FREOPEN(3) Library Functions Manual FREOPEN(3)

freopen — open a stream

#include <stdio.h>

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

[CX] 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 (“POSIX.1”) defers to the ISO C standard.

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(3).

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, the encoding rule shall be cleared, and the associated object shall be set to describe an initial conversion state.

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

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

The freopen () function shall fail if:

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

The freopen () function may fail if:

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

The following example logs all standard output to the file.

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

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

None.

None.

fclose(3) , fopen(3) , fdopen(3) , mbsinit(3) , the Base Definitions volume of IEEE Std 1003.1-2001 (“POSIX.1”), <stdio.h>

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

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.

Extensions beyond the ISO C standard are marked.

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

  • In the DESCRIPTION, text is added to indicate setting of the offset maximum in the open file description. This change is to support large files.
  • In the ERRORS section, the [EOVERFLOW] condition is added. This change is to support large files.
  • The [ELOOP] mandatory error condition is added.
  • A second [ENAMETOOLONG] is added as an optional error condition.
  • The [EINVAL], [ENOMEM], [ENXIO], and [ETXTBSY] optional error conditions are added.

The following changes are made for alignment with the ISO/IEC 9899:1999 (“ISO C99”) standard:

  • The freopen () prototype is updated.
  • The DESCRIPTION is updated.

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".

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December 6, 2001 posix.fail