FREOPEN(3)                 POSIX Programmer's Manual                FREOPEN(3)



PROLOG
       This manual page is part of the POSIX Programmer's Manual.  The OS
       implementation of this interface may differ (consult the corresponding
       OS manual page for details of OS behavior), or the interface may not be
       implemented on OS.

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 aa 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 .



IEEE/The Open Group                  2017                           FREOPEN(3)
