/usr/ucb/cc [flag . . . ] file . . .#include <signal.h>
void (*signal(int sig, void *func))();
A signal is generated by some abnormal event, initiated by a user at a terminal (quit, interrupt, stop), by a program error (bus error, and so on), 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(7)). 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 | hangup | |
| SIGINT | interrupt | |
| SIGQUIT | * | quit |
| SIGILL | * | illegal instruction |
| SIGTRAP | * | trace trap |
| SIGABRT | * | abort (generated by abort(3C) 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) |
| 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)) |
| SIGPWR |
| power fail/restart |
| SIGXCPU | * | cpu time limit exceeded (see getrlimit(2)) |
| SIGXFSZ | * | file size limit exceeded (see |
| getrlimit(2)) | ||
| SIGVTALRM | virtual time alarm (see getitimer(3C)) | |
| SIGPROF | profiling timer alarm (see getitimer(3C)) | |
| SIGWINCH |
| window changed (see termio(7)) |
| SIGUSR1 | user-defined signal 1 | |
| SIGUSR2 | user-defined signal 2 |
SIGHUP hangup
SIGINT interrupt
SIGQUIT * quit
SIGILL * illegal instruction
SIGTRAP * trace trap
SIGABRT * abort (generated by
abort(3C) 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)
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)
SIGPWR • power fail/restart
SIGXCPU * cpu time limit
exceeded (see
getrlimit(2)
SIGXFSZ * file size limit
exceeded (see
getrlimit(2)
SIGVTALRM virtual time alarm
(see getitimer(3C)
SIGPROF profiling timer
alarm (see
getitimer(3C)
SIGWINCH • window changed (see
termio(7)
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
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.
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(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 (see exec(2)) resets all caught signals to the default action; ignored signals remain ignored.
signal will fail and no action will take place if one of the following occur:
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(3bsd) for more details.