command

    A pointer to a null-terminated string containing a shell command
    line.

 type

    A pointer to a null-terminated string containing an I/O mode.
    Because open files are shared, you can use a type r command as
    an input filter and a type w command as an output filter. Specify
    one of the following values for the type argument:

    o  r-the calling program can read from the standard output of the
       command by reading from the returned file stream.

    o  w-the calling program can write to the standard input of the
       command by writing to the returned file stream.