SIGNAL(3V)

NAME
signal - simplified software signal facilities

SYNOPSIS

#include <signal.h>

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

DESCRIPTION

signal() is a simplified interface to the more general sigvec(2) facility. Programs that use signal() in preference to sigvec() are more likely to be portable to all systems.

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 termio(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 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
    SIGABRT	6*	abort (generated by abort(3) routine)
    SIGEMT	7*	emulator trap
    SIGFPE	8*	arithmetic exception
    SIGKILL	9	kill (cannot be caught, blocked, or ignored)
    SIGBUS	10*	bus error
    SIGSEGV	11*	segmentation violation
    SIGSYS	12*	bad argument to system call
    SIGPIPE	13	write on a pipe or other socket 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, blocked, 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(2V))
    SIGXCPU	24	cpu time limit exceeded (see
    getrlimit(2))
    SIGXFSZ	25	file size limit exceeded (see
    getrlimit(2))
    SIGVTALRM	26	virtual time alarm (see
    getitimer(2))
    SIGPROF	27	profiling timer alarm (see
    getitimer(2))
    SIGWINCH	28@	window changed (see termio(4) and win(4S))
    SIGLOST	29*	resource lost (see
    lockd(8C))
    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, terminating the call prematurely, the call is automatically restarted. In particular this can occur during a read(2V) or write(2V) on a slow device (such as a terminal; but not a file) and during a wait(2V).

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

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

SYSTEM V DESCRIPTION

If func is SIG_IGN the signal is subsequently ignored and pending instances of the signal are discarded. Otherwise, when the signal occurs, func is called. Further occurrences of the signal are not automatically blocked. The value of func for the caught signal is reset to SIG_DFL before func is called, unless the signal is SIGILL or SIGTRAP.

A return from the function continues the process at the point at which it was interrupted. The handler func does not remain 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 interrupted. In particular this can occur during a read(2V) or write(2V) on a slow device (such as a terminal; but not a file) and during a wait(2V). After the signal catching function returns, the interrupted system call may return a -1 to the calling process with errno set to EINTR.

RETURN VALUES

signal() returns the previous action on success. On failure, it returns -1 and sets errno to indicate the error.

ERRORS

signal() will fail and no action will take place if one of the following occurs:

EINVAL
    sig was not a valid signal number.

    An attempt was made to ignore or supply a handler for SIGKILL or SIGSTOP.

SEE ALSO
kill(1), execve(2V), fork(2V), getitimer(2), getrlimit(2), kill(2V), ptrace(2), read(2V), sigblock(2), sigpause(2V), sigsetmask(2), sigstack(2), sigvec(2), vfork(2), wait(2V), write(2V), setjmp(3V), termio(4)

NOTES

The handler routine can be declared:

    void handler(sig, code, scp, addr)
    int sig, code;
    struct sigcontext *scp;
    char *addr;

Here sig is the signal number; code is a parameter of certain signals that provides additional detail; scp is a pointer to the sigcontext structure (defined in <signal.h>), used to restore the context from before the signal; and addr is additional address information. See sigvec(2) for more details.