FREOPEN
Section: POSIX Programmer's Manual (3P)
Updated: 2017
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PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
NAME
freopen
--- open a stream
SYNOPSIS
#include <stdio.h>
FILE *freopen(const char *restrict pathname, const char *restrict mode,
FILE *restrict stream);
DESCRIPTION
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 POSIX.1-2017 defers to the ISO C standard.
The
freopen()
function shall first attempt to flush the stream associated with
stream
as if by a call to
fflush(stream).
Failure to flush the stream successfully shall be ignored. If
pathname
is not a null pointer,
freopen()
shall close any file descriptor associated with
stream.
Failure to 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
pathname
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
pathname
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. In this case, the file descriptor associated with the stream
need not be closed if the call to
freopen()
succeeds. 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
mbstate_t
object shall be set to describe an initial conversion state.
If
pathname
is not a null pointer, or if
pathname
is a null pointer and the specified mode change necessitates the file
descriptor associated with the stream to be closed and reopened, the
file descriptor associated with the reopened stream shall be allocated
and opened as if by a call to
open()
with the following flags:
| freopen() Mode | open() Flags
|
|
| r or rb | O_RDONLY
|
| w or wb | O_WRONLY|O_CREAT|O_TRUNC
|
| a or ab | O_WRONLY|O_CREAT|O_APPEND
|
| r+ or rb+ or r+b | O_RDWR
|
| w+ or wb+ or w+b | O_RDWR|O_CREAT|O_TRUNC
|
| a+ or ab+ or a+b | O_RDWR|O_CREAT|O_APPEND
|
|
RETURN VALUE
Upon successful completion,
freopen()
shall return the value of
stream.
Otherwise, a null pointer shall be returned,
and
errno
shall be set to indicate the error.
ERRORS
The
freopen()
function shall 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 file descriptor underlying the stream is not a valid file
descriptor when
pathname
is a null pointer.
- EINTR
-
A signal was caught during
freopen().
- EISDIR
-
The named file is a directory and
mode
requires write access.
- ELOOP
-
A loop exists in symbolic links encountered during resolution of the
path
argument.
- EMFILE
-
All file descriptors available to the process are currently open.
- ENAMETOOLONG
-
The length of a component of a pathname is longer than
{NAME_MAX}.
- ENFILE
-
The maximum allowable number of files is currently open in the system.
- ENOENT
-
The
mode
string begins with
'r'
and a component of
pathname
does not name an existing file, or
mode
begins with
'w'
or
'a'
and a component of the path prefix of
pathname
does not name an existing file, or
pathname
is an empty string.
- ENOENT or ENOTDIR
-
The
pathname
argument contains at least one non-<slash>
character and ends with one or more trailing
<slash>
characters. If
pathname
without the trailing
<slash>
characters would name an existing file, an
[ENOENT]
error shall not occur.
- 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 names an existing file that is neither
a directory nor a symbolic link to a directory, or the
pathname
argument contains at least one non-<slash>
character and ends with one or more trailing
<slash>
characters and the last pathname component names an existing file that
is neither a directory nor a symbolic link to 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 named file is a regular file and the size of the file 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:
- EBADF
-
The mode with which the file descriptor underlying the stream was
opened does not support the requested mode when
pathname
is a null pointer.
- EINVAL
-
The value of the
mode
argument is not valid.
- ELOOP
-
More than
{SYMLOOP_MAX}
symbolic links were encountered during resolution of the
path
argument.
- ENAMETOOLONG
-
The length of a pathname exceeds
{PATH_MAX},
or pathname resolution of a symbolic link produced an intermediate
result with a length that exceeds
{PATH_MAX}.
- ENOMEM
-
Insufficient storage space is available.
- ENXIO
-
A request was made of a nonexistent 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.
The following sections are informative.
EXAMPLES
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);
...
APPLICATION USAGE
The
freopen()
function is typically used to attach the pre-opened
streams
associated with
stdin,
stdout,
and
stderr
to other files.
Since implementations are not required to support any stream mode changes
when the
pathname
argument is NULL, portable applications cannot rely on the use of
freopen()
to change the stream mode, and use of this feature is discouraged. The
feature was originally added to the ISO C standard in order to facilitate changing
stdin
and
stdout
to binary mode. Since a
'b'
character in the mode has no effect on POSIX systems, this use of the
feature is unnecessary in POSIX applications. However, even though the
'b'
is ignored, a successful call to
freopen(NULL, "wb", stdout) does have an effect. In particular,
for regular files it truncates the file and sets the file-position
indicator for the stream to the start of the file. It is possible that
these side-effects are an unintended consequence of the way the feature
is specified in the ISO/IEC 9899:1999 standard, but unless or until the ISO C standard is changed,
applications which successfully call
freopen(NULL, "wb", stdout) will behave in unexpected ways on
conforming systems in situations such as:
-
{ appl file1; appl file2; } > file3
which will result in
file3
containing only the output from the second invocation of
appl.
RATIONALE
None.
FUTURE DIRECTIONS
None.
SEE ALSO
Section 2.5, Standard I/O Streams,
fclose(),
fdopen(),
fflush(),
fmemopen(),
fopen(),
mbsinit(),
open(),
open_memstream()
The Base Definitions volume of POSIX.1-2017,
<stdio.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
In the event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- Directing Standard Output to a File
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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