Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help
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 (possibly followed	by additional characters, as described
       below):

       r      Open text	file for reading.  The stream is positioned at the be-
	      ginning 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 al-
	      ways 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 the call:

	   fseek(stream, 0, SEEK_END);

       The  fdopen()  function	associates a stream with the existing file de-
       scriptor, 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 de-
       scriptor.  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  de-
       scriptor	 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 errno is set to indicate
       the error.

ERRORS
       EINVAL The mode provided	to fopen(), fdopen(),  or  freopen()  was  in-
	      valid.

       The fopen(), fdopen() and freopen() functions may also fail and set er-
       rno 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  er-
       rors 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 op-
	      erations,	thread cancellation points.  This flag is ignored  for
	      fdopen().

       e (since	glibc 2.7)
	      Open the file with the O_CLOEXEC flag.  See open(2) for more in-
	      formation.  This flag is ignored for fdopen().

       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 at-
	      tempted only 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().

       In addition to the above	characters, fopen() and	freopen() support  the
       following syntax	in mode:

	   ,ccs=string

       The  given string is taken as the name of a coded character set and the
       stream is marked	as  wide-oriented.   Thereafter,  internal  conversion
       functions  convert  I/O	to  and	from the character set string.	If the
       ,ccs=string syntax is not specified, then the wide-orientation  of  the
       stream is determined by the first file operation.  If that operation is
       a wide-character	operation, the stream  is  marked  wide-oriented,  and
       functions to convert to the coded character set are loaded.

BUGS
       When  parsing for individual flag characters in mode (i.e., the charac-
       ters preceding the "ccs"	specification),	the  glibc  implementation  of
       fopen()	and freopen() limits the number	of characters examined in mode
       to 7 (or, in glibc versions before 2.14,	to 6, which was	not enough  to
       include possible	specifications such as "rb+cmxe").  The	current	imple-
       mentation of fdopen() parses at most 5 characters in mode.

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

COLOPHON
       This page is part of release 3.74 of the	Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.

GNU				  2012-04-22			      FOPEN(3)

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

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=fopen&sektion=3&manpath=Debian+8.1.0>

home | help