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 following sequences (Additional characters may  follow
       these sequences.):

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

       r+     Open for reading and writing.  The stream is  posi­
              tioned 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 writing.  The file is created if  it  does
              not  exist.  The stream is positioned at the end of
              the file.

       a+     Open for reading and writing.  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 charac­
       ters in any of the two-character strings described  above.
       This  is  strictly for compatibility with ANSI C3.159-1989
       (``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 posi­
       tioning  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 neces­
       sary 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.

       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 com­
       patible 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 indica­
       tors  are cleared.  Modes "w" or "w+" do not cause trunca­
       tion 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 func­
       tion 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 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).

SEE ALSO

       open(2), fclose(3)

CONFORMING TO

       The  fopen  and  freopen   functions   conform   to   ANSI
       C3.159-1989 (``ANSI C'').  The fdopen function conforms to
       IEEE Std1003.1-1988 (``POSIX.1'').