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Linux 2.6 - man page for freopen (linux section 3)

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 asso-
       ciates a stream with it.

       The argument mode points to a string beginning with one of the following sequences (possi-
       bly followed by additional characters, as described below):

       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  posi-
	      tioned 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 charac-
       ter between the characters in any of the two-character strings described above.	 This  is
       strictly  for  compatibility  with  C89 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.)

       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 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 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  as-
       sociates  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.   Other-
       wise, 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() function conforms to
       POSIX.1-1990.

NOTES
   Glibc notes
       The GNU C library allows the following extensions for the string specified in mode:

       c (since glibc 2.3.3)
	      Do not make the open operation, or subsequent read  and  write  operations,  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 information.  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 attempted only for a file opened for read-
	      ing.

       x      Open the file exclusively (like the O_EXCL flag of open(2)).  If the  file  already
	      exists,  fopen()	fails,	and  sets  errno  to  EEXIST.	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-orienta-
       tion 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  characters  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 implementa-
       tion 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 3.55 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					    2012-04-22					 FOPEN(3)


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