Linux Signals Reference

A common concept of Linux systems programming is the idea of sending and receiving signals. You can think of a signal as a software interrupt that’s used to announce some sort of event, asynchronously, to a process.

We do not define our own signals. Instead, they are defined by the kernel. Each signal has a number, but they also have a name that begins with SIG. We typically refer to a single by its name rather than its number.

As an example, signal 11 is called SIGSEGV and is the signal that is sent to a process when it experiences a segmentation fault.

The Standard Linux Signals

The first 31 signals are standardized on Linux. Some of these are from the POSIX standard, but not all of them.

#	Signal Name	Default Action	Description	From POSIX?
1	SIGHUP	Terminate	Historically, indicates that a terminal connection has been "hung up". May be used differently on modern systems	Yes
2	SIGINT	Terminate	Indicates an interrupt from the keyboard, such as issuing `Ctrl-C`.	Yes
3	SIGQUIT	Dump	Indicates that the receiving process should be terminated and a core dump generated (if enabled). Triggered by `Ctrl-\` on most terminals.	Yes
4	SIGILL	Dump	Indicates an illegal instruction.	Yes
5	SIGTRAP	Dump	Indicates that a debugging breakpoint was hit.	No
6	SIGABRT	Dump	Indicates an abnormal termination.	Yes
6	SIGIOT	Dump	Functionally equivalent to SIGABRT.	No
7	SIGBUS	Dump	Indicates a bus error.	No
8	SIGFPE	Dump	Indicates a floating-point exception.	Yes
9	SIGKILL	Terminate	Indicates that the process has been force terminated.	Yes
10	SIGUSR1	Terminate	No pre-defined meaning. Available for the process to use.	Yes
11	SIGSEGV	Dump	Indicates an invalid memory reference (segfault).	Yes
12	SIGUSR2	Terminate	Similar to SIGUSR1, has no pre-defined meaning. Available for the process to use.	Yes
13	SIGPIPE	Terminate	Indicates that the process attempted to write to a pipe or socket that has been closed by the reader.	Yes
14	SIGALRM	Terminate	Sent by the kernel when a timer set by the `alarm()` syscall expires. Used to implement timeouts and periodic actions in programs.	Yes
15	SIGTERM	Terminate	Indicates process termination.	Yes
16	SIGSTKFLT	Terminate	Indicates a stack error from a coprocessor.	No
17	SIGCHLD	Ignore	Indicates that a child process has stopped, been terminated, or continued after being stopped.	Yes
18	SIGCONT	Continue	Resume execution, if stopped.	Yes
19	SIGSTOP	Stop	Indicates that a process should stop execution. Triggered by `Ctrl-Z` on most terminals.	Yes
20	SIGTSTP	Stop	Stop process issued from a TTY.	Yes
21	SIGTTIN	Stop	Indicates that a background process requires input.	Yes
22	SIGTTOU	Stop	Indicates that a background process requires output.	Yes
23	SIGURG	Ignore	Indicates that there is an urgent condition on a socket.	No
24	SIGXCPU	Dump	Indicates that a CPU time limit has been exceeded.	No
25	SIGXFSZ	Dump	Sent when a process attempts to write beyond the maximum file size limit.	No
26	SIGVTALRM	Terminate	Indicates that a process has consumed a certain amount of CPU time. Unlike SIGALRM (which uses the real/wall-clock time), SIGVTALRM tracks the time that the process has been executing on the CPU. It only counts the time when the process is running.	No
27	SIGPROG	Terminate	Sent when the profiling timer expires.	No
28	SIGWINCH	Ignore	Sent when the process's controlling terminal window changes size.	No
29	SIGIO	Terminate	Indicates that I/O is now possible.	No
29	SIGPOLL	Terminate	Functionally equivalent to SIGIO. SIGPOLL and SIGIO are actually the same signal.	No
30	SIGPWR	Terminate	Indicates a power supply failure.	No
31	SIGSYS	Dump	Indicates a bad system call.	No

What Happens When a Signal is Sent?

When the kernel sends a signal to a process, it must be handled. There are three possible ways to do this:

Ignore It. Many of the signals can be safely ignored (but not all of them). For example, you cannot ignore a SIGKILL signal.
Catch and Handle it. The process can listen for a signal and handle it itself. The program may use this to terminate things gracefully or even handle it without terminating the program.
Use the Default Action. Every signal has a default action that the process can use.