DragonFly On-Line Manual Pages

FOPEN(3)              DragonFly Library Functions Manual              FOPEN(3)

NAME
     fopen, fdopen, freopen -- 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 * restrict path, const char * restrict mode,
         FILE * restrict stream);

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
     sequences (Additional characters may follow these sequences.):

     ``r''   Open file for reading.  The stream is positioned at the beginning
             of the file.

     ``w''   Open file for writing.  Creates the file if it does not already
             exist, truncates the file if it does.

     ``a''   Open file for append.  Creates the file if it does not already
             exist.  Seeks to 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 operation.

     Additional flags may modify the primary mode as follows:

     ``+''   The file will be opened for both reading and writing.  For
             example 'r+' or 'w+'.

     ``x''   The open will fail if the file already exists (C11).  For example
             'wx'.

     ``e''   Close-on-exec.  The underlying descriptor will automatically be
             closed on any exec() (glibc compatibility).

     ``b''   Binary mode.  On UNIX systems file reading and writing is always
             in binary mode so this flag has no effect.

     The mode string can also include the letter ``b'' either as a third
     character or as a character between the characters in any of the two-
     character strings described above.  This is strictly for compatibility
     with ISO/IEC 9899:2011 (``ISO C11'') and has no effect; the ``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,
     and do not require an intermediate seek as in previous versions of stdio.
     This is not portable to other systems, however; ANSI C requires that a
     file positioning function intervene between output and input, unless an
     input operation encounters end-of-file.

     The fdopen() function associates a stream with the existing file
     descriptor, fildes.  The mode of the stream must be compatible with the
     mode of the file descriptor.  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:

           o   Streams originally opened with mode ``r'' can only be reopened
               with that same mode.

           o   Streams originally opened with mode ``a'' can be reopened with
               the same mode, or mode ``w''.

           o   Streams originally opened with mode ``w'' can be reopened with
               the same mode, or mode ``a''.

           o   Streams originally opened with mode ``r+'', ``w+'', or ``a+''
               can be reopened with any mode.

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

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

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

     The fopen(), fdopen() and freopen() 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).

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

STANDARDS
     The fopen() and freopen() functions conform to ISO/IEC 9899:2011
     (``ISO C11'').  The fdopen() function conforms to IEEE Std 1003.1-1988
     (``POSIX.1'').

DragonFly 5.5                  February 22, 2018                 DragonFly 5.5