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FREOPEN(3P) POSIX Programmer's Manual FREOPEN(3P)
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.
freopen — open a stream
#include <stdio.h>
FILE *freopen(const char *restrict pathname, const char *restrict mode,
FILE *restrict stream);
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.1‐2008 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 │
└─────────────────┴───────────────────────────┘
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.
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 names 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.
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 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.
None.
None.
Section 2.5, Standard I/O Streams, fclose(3p), fdopen(3p),
fflush(3p), fmemopen(3p), fopen(3p), mbsinit(3p), open(3p),
open_memstream(3p)
The Base Definitions volume of POSIX.1‐2008, stdio.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 .
IEEE/The Open Group 2013 FREOPEN(3P)
Pages that refer to this page: stdio.h(0p), fmemopen(3p), fopen(3p), open_memstream(3p)