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' 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
sigaction(2) 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; or 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),
bsd_signal(3),
psignal(3),
setjmp(3),
strsignal(3),
tty(4),
signal(7)
HISTORY
This
signal() facility appeared in
4.0BSD.