C.3 Signals

Signals are a simple UNIX mechanism for controlling processes. A signal is a 5-bit message to a process that requires immediate attention. Each signal has associated with it a default action; for some signals, you can change this default action. Signals are generated by exceptions, which include:

• Attempts to use illegal instructions

• Certain kinds of mathematical operations

• Window resize events

• Predefined alarms

• The user pressing an interrupt key on a terminal

• Another program using the kill() l or killpg() system calls

• A program running in the background attempting to read from or write to its controlling terminal

• A child process calling exit or terminating abnormally

The system default may be to ignore the signal, to terminate the process receiving the signal (and, optionally, generate a core file), or to suspend the process until it receives a continuation signal. Some signals can be caught - that is, a program can specify a particular function that should be run when the signal is received. By design, UNIX supports exactly 31 signals. They are listed in the files /usr/include/signal.h and /usr/include/sys/signal.h. Table 27.4 contains a summary.

Table C.6: UNIX Signals

Signal Name	Number[7]	Key	Meaning[8]
SIGHUP	1		Hangup (sent to a process when a modem or network connection is lost)
SIGINT	2		Interrupt (generated by CTRL-C (Berkeley UNIX) or RUBOUT (System V).
SIGQUIT	3	*	Quit
SIGILL	4	*	Illegal instruction
SIGTRAP	5	*	Trace trap
SIGIOT	6	*	I/O trap instruction; used on PDP-11 UNIX
SIGEMT	7	*	Emulator trap instruction; used on some computers without floating-point hardware support
SIGFPE	8	*	Floating-point exception
SIGKILL	9	!	Kill
SIGBUS	10	*	Bus error (invalid memory reference, such as an attempt to read a full word on a half-word boundary)
SIGSEGV	11	*	Segmentation violation (invalid memory reference, such as an attempt to read outside a process's memory map)
SIGSYS	12	*	Bad argument to a system call
SIGPIPE	13		Write on a pipe that has no process to read it
SIGALRM	14		Timer alarm
SIGTERM	15		Software termination signal (default kill signal)
SIGURG	16	@	Urgent condition present
SIGSTOP	17	+!	Stop process
SIGTSTP	18	+	Stop signal generated by keyboard
SIGCONT	19	@	Continue after stop
SIGCHLD	20	@	Child process state has changed
SIGTTIN	21	+	Read attempted from control terminal while process is in background
SIGTTOU	22	+	Write attempted to control terminal while process is in background
SIGIO	23	@	Input/output event
SIGXCPU	24		CPU time limit exceeded
SIGXFSZ	25		File size limit exceeded
SIGVTALRM	26		Virtual time alarm
SIGPROF	27		Profiling timer alarm
SIGWINCH	28	@	tty window has changed size
SIGLOST	29		Resource lost
SIGUSR1	30		User-defined signal #1
SIGUSR2	31		User-defined signal #2

[7] The signal number varies on some systems.

[8] The default action for most signals is to terminate.

Key:

*	If signal is not caught or ignored, generates a core image dump.
@	Signal is ignored by default.
+	Signal causes process to suspend.
!	Signal cannot be caught or ignored.

Signals are normally used between processes for process control. They are also used within a process to indicate exceptional conditions that should be handled immediately (for example, floating-point overflows).