void
*
signal(
int sig
, void *funcintint
)
)
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' 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.
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 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 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).
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 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.
SIG_ERR
is returned and the global variable
errno
is set to indicate the error.
)
will fail and no action will take place if one of the following occur:
EINVAL
]
EINVAL
]
SIGKILL
or
SIGSTOP
.
)
facility appeared in
4.0BSD.