DESCRIPTION is updated regarding failure to close, changing the "file" to "file descriptor".

End of informative text.

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POSIX ® is a registered Trademark of The IEEE.
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