![]() This is quite a list and Unix developers have to keep these problems in mind when dealing with signals. See ppoll(2), pselect(2) and epoll_pwait(2). For this reason, you have system calls that allow you to either wait for a file descriptor to be ready or for a signal to occur in one, atomic step. ![]() It is easy to “lose” or miss signals and thus get stuck waiting for one if a signal occurs between the time you setup a signal handler and call pause(2) to wait for a signal to occur.You can read more details on this in the signal(7) man page. Unfortunately though, there are many system calls (like epoll_wait(2), epoll_pwait(2), poll(2), ppoll(2), select(2), pselect(2), revc(2), send(2), and nanosleep(2)) which are never restarted in spite of SA_RESTART being specified. This leads to the operating system automatically restarting system calls which are interrupted by signals. It is however, possible to automatically restart many system calls by specifying the SA_RESTART flag when setting up a signal handler with the sigaction(2)function (We discuss this in detail later). When a signal occurs while a program is blocked on a system call, the system call returns with an error EINTR.If the signal handler calls the same function, trouble ensues. As an example, non-re-entrant functions should not be called from signal handlers since they could have been called from your main code, got interrupted in the middle of their being executed. The signal-safety(7) man page describes details of this and provides a list of functions that you should not be calling from a signal handler. The functions you call have to be async-signal-safe. ![]() You’ve got to watch which functions you call from within your signal handler.This is because signal handlers can be called anytime asynchronously and variables that are updated with multiple CPU instructions can be in an inconsistent state when a signal handler is invoked. You might also have to worry about using a type that is updated atomically when accessed from both regular code and also from your signal handlers.You might have to remember to use volatile class variables when dealing with signal handlers since they are executed asynchronously.Notification about the termination of a child used to be in this category, until in Linux kernel version 5.2, we got support for pidfd. ![]() Terminal size change notification is a good example of this ( SIGWINCH). For decent sized programs, you can’t do away with signals.Some pains of signalsīefore we move on and look at alternative, Linux-specific solutions to alleviate some of the pains generally associated with Unix signals, let’s look at all the potential problems programmers have to deal with when working with signals. You might have to carefully model your program to compensate for the asynchronous nature of signals and this is something experienced Unix programmers always take into account. They can occur while you’re blocked in a system call (causing them to fail) or while executing user space instructions (potentially causing race conditions). Signals allow the operating system to tell the process about the occurrence of various events like the execution of illegal CPU instructions, a user typing and thus causing a hardware interrupt at the keyboard ( Ctrl+C) or the termination of a child process, which causes SIGCHLD to be delivered to the parent.īut here’s the thing: signals can be very disruptive to the general flow of your program, since they are asynchronous in nature. In the life-cycle of a process, fortune and misfortune are present in good measure. Anyone introduced to Unix programming gets to marvel at the clever construct of signals.
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