Unix Signals

A signal is a message that can be sent to a running process.

Programs, users, or administrators can initiate signals.

For example, the proper method of telling the Internet Daemon (inetd) to re-read its configuration file is to send it a SIGHUP signal.

For example, if the current process ID (PID) of inetd is 4140, type:

kill -SIGHUP 4140

Another common use of signals is to stop a running process. To stop the inetd process completely, use this command:

kill 4140

By default, the kill command sends the SIGTERM signal. If SIGTERM fails, escalate to using the SIGKILL signal to stop the process:

kill -9 4140

Because SIGKILL cannot be handled, stopping a process with SIGKILL is generally considered a bad idea. Using SIGKILL prevents a process from cleaning up after itself and exiting gracefully.

Handling Signals

Each Unix signal has a default set of effects on a Unix program. Programmers can code their applications to respond in customized ways to most signals. These custom code pieces are called signal handlers.

A signal handler is unable to redefine two signals. SIGKILL always stops a process and SIGSTOP always moves a process from the foreground to the background. A signal handler cannot “catch” these two signals.

FreeBSD Signals

Signal Name	Signal Number	Signal Description
SIGHUP	1	Terminal line hangup
SIGINT	2	Interrupt program
SIGQUIT	3	Quit program
SIGILL	4	Illegal instruction
SIGTRAP	5	Trace trap
SIGABRT	6	Abort
SIGEMT	7	Emulate instruction executed
SIGFPE	8	Floating-point exception
SIGKILL	9	Kill program
SIGBUS	10	Bus error
SIGSEGV	11	Segmentation violation
SIGSYS	12	Bad argument to system call
SIGPIPE	13	Write on a pipe with no one to read it
SIGALRM	14	Real-time timer expired
SIGTERM	15	Software termination signal
SIGURG	16	Urgent condition on I/O channel
SIGSTOP	17	Stop signal not from terminal
SIGTSTP	18	Stop signal from terminal
SIGCONT	19	A stopped process is being continued
SIGCHLD	20	Notification to parent on child stop or exit
SIGTTIN	21	Read on terminal by background process
SIGTTOU	22	Write to terminal by background process
SIGIO	23	I/O possible on a descriptor
SIGXCPU	24	CPU time limit exceeded
SIGXFSZ	25	File-size limit exceeded
SIGVTALRM	26	Virtual timer expired
SIGPROF	27	Profiling timer expired
SIGWINCH	28	Window size changed
SIGINFO	29	Information request
SIGUSR1	30	User-defined signal 1
SIGUSR2	31	User-defined signal 2
SIGTHR	32	Thread interrupt

Solaris Signals

Signal Name	Signal Number	Signal Description
SIGHUP	1	Hangs up
SIGINT	2	Interrupts
SIGQUIT	3	Quits
SIGILL	4	Illegal instruction
SIGTRAP	5	Trace trap
SIGABRT	6	Used by abort
SIGEMT	7	EMT instruction
SIGFPE	8	Floating-point exception
SIGKILL	9	Kill (cannot be caught or ignored)
SIGBUS	10	Bus error
SIGSEGV	11	Segmentation violation
SIGSYS	12	Bad argument to system call
SIGPIPE	13	Writes on a pipe with no one to read it
SIGALRM	14	Alarm clock
SIGTERM	15	Software termination
SIGUSR1	16	User-defined signal 1
SIGUSR2	17	User-defined signal 2
SIGCHLD	18	Child status change alias (POSIX)
SIGPWR	19	Power-fail restart
SIGWINCH	20	Window size change
SIGURG	21	Urgent socket condition
SIGPOLL /SIGIO	22	Pollable event occurred or Socket I/O possible
SIGSTOP	23	Stop (cannot be caught or ignored)
SIGTSTP	24	User stop requested from TTY
SIGCONT	25	Stopped process has been continued
SIGTTIN	26	Background TTY read attempted
SIGTTOU	27	Background TTY write attempted
SIGVTALRM	28	Virtual timer expired
SIGPROF	29	Profiling timer expired
SIGXCPU	30	Exceeded CPU limit
SIGXFSZ	31	Exceeded file size limit
SIGWAITING	32	Process’ LWPs are blocked
SIGLWP	33	Special signal used by thread library
SIGFREEZE	34	Special signal used by CPR
SIGTHAW	35	Special signal used by CPR
SIGCANCEL	36	Thread cancellation signal used by libthread
SIGLOST	37	Resource lost
SIGRTMIN	38	Highest priority real-time signal
SIGRTMAX	45	Lowest priority real-time signal