FOPEN(3S)            INTERACTIVE UNIX System            FOPEN(3S)



          NAME
               fopen, freopen, fdopen - open a stream

          SYNOPSIS
               #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 fopen function opens the file named by filename and
               associates a stream with it.  The fopen function 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 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

                    "r+"      open for update (reading and writing)

                    "w+"      truncate or create for update

                    "a+"      append; open or create for update at end-of-
                              file

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

               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 return pointers to a FILE


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          FOPEN(3S)            INTERACTIVE UNIX System            FOPEN(3S)



               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 fseek or
               rewind, and input may not be directly followed by output
               without an intervening fseek, rewind, or an input operation
               which encounters end-of-file.

               When a file is opened for append (i.e., when type is "a" or
               "a+"), it is impossible to overwrite information already in
               the file.  The fseek function 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 disre-
               garded.  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.

          SEE ALSO
               creat(2), dup(2), open(2), pipe(2), fclose(3S), fseek(3S),
               stdio(3S).

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

























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