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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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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.1‐2017 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 │
└──────────────────┴───────────────────────────┘
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 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.
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.1‐2017, stdio.h(0p)
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 .
IEEE/The Open Group 2017 FREOPEN(3P)
Pages that refer to this page: stdio.h(0p), fclose(3p), fmemopen(3p), fopen(3p), open_memstream(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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