SIGNAL(3C)

NAME
signal - simplified software signal facilities

SYNOPSIS
#include <signal.h>

int (*signal(sig, func))()
int (*func)();

DESCRIPTION
Signal is a simplified interface to the more general sigvec(2) facility.

A signal is generated by some abnormal event, initiated by a user at a terminal (quit, interrupt, stop), by a program error (bus error, etc.), by request of another program (kill), or when a process is stopped because it wishes to access its control terminal while in the background (see tty(4)). Signals are optionally generated when a process resumes after being stopped, when the status of child processes changes, or when input is ready at the control terminal. Most signals cause termination of the receiving process if no action is taken; some signals instead cause the process receiving them to be stopped, or are simply discarded if the process has not requested otherwise. Except for the SIGKILL and SIGSTOP signals, the signal call allows signals either to be ignored or to cause an interrupt to a specified location. The following is a list of all signals with names as in the include file <signal.h>:

SIGHUP	1	hangup
SIGINT	2	interrupt
SIGQUIT	3*	quit
SIGILL	4*	illegal instruction
SIGTRAP	5*	trace trap
SIGIOT	6*	IOT instruction
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	write on a pipe with no one to read it
SIGALRM	14	alarm clock
SIGTERM	15	software termination signal
SIGURG	16@	urgent condition present on socket
SIGSTOP	17'|+'	stop (cannot be caught or ignored)
SIGTSTP	18'|+'	stop signal generated from keyboard
SIGCONT	19@	continue after stop
SIGCHLD	20@	child status has changed
SIGTTIN	21'|+'	background read attempted from control terminal
SIGTTOU	22'|+'	background write attempted to control terminal
SIGIO	23@	i/o is possible on a descriptor (see fcntl(2))
SIGXCPU	24	cpu time limit exceeded (see setrlimit(2))
SIGXFSZ	25	file size limit exceeded (see setrlimit(2))
SIGVTALRM	26	virtual time alarm (see setitimer(2))
SIGPROF	27	profiling timer alarm (see setitimer(2))
SIGWINCH	28@	used by PCI to signal a window size change
SIGUSR1	30	user-defined signal 1
SIGUSR2	31	user-defined signal 2

The starred signals in the list above cause a core image if not caught or ignored.

If func is SIG_DFL, the default action for signal sig is reinstated; this default is termination (with a core image for starred signals) except for signals marked with @ or '|+'. Signals marked with @ are discarded if the action is SIG_DFL; signals marked with '|+' cause the process to stop. If func is SIG_IGN the signal is subsequently ignored and pending instances of the signal are discarded. Otherwise, when the signal occurs further occurrences of the signal are automatically blocked and func is called.

A return from the function unblocks the handled signal and continues the process at the point it was interrupted. Unlike previous signal facilities, the handler func remains installed after a signal has been delivered.

If a caught signal occurs during certain system calls, causing the call to terminate prematurely, the call is automatically restarted. In particular this can occur during a read or write(2) on a slow device (such as a terminal; but not a file) and during a wait(2).

The value of signal is the previous (or initial) value of func for the particular signal.

After a fork(2) or vfork(2) the child inherits all signals. Execve(2) resets all caught signals to the default action; ignored signals remain ignored.

NOTES
Note that floating-point context is saved and restored across all signal handlers EXCEPT SIGFPE handlers. In the case of a SIGFPE, the corresponding handler inherits the floating-point context of the interrupted procedure. And, on return from the SIGFPE handler, the interrupted procedure inherits any modifications the signal handler made to its floating-point state. Thus, floating-point applications can reset the faulting state directly in the fpu/fpa hardware from within the SIGFPE handler. All other signal handlers receive initialized fpu/fpa state and, on normal return, the fpu/fpa context of the interrupted procedure is restored.

RETURN VALUE
The previous action is returned on a successful call. Otherwise, -1 is returned and errno is set to indicate the error.

ERRORS
Signal will fail and no action will take place if one of the following occur:

[EINVAL]
    Sig is not a valid signal number.

[EINVAL]
    An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.

[EINVAL]
    An attempt is made to ignore SIGCONT (by default SIGCONT is ignored).

SEE ALSO
kill(1), ptrace(2), kill(2), sigvec(2), sigblock(2), sigsetmask(2), sigpause(2), sigstack(2), setjmp(3), tty(4)