SIGNAL(3) | Library Functions Manual | SIGNAL(3) |
signal
—
simplified software signal facilities
Standard C Library (libc, -lc)
#include
<signal.h>
void (*
signal
(int sig,
void (*func)(int));
)(int);
or in the equivalent but easier to read typedef'd version:
typedef void (*sig_t) (int);
sig_t
signal
(int
sig, sig_t
func);
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. These signals are defined in the file
<signal.h>
:
Name | Default Action | Description | |
1 | SIGHUP |
terminate process | terminal line hangup |
2 | SIGINT |
terminate process | interrupt program |
3 | SIGQUIT |
create core image | quit program |
4 | SIGILL |
create core image | illegal instruction |
5 | SIGTRAP |
create core image | trace trap |
6 | SIGABRT |
create core image | abort program (formerly SIGIOT ) |
7 | SIGEMT |
create core image | emulate instruction executed |
8 | SIGFPE |
create core image | floating-point exception |
9 | SIGKILL |
terminate process | kill program |
10 | SIGBUS |
create core image | bus error |
11 | SIGSEGV |
create core image | segmentation violation |
12 | SIGSYS |
create core image | non-existent system call invoked |
13 | SIGPIPE |
terminate process | write on a pipe with no reader |
14 | SIGALRM |
terminate process | real-time timer expired |
15 | SIGTERM |
terminate process | software termination signal |
16 | SIGURG |
discard signal | urgent condition present on socket |
17 | SIGSTOP |
stop process | stop (cannot be caught or ignored) |
18 | SIGTSTP |
stop process | stop signal generated from keyboard |
19 | SIGCONT |
discard signal | continue after stop |
20 | SIGCHLD |
discard signal | child status has changed |
21 | SIGTTIN |
stop process | background read attempted from control terminal |
22 | SIGTTOU |
stop process | background write attempted to control terminal |
23 | SIGIO |
discard signal | I/O is possible on a descriptor (see fcntl(2)) |
24 | SIGXCPU |
terminate process | cpu time limit exceeded (see setrlimit(2)) |
25 | SIGXFSZ |
terminate process | file size limit exceeded (see setrlimit(2)) |
26 | SIGVTALRM |
terminate process | virtual time alarm (see setitimer(2)) |
27 | SIGPROF |
terminate process | profiling timer alarm (see setitimer(2)) |
28 | SIGWINCH |
discard signal | Window size change |
29 | SIGINFO |
discard signal | status request from keyboard |
30 | SIGUSR1 |
terminate process | User defined signal 1 |
31 | SIGUSR2 |
terminate process | User defined signal 2 |
The sig argument specifies which signal was
received. 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.
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. Any handler installed with
signal(3) will have the
SA_RESTART
flag set, meaning that any restartable
system call will not return on receipt of a signal. 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). These semantics could be changed with
siginterrupt(3).
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.
If a process explicitly specifies SIG_IGN
as the action for the signal SIGCHLD
, the system
will not create zombie processes when children of the calling process exit.
As a consequence, the system will discard the exit status from the child
processes. If the calling process subsequently issues a call to
wait(2) or equivalent, it will block
until all of the calling process's children terminate, and then return a
value of -1 with errno set to
ECHILD
.
See sigaction(2) for a list of functions that are considered safe for use in signal handlers.
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.
The signal
() function will fail and no
action will take place if one of the following occur:
kill(1), kill(2), ptrace(2), sigaction(2), sigaltstack(2), sigprocmask(2), sigsuspend(2), wait(2), fpsetmask(3), setjmp(3), siginterrupt(3), tty(4)
The signal
facility appeared in
4.0BSD. The option to avoid the creation of child
zombies through ignoring SIGCHLD
appeared in
FreeBSD 5.0.
June 7, 2004 | macOS 14.6 |