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FOPEN(3S)							     FOPEN(3S)

NAME    [Toc]    [Back]

     fopen, freopen, fdopen - open a stream

SYNOPSIS    [Toc]    [Back]

     #include <stdio.h>

     FILE *fopen (const	char *filename,	const char *type);

     FILE *freopen (const char *filename, const	char *type, FILE *stream);

     FILE *fdopen (int fildes, const char *type);

DESCRIPTION    [Toc]    [Back]

     fopen opens the file named	by filename and	associates a stream with it.
     fopen returns a pointer to	the FILE structure associated with the stream.

     filename points to	a character string that	contains the name of the file
     to	be opened.

     type is a character string.  The initial portion of type must consist of
     one of the	following character sequences:

	  r or rb	 open for reading

	  w or wb	 truncate or create for	writing

	  a or ab	 append: open for writing at end of file or create for

	  r+,r+b or rb+	 open for update (reading and writing)

	  w+,w+b or wb+	 truncate or create for	update

	  a+,a+b or ab+	 append: open for update at end-of-file	or create for

     As	this implementation does not distinguish between binary	and text
     files, the	character b in the string type,	(which is used to indicate
     that the file being opened	is a binary file) is inconsequential.

     Opening a file for	reading	(when r	is the first character of type)	will
     fail if the file does not exist or	is unreadable.

     When a file is opened for update (when + appears as the second (or	third,
     in	the case of fopen or freopen) character	of type) both input and	output
     may be done on the	resulting stream.  However, output may not be directly
     followed by input without an intervening fseek, fsetpos, or rewind.
     Similarly,	input may not be directly followed by output without an
     intervening call to one of	these functions, unless	the input operation
     left the file positioned at end-of-file.  (See note under BUGS below.)

									Page 1

FOPEN(3S)							     FOPEN(3S)

     When a file is opened for append (i.e., when type is a, ab, a+b, ab+ or
     a+), it is	impossible to overwrite	information already in the file.
     fseek may be used to reposition the file pointer to any position in the
     file, but when output is written to the file, the current file pointer is
     disregarded.  All output is written at the	end of the file	and causes the
     file pointer to be	repositioned at	the end	of the output.	If two
     separate processes	open the same file for append, each process may	write
     freely to the file	without	fear of	destroying output being	written	by the
     other.  The output	from the two processes will be intermixed in the file
     in	the order in which it is written.

     freopen substitutes the named file	in place of the	open stream.  An
     attempt is	made to	close the original stream, using fclose(3s).  If this
     close attempt is unsuccessful, the	failure	is ignored. freopen then
     attempts to open the file indicated by filename, returning	the result.
     The character string type must have the same form as for fopen.  If the
     open is successful, the end-of-file and error indicators for stream are

     freopen is	typically used to attach the preopened streams associated with
     stdin, stdout and stderr to other files.

     fdopen associates a stream	with a file descriptor.	 File descriptors are
     obtained from open, dup, creat, or	pipe(2), which open files but do not
     initialize	a stream, which	is the object manipulated by many of the
     Section 3S	library	routines.  fdopen initializes a	stream for the open
     file descriptor fildes, and returns a pointer to the corresponding	FILE
     structure.	 The character string type has the same	form as	that for
     fopen, with the exception that the	(superfluous) binary file
     specification character b,	is not allowed.	 (This restricts the initial
     portion of	type to	one of r, w, a,	r+, w+,	or a+.)	 The type specified
     for the stream must agree with the	mode of	the open file indicated	by
     fildes (see open(2)).

SEE ALSO    [Toc]    [Back]

     creat(2), close(2), dup(2), open(2), pipe(2), write(2), fclose(3S),
     fseek(3S),	setbuf(3s), stdio(3S).

DIAGNOSTICS    [Toc]    [Back]

     fopen, fdopen, and	freopen	return a NULL pointer on failure.

     Depending on which	ABI a program is compiled in, there may	be a limit on
     the number	of open	stdio streams, or a limit on which file	descriptors
     may be associated with stdio streams.  When compiling in n32 or n64
     modes, there are no limits, however in o32	mode the functions fopen or
     fdopen may	fail and not set errno if there	are no free stdio streams. No
     more than 255 files may be	opened via fopen, and only file	descriptors 0
     through 255 are valid with	stdio streams.

     In	o32 mode file descriptors used by fdopen must be less than 255.

									Page 2

FOPEN(3S)							     FOPEN(3S)

BUGS    [Toc]    [Back]

     When operating on a file opened for update	on which the last operation
     was output, an input operation may	be performed if	there is an
     intervening call to a file	positioning function.  An input	operation
     should also be possible under these circumstances if an intervening call
     is	made to	fflush.	 If this sequence of operations	(i.e., output, fflush,
     input) is performed, however, the input operation fails with the
     misleading	error EBADF.

									PPPPaaaaggggeeee 3333
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