Signals are a simple form of ``interruption'' in the Unix-like OS world,
and are an ancient part of Unix.
A process can set a ``signal'' on another process (say using
kill(1) or kill(2)), and that other process would receive and
handle the signal asynchronously.
For a process to have permission to send a signal to some other process,
the sending process must either have root privileges, or
the real or effective user ID of the sending process
must equal the real or saved set-user-ID of the receiving process.
Although signals are an ancient part of Unix, they've had different
semantics in different implementations.
Basically, they involve questions such as ``what happens when a signal
occurs while handling another signal''?
The older Linux libc 5 used a different set of semantics for some signal
operations than the newer GNU libc libraries.
For more information, see the glibc FAQ (on some systems a local
copy is available at /usr/doc/glibc-*/FAQ).
For new programs, just use the POSIX signal system
(which in turn was based on BSD work); this set is widely supported
and doesn't have the problems that some of the older signal systems did.
The POSIX signal system is based on using the sigset_t datatype, which can
be manipulated through a set of operations: sigemptyset(),
sigfillset(), sigaddset(), sigdelset(), and sigismember().
You can read about these in sigsetops(3).
Then use sigaction(2), sigaction(2), sigprocmask(2),
sigpending(2), and sigsuspend(2) to set up an manipulate signal handling
(see their man pages for more information).
In general, make any signal handlers very short and simple, and
look carefully for race conditions.
Signals, since they are by nature asynchronous,
can easily cause race conditions.
A common convention exists for servers: if you receive SIGHUP, you should
close any log files, reopen and reread configuration files, and then
re-open the log files.
This supports reconfiguration without halting the server and
log rotation without data loss.
If you are writing a server where this convention makes sense,
please support it.
