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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	fd, const char *mode);

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

   Feature Test	Macro Requirements for glibc (see feature_test_macros(7)):

       fdopen(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE

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 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 char-
       acter or	as a character between the characters in any of	the  two-char-
       acter strings described above.  This is strictly	for compatibility with
       C89 and has no effect; the 'b' is ignored on all	POSIX conforming  sys-
       tems,  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.)

       See NOTES below for details of glibc extensions for mode.

       Any created files will have mode	S_IRUSR	| S_IWUSR | S_IRGRP |  S_IWGRP
       |  S_IROTH  |  S_IWOTH (0666), as modified by the process's 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(3)	or fgetpos(3) 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,  fd.	  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 fd, 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 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 C89.  The	fdopen() func-
       tion conforms to	POSIX.1-1990.

NOTES
   Glibc Notes
       The GNU C library allows	the following extensions for the string	speci-
       fied in mode:

       c (since	glibc 2.3.3)
	      Do not make the open operation, or  subsequent  read  and	 write
	      operations, thread cancellation points.

       e (since	glibc 2.7)
	      Open  the	 file  with  the O_CLOEXEC flag.  See open(2) for more
	      information.

       m (since	glibc 2.3)
	      Attempt to access	the file using mmap(2),	rather than I/O	system
	      calls  (read(2),	write(2)).   Currently,	use of mmap(2) is only
	      attempted	for a file opened for reading.

       x      Open the file exclusively	(like the O_EXCL flag of open(2)).  If
	      the  file	 already exists, fopen() fails,	and sets errno to EEX-
	      IST.  This flag is ignored for fdopen().

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

COLOPHON
       This page is part of release 3.25 of the	Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found	at http://www.kernel.org/doc/man-pages/.

GNU				  2009-02-23			      FOPEN(3)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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