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signal(2) [v7 man page]

SIGNAL(2)							System Calls Manual							 SIGNAL(2)

NAME
signal - catch or ignore signals SYNOPSIS
#include <signal.h> (*signal(sig, func))() (*func)(); DESCRIPTION
A signal is generated by some abnormal event, initiated either by user at a typewriter (quit, interrupt), by a program error (bus error, etc.), or by request of another program (kill). Normally all signals cause termination of the receiving process, but a signal call allows them either to be ignored or to cause an interrupt to a specified location. Here is the list of signals with names as in the include file. SIGHUP 1 hangup SIGINT 2 interrupt SIGQUIT 3* quit SIGILL 4* illegal instruction (not reset when caught) SIGTRAP 5* trace trap (not reset when caught) 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 or link with no one to read it SIGALRM 14 alarm clock SIGTERM 15 software termination signal 16 unassigned 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, sometimes with a core image. If func is SIG_IGN the signal is ignored. Otherwise when the signal occurs func will be called with the signal number as argument. A return from the function will continue the process at the point it was interrupted. Except as indicated, a signal is reset to SIG_DFL after being caught. Thus if it is desired to catch every such signal, the catching routine must issue another signal call. When a caught signal occurs during certain system calls, the call terminates prematurely. In particular this can occur during a read or write(2) on a slow device (like a typewriter; but not a file); and during pause or wait(2). When such a signal occurs, the saved user sta- tus is arranged in such a way that when return from the signal-catching takes place, it will appear that the system call returned an error status. The user's program may then, if it wishes, re-execute the call. The value of signal is the previous (or initial) value of func for the particular signal. After a fork(2) the child inherits all signals. Exec(2) resets all caught signals to default action. SEE ALSO
kill(1), kill(2), ptrace(2), setjmp(3) DIAGNOSTICS
The value (int)-1 is returned if the given signal is out of range. BUGS
If a repeated signal arrives before the last one can be reset, there is no chance to catch it. The type specification of the routine and its func argument are problematical. ASSEMBLER
(signal = 48.) sys signal; sig; label (old label in r0) If label is 0, default action is reinstated. If label is odd, the signal is ignored. Any other even label specifies an address in the process where an interrupt is simulated. An RTI or RTT instruction will return from the interrupt. SIGNAL(2)

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SIGNAL(3)						   BSD Library Functions Manual 						 SIGNAL(3)

NAME
signal -- simplified software signal facilities LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <signal.h> void (* signal(int sig, void (*func)(int)))(int); DESCRIPTION
This signal() facility is a simplified interface to the more general sigaction(2) facility. 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' char- acter. 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. Except for the SIGKILL and SIGSTOP signals, the signal() function allows for a signal to be caught, to be ignored, or to generate an interrupt. See signal(7) for comprehensive list of supported signals. 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 subse- quent 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 when the function returns and the process continues from where it left off when the signal occurred. Unlike previous signal facilities, the handler func() remains installed after a signal has been delivered. 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 handler is installed using the SA_RESTART flag with sigaction(2)). 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). How- ever, 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 may be reset to their default action by a call to the execve(2) function; ignored signals remain ignored. Only functions that are async-signal-safe can safely be used in signal handlers, see signal(7) for a complete list. RETURN VALUES
The previous action is returned on a successful call. Otherwise, SIG_ERR 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] Specified sig is not a valid signal number. [EINVAL] An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP. SEE ALSO
kill(1), kill(2), ptrace(2), sigaction(2), sigaltstack(2), sigprocmask(2), sigsuspend(2), psignal(3), setjmp(3), strsignal(3), tty(4), signal(7) HISTORY
This signal() facility appeared in 4.0BSD. BSD
June 11, 2004 BSD

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