freopen(3p) — Linux manual page

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.1‐2017 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         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 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         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.1‐2017, stdio.h(0p)

COPYRIGHT         top

       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)