fopen(3S)                                               fopen(3S)



NAME
     fopen, freopen, fdopen - open a stream

SYNOPSIS
     #include <stdio.h>

     FILE *fopen (file-name, type)
     char *file-name, *type;

     FILE *freopen (file-name, type, stream)
     char *file-name, *type;
     FILE *stream;

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

DESCRIPTION
     fopen opens the file named by file-name and associates a
     stream with it.  fopen returns a pointer to the FILE struc-
     ture associated with the stream.

     File-name 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),



Page 1                        CX/UX Programmer's Reference Manual

fopen(3S)                                               fopen(3S)



     which open files but do not return pointers to a FILE struc-
     ture 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.

     In the att universe, 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.  Fseek may be used to repo-
     sition 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.

     In the ucb universe, when a file is opened for append, it is
     possible to overwrite information already in the file.  In
     this case, fopen merely sets the file pointer to the end of
     the file.  Fseek may be used to reposition the file pointer
     to any position in the file, and output is always written to
     the file at the current position of the file pointer.

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

DIAGNOSTICS
     Fopen, fdopen, and freopen return a NULL pointer on failure.
     When given an invalid mode string, fopen sets errno to EIN-
     VAL.
















Page 2                        CX/UX Programmer's Reference Manual