How to do a pipe & fork?

[a.v.reed]

In article <4629 at mtgzz.att.com>, I wrote:
< In article <Nov.2.14.51.36.1988.8260 at zztop.rutgers.edu>, pratt at zztop.rutgers.edu (Lorien Y. Pratt) writes:
< > I have two processes that I want to communicate.  I want the parent to
< > be able to fprintf to the child's stdin and to fscanf from the child's
< > stdout.  I know that I need to fork and then in the child do an execlp,
< > but I can't figure out how to set up the file descriptors so I can do
< > what I want. The Sun IPC guide does not help me here.  I know that I
< > need to do a dup and a fork and a pipe, but I'm not sure in what order
< > and with what parameters.
< 
< The most legible, general, portable (at least to other Sun and AT&T
< derived systems), easy to maintain and modify, and only slightly less
< then optimally efficient solution is to start up your process from a
< shell script after creating a named pipe, and passing its name as an
< argument to the process:
< 
< if /etc/mknod /usr/tmp/pipe$$ p
< then exec $LIB/yourprocess -p /usr/tmp/pipe$$
< fi
< echo "yourprocess: can't make named pipe /usr/tmp/pipe$$; exiting"
< exit 5 # EIO
< 
< In the executable $LIB/yourprocess, read the pipe name with getopt(),
< fopen it for "r+", then fork and exec, write in the parent and read in
< the child. When the child is finished fclose() the pipe fd, and,
< in the parent, fclose() and unlink the named pipe when an attempt to
< write sets errno to EPIPE.
< 				Adam Reed (avr at mtgzz.ATT.COM)

To clarify: popen(), about which you can read in section 3S of the
manual, or just with $ man popen, will do what Lorien Y. Pratt wants to
do, perfectly well, as long as only ONE pipe, in ONE direction, is all
that is needed. The solution I outlined above has the advantage of
generality, in that it can be readily extended to bidirectional
communication (one named pipe in each direction) and to an arbitrarily
complex network of communicating processes with arbitrarily many pipes.

				Adam Reed (avr at mtgzz.ATT.COM)