• how to design a signal handler, which necessitates a discussion of reentrancy and async-signal-safe functions
• alternatives to performing a normal return from a signal handler, in particular, the use of a nonlocal goto for this purpose
• handling of signals on an alternate stack
• the use of the sigaction() SA_SIGINFO flag to allow a signal handler to obtain more detailed information about the signal that caused its invocation
• how a blocking system call may be interrupted by a signal handler, and how the call can be restarted if desired

Designing Signal Handlers

Other methods of terminating a signal hadnler

Handling a signal on an alternate stack

The SA_SIGINFO flag

Interruption and restarting of syscalls

Summary

Because signals are not queued, a signal handler must sometimes be coded to deal with the possibility that multiple events of a particular type ahve occurred, even through only one signal was delivered. The issue of reentrancy affects how we update global variables and limits the set of functions that we can safely call from a signal handler.

Instead of returning, a signal handler can terminate in a variety of other ways, including calling __exit(), terminating the process by sending a signal (kill(), raise(), or abort()), or performing a nonlocal goto. Using sigsetjmp() and siglongjmp() provides a program with explicit control of the treatment of the process signal mask when a nonlocal goto is performed.

We can use sigaltstack() to define an alternative signal stack for a process. This is an area of memory that is used instead of the standard process stack when invoking a signal handler. An alternate signal stack is useful in cases where the standard stack has been exhausted by growing too large (at which point the kernel sends a SIGSEGV signal to the process).

The sigaction() SA_SIGINFO flag allows us to establish a signal handler that receives additional information about a signal. This information is supplied via a siginfo_t structure whose address is passed as an argument to signal handler.

When a signal handler interrupts a blocked syscall, the syscall fails with the error EINTR. We can take advantage of this behavior to, for example, set a timer on a blocking system call. Interrupted system calls can be manually restarted if desired. Alternatively, establishing the signal handler with the sigaction() SA_RESTART flag causes many (but not all) syscalls to be automatically restarted.