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Document number: N3618

Date: 2013-03-17

What Can Signal Handlers Do? (CWG 1441)

by Hans-J. Boehm

Excerpt:

CWG Issue 1441 points out that in the process of relaxing the restrictions on asynchronous signal handlers to allow use of atomics, we inadvertently made it impossible to use even local variables of non-volatile, non-atomic type. As a result of an initial discussion within CWG, Jens Maurer generated a proposed resolution, which addresses that specific issue.

Later discussion in SG1, both in Portland and during the February 2013 SG1 teleconference, raised a number of additional issues. Both Jens' solution and all prior versions of the standard still give undefined behavior to code involving signal handlers which we believe should clearly be legal. For example, a signal handler should be allowed to access "read-only" data that has not been modified since the signal handler was installed. Our goal is to correct such oversights, and allow some realistic signal handlers to be portable, while preserving a significant amount of implementation freedom with respect to what is allowable in a signal handler. In particular, we do not want to reinvent Posix' notion of async-signal-safe functions here.

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We give several proposed changes and summarize the reasoning behind the change as well as some of the past discussion: ...