How to prevent SIGPIPEs (or handle them properly)

You generally want to ignore the SIGPIPE and handle the error directly in your code. This is because signal handlers in C have many restrictions on what they can do.

The most portable way to do this is to set the SIGPIPE handler to SIG_IGN. This will prevent any socket or pipe write from causing a SIGPIPE signal.

To ignore the SIGPIPE signal, use the following code:

signal(SIGPIPE, SIG_IGN);

If you're using the send() call, another option is to use the MSG_NOSIGNAL option, which will turn the SIGPIPE behavior off on a per call basis. Note that not all operating systems support the MSG_NOSIGNAL flag.

Lastly, you may also want to consider the SO_SIGNOPIPE socket flag that can be set with setsockopt() on some operating systems. This will prevent SIGPIPE from being caused by writes just to the sockets it is set on.

Another method is to change the socket so it never generates SIGPIPE on write(). This is more convenient in libraries, where you might not want a global signal handler for SIGPIPE.

On most BSD-based (MacOS, FreeBSD...) systems, (assuming you are using C/C++), you can do this with:

int set = 1;
setsockopt(sd, SOL_SOCKET, SO_NOSIGPIPE, (void *)&set, sizeof(int));

With this in effect, instead of the SIGPIPE signal being generated, EPIPE will be returned.

In this post I described possible solution for Solaris case when neither SO_NOSIGPIPE nor MSG_NOSIGNAL is available.

Instead, we have to temporarily suppress SIGPIPE in the current thread that executes library code. Here's how to do this: to suppress SIGPIPE we first check if it is pending. If it does, this means that it is blocked in this thread, and we have to do nothing. If the library generates additional SIGPIPE, it will be merged with the pending one, and that's a no-op. If SIGPIPE is not pending then we block it in this thread, and also check whether it was already blocked. Then we are free to execute our writes. When we are to restore SIGPIPE to its original state, we do the following: if SIGPIPE was pending originally, we do nothing. Otherwise we check if it is pending now. If it does (which means that out actions have generated one or more SIGPIPEs), then we wait for it in this thread, thus clearing its pending status (to do this we use sigtimedwait() with zero timeout; this is to avoid blocking in a scenario where malicious user sent SIGPIPE manually to a whole process: in this case we will see it pending, but other thread may handle it before we had a change to wait for it). After clearing pending status we unblock SIGPIPE in this thread, but only if it wasn't blocked originally.

Example code at https://github.com/kroki/XProbes/blob/1447f3d93b6dbf273919af15e59f35cca58fcc23/src/libxprobes.c#L156

You cannot prevent the process on the far end of a pipe from exiting, and if it exits before you've finished writing, you will get a SIGPIPE signal. If you SIG_IGN the signal, then your write will return with an error - and you need to note and react to that error. Just catching and ignoring the signal in a handler is not a good idea -- you must note that the pipe is now defunct and modify the program's behaviour so it does not write to the pipe again (because the signal will be generated again, and ignored again, and you'll try again, and the whole process could go on for a long time and waste a lot of CPU power).

Or should I just catch the SIGPIPE with a handler and ignore it?

I believe that is right on. You want to know when the other end has closed their descriptor and that's what SIGPIPE tells you.

Sam