FOPEN(3)                 BSD Library Functions Manual                 FOPEN(3)

NAME
     fopen, fdopen, freopen, fmemopen -- stream open functions

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdio.h>

     FILE *
     fopen(const char * restrict path, const char * restrict mode);

     FILE *
     fdopen(int fildes, const char *mode);

     FILE *
     freopen(const char *path, const char *mode, FILE *stream);

     FILE *
     fmemopen(void *restrict *buf, size_t size, const char * restrict mode);

DESCRIPTION
     The fopen() function opens the file whose name is the string pointed to
     by path and associates a stream with it.

     The argument mode points to a string beginning with one of the following
     letters:

     ``r''   Open for reading.  The stream is positioned at the beginning of
             the file.  Fail if the file does not exist.

     ``w''   Open for writing.  The stream is positioned at the beginning of
             the file.  Create the file if it does not exist.

     ``a''   Open for writing.  The stream is positioned at the end of the
             file.  Subsequent writes to the file will always end up at the
             then current end of file, irrespective of any intervening
             fseek(3) or similar.  Create the file if it does not exist.

     An optional ``+'' following ``r'', ``w'', or ``a'' opens the file for
     both reading and writing.  An optional ``x'' following ``w'' or ``w+''
     causes the fopen() call to fail if the file already exists.  An optional
     ``e'' following the above causes the fopen() call to set the FD_CLOEXEC
     flag on the underlying file descriptor.

     The mode string can also include the letter ``b'' after either the ``+''
     or the first letter.  This is strictly for compatibility with ISO/IEC
     9899:1990 (``ISO C90'') and has effect only for fmemopen() ; otherwise
     ``b'' is ignored.

     Any created files will have mode ``S_IRUSR | S_IWUSR | S_IRGRP | S_IWGRP
     | S_IROTH | S_IWOTH'' (0666), as modified by the process' umask value
     (see umask(2)).

     Reads and writes may be intermixed on read/write streams in any order;
     however, a file positioning function must be called when switching
     between output and input, unless an input operation encounters end-of-
     file.

     The fdopen() function associates a stream with the existing file descrip-
     tor, fildes.  The mode of the stream must be compatible with the mode of
     the file descriptor.  The ``x'' mode option is ignored.  If the ``e''
     mode option is present, the FD_CLOEXEC flag is set, otherwise it remains
     unchanged.  When the stream is closed via fclose(3), fildes is closed
     also.

     The freopen() function opens the file whose name is the string pointed to
     by path and associates the stream pointed to by stream with it.  The
     original stream (if it exists) is closed.  The mode argument is used just
     as in the fopen() function.

     If the path argument is NULL, freopen() attempts to re-open the file
     associated with stream with a new mode.  The new mode must be compatible
     with the mode that the stream was originally opened with: Streams open
     for reading can only be re-opened for reading, streams open for writing
     can only be re-opened for writing, and streams open for reading and writ-
     ing can be re-opened in any mode.  The ``x'' mode option is not meaning-
     ful in this context.

     The primary use of the freopen() function is to change the file associ-
     ated with a standard text stream (stderr, stdin, or stdout).

     The fmemopen() function associates the buffer given by the buf and size
     arguments with a stream.  The buf argument is either a null pointer or a
     pointer to a buffer that is at least size bytes long.  If a null pointer
     is specified as the buf argument, fmemopen() allocates size bytes of mem-
     ory, and this allocation is automatically freed when the stream is
     closed.  If a non-null pointer is specified, the caller retains ownership
     of the buffer and is responsible for disposing of it after the stream has
     been closed.  Buffers can be opened in text-mode (default) or binary-mode
     (if ``b'' is present in the second or third position of the mode argu-
     ment). Buffers opened in text-mode make sure that writes are terminated
     with a NULL byte, if the last write hasn't filled up the whole buffer.
     Buffers opened in binary-mode never append a NULL byte.

RETURN VALUES
     Upon successful completion fopen(), fdopen(), freopen() and fmemopen()
     return a FILE pointer.  Otherwise, NULL is returned and the global vari-
     able errno is set to indicate the error.

ERRORS
     [EINVAL]           The mode argument to fopen(), fdopen(), freopen(), or
                        fmemopen() was invalid.

     The fopen(), fdopen(), freopen() and fmemopen() functions may also fail
     and set errno for any of the errors specified for the routine malloc(3).

     The fopen() function may also fail and set errno for any of the errors
     specified for the routine open(2).

     The fdopen() function may also fail and set errno for any of the errors
     specified for the routine fcntl(2).

     The freopen() function may also fail and set errno for any of the errors
     specified for the routines open(2), fclose(3) and fflush(3).

     The fmemopen() function may also fail and set errno if the size argument
     is 0.

SEE ALSO
     open(2), fclose(3), fileno(3), fseek(3), funopen(3)

STANDARDS
     The fopen() and freopen() functions conform to ISO/IEC 9899:1990
     (``ISO C90''), with the exception of the ``x'' mode option which conforms
     to ISO/IEC 9899:2011 (``ISO C11'').  The fdopen() function conforms to
     IEEE Std 1003.1-1988 (``POSIX.1'').  The ``e'' mode option does not con-
     form to any standard but is also supported by glibc.  The fmemopen()
     function conforms to IEEE Std 1003.1-2008 (``POSIX.1'').  The ``b'' mode
     does not conform to any standard but is also supported by glibc.

BSD                            January 30, 2013                            BSD