Thread-Specific Data and Signal Handling in Multi-Threaded Applications
Listing 4 shows how to deal with signals in a multi-threading environment that handles threads in a POSIX compliant way.
Personally, I like the kernel-level package “LinuxThreads” that makes use of Linux 2.0's clone() system call to create new threads. At some point in the future, the clone() call may implement the CLONE_PID flag which would allow all the threads to share a process ID. Until then each thread created using “LinuxThreads” (or any other packages which chooses to use clone() to create threads) will have its own unique process ID. As such, there is no concept of sending a signal to “the process.” If one thread calls sigwait() and all other threads block signals, only those signals which are specifically sent to the sigwait()-ing thread will be processed. Depending on your application, this could mean that you have no choice other than to include an asynchronous signal handler in each of the threads.
Thread specific data is easy to use—far easier than many people's first experiences may suggest. In a way, this ease of use is a disadvantage, since very often there are more elegant solutions to a problem. But in times of need, thread specific data is your friend.
On the other hand, signal handling in anger can be a little hairy. Anyone who thinks otherwise has overlooked something—either that or they're far too clever for their own good. Make life easier for yourself by consigning all the handling of asynchronous signals to one thread that sits on sigwait().
Martin McCarthy discovered multi-threaded programming while writing the server for a high-speed, multi-user, distributed, virtual-reality system. Of course, he only took that job so that he could squeeze as many buzzwords into his job description as possible. He can be reached at email@example.com.