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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
       following   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
              created 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 initial file position
              for  reading  is  at  the  beginning  of the file, but output is
              always appended to 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
       systems  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
       descriptor, 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 descriptor.  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
       created 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)