signal(3-BSD) MISC. REFERENCE MANUAL PAGES signal(3-BSD)
NAME
signal - simplified software signal facilities
SYNOPSIS
cc [ flag... ] file ... -lucb
#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 dis-
carded 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 hangup
SIGINT interrupt
SIGQUIT * quit
SIGILL * illegal instruction
SIGTRAP * trace trap
SIGABRT * abort (generated by abort(3) routine)
SIGEMT * emulator trap
SIGFPE * arithmetic exception
SIGKILL kill (cannot be caught, blocked, or ignored)
SIGBUS * bus error
SIGSEGV * segmentation violation
SIGSYS * bad argument to system call
SIGPIPE write on a pipe or other socket with no one to read it
SIGALRM alarm clock
SIGTERM software termination signal
SIGURG @ urgent condition present on socket
SIGSTOP † stop (cannot be caught, blocked, or ignored)
SIGTSTP † stop signal generated from keyboard
SIGCONT @ continue after stop (cannot be blocked)
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SIGCHLD @ child status has changed
SIGTTIN † background read attempted from control terminal
SIGTTOU † background write attempted to control terminal
SIGIO @ I/O is possible on a descriptor [see fcntl(2)]
SIGXCPU * cpu time limit exceeded [see getrlimit(2)
SIGXFSZ * file size limit exceeded [see getrlimit(2)
SIGVTALRM virtual time alarm [see getitimer(2)
SIGPROF profiling timer alarm [see getitimer(2)]
SIGWINCH @ window changed [see termio(4)]
SIGUSR1 user-defined signal 1
SIGUSR2 user-defined signal 2
The starred signals in the list above cause a core image if
not caught or ignored.
If func is SIGDFL, 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
SIGDFL; signals marked with † cause the process to stop.
If func is SIGIGN 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.
If a caught signal occurs during certain system calls, ter-
minating the call prematurely, the call is automatically
restarted. In particular this can occur during a read(2) 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.
An execve(2) resets all caught signals to the default
action; ignored signals remain ignored.
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 cer-
tain signals that provides additional detail; scp is a
pointer to the sigcontext structure (defined in <signal.h>),
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signal(3-BSD) MISC. REFERENCE MANUAL PAGES signal(3-BSD)
used to restore the context from before the signal; and addr
is additional address information. See sigvec(2) for more
details.
RETURN VALUE
The previous action is returned on a successful call. Oth-
erwise, -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, or is
SIGKILL or SIGSTOP.
SEE ALSO
setjmp(3), sigblock(3), sigpause(3), sigsetmask(3), sig-
stack(3), sigvec(3), wait(3)
execve(2), fork(2), getitimer(2), getrlimit(2), kill(2),
ptrace(2), read(2), sigaction(2) wait(2), write(2),
setjmp(3C), in the Programmer's Reference Manual.
kill(1), in the User's Reference Manual.
termio(7) in the System Administrator's Reference Manual.
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