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fopen(3s)							     fopen(3s)

Name
       fopen, freopen, fdopen - open a stream

Syntax
       #include <stdio.h>

       FILE ∗fopen (filename, type)
       char ∗filename, ∗type;

       FILE ∗freopen (filename, type, stream)
       char ∗filename, ∗type;
       FILE ∗stream;

       FILE ∗fdopen (fildes, type)
       int fildes;
       char ∗type;

Description
       The  routine  opens  the file named by filename and associates a stream
       with it.	 The routine returns a pointer to the FILE  structure  associ‐
       ated with the stream.

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

       The type is a character string having one of the following values:

	  "r"	    Open for reading

	  "w"	    Truncate or create for writing

	  "a"	    Append; open for writing at end of	file,  or  create  for
		    writing

	  "A"	    Append with no overwrite; open for writing at end-of-file,
		    or create for writing

	  "r+"	    Open for reading and writing

	  "w+"	    Truncate or create for reading and writing

	  "a+"	    Append; open or create for reading and writing at  end-of-
		    file

	  "A+"	    Append  with  no  overwrite,  open or create for update at
		    end-of-file

       The letter "b" can also follow r, w, or a. In some  C  implementations,
       the  "b" is needed to indicate a binary file, however, it is not needed
       in ULTRIX.  If "+" is used, the "b" may occur on	 either	 side,	as  in
       "rb+" or "w+b".

       The  routine  substitutes  the  named file in place of the open stream.
       The original stream is closed, regardless of  whether  the  open	 ulti‐
       mately  succeeds.   The routine returns a pointer to the FILE structure
       associated with stream.

       The routine is typically used to attach the preopened  streams  associ‐
       ated with stdin, stdout and stderr to other files.

       The  routine associates a stream with a file descriptor.	 File descrip‐
       tors are obtained from or which open files but do not  return  pointers
       to  a  FILE  structure stream.  Streams are necessary input for many of
       the Section 3s library routines.	 The type of stream  must  agree  with
       the mode of the open file.

       When  a file is opened for update, both input and output may be done on
       the resulting stream.  However, output may not be directly followed  by
       input  without an intervening or and input may not be directly followed
       by output without an intervening or an input operation which encounters
       end-of-file.

       When  a	file is opened for append with no overwrite (that is when type
       is "A" or "A+"), it is impossible to overwrite information  already  in
       the  file.   The	 routine 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.

Return Values
       The and routines return a NULL pointer on failure.

Environment
   SYSTEM_V
       When  your  program  is compiled using the System V environment, append
       with no overwrite is specified by using the "a" or  "a+"	 type  string,
       and the "A" and "A+" type strings are not allowed.

   POSIX
       In  the	POSIX  environment,  the "a" and "a+" strings, and the "A" and
       "A+" strings specify append with no overwrite.

See Also
       creat(2), dup(2), open(2), pipe(2), fclose(3s), fseek(3s).

								     fopen(3s)
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