PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT

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

PROLOG         top

       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         top

       freopen — open a stream

SYNOPSIS         top

       #include <stdio.h>

       FILE *freopen(const char *restrict pathname, const char *restrict mode,
           FILE *restrict stream);

DESCRIPTION         top

       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   │
                  └─────────────────┴───────────────────────────┘

RETURN VALUE         top

       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         top

       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.

EXAMPLES         top

   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         top

       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         top

       None.

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       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)

COPYRIGHT         top

       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)