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fdopen

FOPEN(3)                   Linux Programmer's Manual                  FOPEN(3)



NAME
       fopen, fdopen, freopen - stream open functions

SYNOPSIS
       #include <stdio.h>

       FILE *fopen(const char *path, const char *mode);
       FILE *fdopen(int fildes, const char *mode);
       FILE *freopen(const char *path, const char *mode, FILE *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  follow-
       ing sequences (Additional characters may follow these sequences.):

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

       r+     Open for reading and writing.  The stream is positioned  at  the
              beginning of the file.

       w      Truncate  file  to  zero length or create text file for writing.
              The stream is positioned at the beginning of the file.

       w+     Open for reading and writing.  The file is created  if  it  does
              not  exist, otherwise it is truncated.  The stream is positioned
              at the beginning of the file.

       a      Open for appending (writing at end of file).  The file  is  cre-
              ated  if it does not exist.  The stream is positioned at the end
              of the file.

       a+     Open for reading and appending (writing at end  of  file).   The
              file  is created if it does not exist.  The stream is positioned
              at the end of the file.

       The mode string can also include the letter  ``b''  either  as  a  last
       character  or  as a character between the characters in any of the two-
       character strings described above.  This is strictly for  compatibility
       with  ANSI  X3.159-1989  (``ANSI  C'')  and has no effect; the ``b'' is
       ignored on all POSIX conforming systems, including Linux.  (Other  sys-
       tems  may treat text files and binary files differently, and adding the
       ``b'' may be a good idea if you do I/O to a binary file and expect that
       your program may be ported to non-Unix environments.)

       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.
       Note that ANSI C requires that a file  positioning  function  intervene
       between  output and input, unless an input operation encounters end-of-
       file.  (If this condition is not met, then a read is allowed to  return
       the result of writes other than the most recent.)  Therefore it is good
       practice (and indeed sometimes necessary under Linux) to put  an  fseek
       or  fgetpos  operation  between  write  and  read  operations on such a
       stream.  This operation may be an apparent no-op (as in fseek(...,  0L,
       SEEK_CUR) called for its synchronizing side effect.

       Opening a file in append mode (a as the first character of mode) causes
       all subsequent write operations to this stream to occur at end-of-file,
       as if preceded by an
              fseek(stream,0,SEEK_END);
       call.

       The fdopen function associates a stream with the existing file descrip-
       tor, fildes.  The mode of the stream (one of the values "r", "r+", "w",
       "w+",  "a", "a+") must be compatible with the mode of the file descrip-
       tor.  The file position indicator of the new  stream  is  set  to  that
       belonging  to  fildes,  and  the  error  and end-of-file indicators are
       cleared.  Modes "w" or "w+" do not cause truncation of the  file.   The
       file  descriptor is not dup'ed, and will be closed when the stream cre-
       ated by fdopen is closed.  The result of applying fdopen  to  a  shared
       memory object is undefined.

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

RETURN VALUE
       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 provided 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).

CONFORMING TO
       The fopen and freopen functions conform  to  ANSI  X3.159-1989  (``ANSI
       C'').    The   fdopen   function   conforms   to   IEEE  Std1003.1-1988
       (``POSIX.1'').

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



BSD MANPAGE                       2002-01-03                          FOPEN(3)