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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 .