FOPEN(3S)                            SysV                            FOPEN(3S)



NAME
     fopen, freopen, fdopen - open a stream

SYNOPSIS
     #include <stdio.h>

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

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

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

DESCRIPTION
     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 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.

     Each type specified to fopen and freopen may be followed by a "b",
     indicating a binary file.  The "b" may precede or follow the "+", if
     present.  Domain/OS SysV, however, ignores the "b" flag.

     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 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 (that is, when type is "a" 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.

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.