FREOPEN(3C) Standard C Library Functions FREOPEN(3C)


NAME


freopen - open a stream

SYNOPSIS


#include <stdio.h>

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


DESCRIPTION


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. This behavior is not
supported by streams that are backed by memory, such as fmemopen(3C). The
following changes of mode are permitted, depending upon the access mode
of the file descriptor underlying the stream:

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

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

o 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.

ERRORS


The freopen() function will fail if:

EACCES
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.


EBADF
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.


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


EINTR
A signal was caught during freopen().


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


ELOOP
Too many symbolic links were encountered in resolving
path.


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


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


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


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


ENOSPC
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.


ENOTDIR
A component of the path prefix is not a directory.


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


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


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


The freopen() function may fail if:

EINVAL
The value of the mode argument is not valid.


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


ENOMEM
Insufficient storage space is available.


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


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


USAGE


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).

ATTRIBUTES


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


+--------------------+-----------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-----------------+
|Interface Stability | Standard |
+--------------------+-----------------+
|MT-Level | MT-Safe |
+--------------------+-----------------+

SEE ALSO


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


March 25, 2020 FREOPEN(3C)