SIGNAL(3)

NAME
signal - simplified software signal facilities

SYNOPSIS
#include <signal.h>

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

or in GNO's equivalent but easier to read typedef'd version:

typedef void (*sig_t) (int, int)

sig_t signal(int sig, sig_t func);

DESCRIPTION
Signals allow the manipulation of a process from outside its domain as well as allowing the process to manipulate itself or copies of itself (children). There are two general types of signals: those that cause termination of a process and those that do not. Signals which cause termination of a program might result from an irrecoverable error or might be the result of a user at a terminal typing the `interrupt' character. Signals are used 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 result in the termination of the process receiving them 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. The signal function allows for a signal to be caught, to be ignored, or to generate an interrupt, except for SIGCONT (which cannot be blocked), and SIGKILL and SIGSTOP (which cannot be caught, blocked, or ignored).

These signals are defined in the file <signal.h>:
    Name		Default Action	Description
    SIGHUP	termination	terminal line hangup
    SIGINT	termination	interrupt program
    SIGQUIT	termination	quit program
    SIGILL	termination	illegal instruction
    SIGTRAP	termination	trace trap
    SIGABRT	termination	abort (generated by abort(3))
    SIGEMT	termination	emulator trap
    SIGFPE	termination	arithmetic exception
    SIGKILL	termination	kill program
    SIGBUS	termination	bus error
    SIGSEGV	termination	segmentation fault
    SIGSYS	termination	bad argument to system call
    SIGPIPE	termination	write on a socket with no readers
    SIGALRM	termination	real-time timer expired
    SIGTERM	termination	software termination
    SIGURG	discarded		urgent condition present on socket
    SIGSTOP	stop			stop
    SIGTSTP	stop			stop signal from keyboard
    SIGCONT	discarded		continue after stop
    SIGCHLD	discarded		child status has changed
    SIGCLD	discarded		SYSV name for SIGCHLD
    SIGTTIN	stop			background read attempted
    SIGTTOU	stop			background write attempted
    SIGIO	discarded		input/output possible on a file descriptor
    SIGPOLL	discarded		SYSV name for SIGIO
    SIGXCPU	termination	exceeded CPU time limit
    SIGUSR1	termination	user defined signal 1
    SIGUSR2	termination	user defined signal 2

The func procedure allows a user to choose the action upon receipt of a signal. To set the default action of the signal to occur as listed above, func should be SIG_DFL. A SIG_DFL resets the default action. To ignore the signal func should be SIG_IGN. This will cause subsequent instances of the signal to be ignored and pending instances to be discarded. If SIG_IGN is not used, further occurrences of the signal are automatically blocked and func is called.

The handled signal is unblocked with the function returns and the process continues from where it left off when the signal occurred. Unlike previous Unix signal facilities, the handler func() remains installed after a signal has been delivered. This behavior remains unchanged from GNO v2.0.4.

For some system calls, if a signal is caught while the call is executing and the call is prematurely terminated, the call is automatically restarted. The affected system calls include read(2), write(2), sendto(2), recvfrom(2), sendmsg(2) and recvmsg(2) on a communications channel or a low speed device and during a ioctl(2) or wait(2). However, calls that have already committed are not restarted, but instead return a partial success (for example, a short read count).

When a process which has installed signal handlers forks, the child process inherits the signals. All caught signals will be reset to their default action by a call to one of the execve(2) family of functions; ignored signals remain ignored.

NOTES
As can be surmised from the prototype above, func should be defined as follows:

    void func(int sig, int code)

sig is the signal that will invoke the handler, and code is additional information about the interrupt condition. Currently, code is always zero. The handler should probably also be compiled using the #pragma databank 1 directive, in the event func is not in the same bank as the C global data segment (func is called with the data bank equal to the program bank).

Orca/C already provides a signal function, but it doesn't do very much. GNO's <signal.h> file replaces the one that comes with Orca/C.

RETURN VALUES
The previous action is returned on a successful call. Otherwise, -1 is returned and the global variable 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), execve(2), fork(2), kill(2), sigblock(2), sigsetmask(2), wait(2), tty(4)

HISTORY
A signal facility appeared in 4.0BSD.