fopen(3S)                                               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
          fopen opens the file named by filename and associates a
          stream with it.  fopen returns a pointer to the FILE struc-
          ture 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 wheth-
          er 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 by format-
          ting a file structure from the file descriptor.  Thus, fdo-



     April, 1990                                                     1

     fopen(3S)                                               fopen(3S)



          pen can be used to access the file descriptors returned by
          open(2), dup(2), creat(2), or pipe(2).  (These calls open
          files but do not return pointers to a FILE structure.)  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.  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 out-
          put 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 pro-
          cess 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 VALUE
          fopen and freopen return a NULL pointer on failure.

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

























     2                                                     April, 1990