5.19 Summary
The first version of our echo client/server
totaled about 150 lines (including the readline and
writen functions), yet provided lots of details to
examine. The first problem we encountered was zombie children and
we caught the SIGCHLD signal to handle this. Our signal
handler then called waitpid and we demonstrated that we
must call this function instead of the older wait
function, since Unix signals are not queued. This led us into some
of the details of POSIX signal handling (additional information on
this topic is provided in Chapter 10 of APUE).
The next problem we encountered was the client
not being notified when the server process terminated. We saw that
our client's TCP was notified, but we did not receive that
notification since we were blocked, waiting for user input. We will
use the select or poll function in Chapter 6 to handle
this scenario, by waiting for any one of multiple descriptors to be
ready, instead of blocking on a single descriptor.
We also discovered that if the server host
crashes, we do not detect this until the client sends data to the
server. Some applications must be made aware of this fact sooner;
in Section 7.5, we
will look at the SO_KEEPALIVE socket option.
Our simple example exchanged lines of text,
which was okay since the server never looked at the lines it
echoed. Sending numeric data between the client and server can lead
to a new set of problems, as shown.
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