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sigsafe library for safe signal handling.

0.1.3

sigsafe is a C library for safely, reliably, and promptly handling signals delivered to specific threads without significant overhead. It includes documentation, a performance benchmark, and a correctness tester that exhaustively searches for race conditions with the ptrace(2) facility.

The meat of the library is a set of alternate system call wrappers. The shows when signals cause system calls to return immediately:

Signal arrival normal syscall + null handler normal syscall + flag handler normal syscall + longjmp() handler sigsafe syscall
Well before entering kernel No (signal lost) Yes Yes Yes
Right before entering kernel No (signal lost) No (signal noted) Yes Yes
While in kernel Yes Yes Yes Yes
Right after exiting kernel No (normal return) No (normal return) Yes (clobbers syscall return) No (normal return)

All sigsafe system calls:

It is not possible to create these guarantees with the standard system call wrappers. And they are extremely useful guarantees - you can handle many signals safely without them, but often with a performance penalty or with great difficulty.

Performance

sigsafe's goal is to allow every system call to have correct behavior when signals arrive, without compromising speed when signals do not arrive. As most system calls should not be interrupted by a signal, this is a necessary and sufficient condition for saying a signal handling method has good performance.

Another common correct way of handling signals is to set up a pipe for signal handling and write to it in the signal handler. If you're already polling for multiple IO sources, this works well. However, if you're using blocking IO, you have to change to non-blocking and precede all read() and write() with a select(). Thus, the system call overhead is doubled in the most common case. For this reason, sigsafe is often superior to this method.

Implementation

sigsafe is implemented through a set of system call wrappers implemented in assembly for each platform. The system call wrappers retrieve a "signal received" flag from memory and return EINTR if it is true shortly before entering the kernel. If a signal is received after this value is retrieved, sigsafe's signal handler manually adjusts the instruction pointer to force EINTR return.

It sounds like a horrible kludge (and maybe it is), but it works reliably and performs well. But don't take my word for it - verify it yourself with the included race condition checker and benchmarks.

Additional information

Availability

sigsafe is available for the following platforms:

I still intend to port it to:

At which point I will be out of machines to port it to. If you want a platform not listed, you'll have to give me access to such a machine or port it yourself.


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