signal - simplified software signal facilities
#include <signal.h>
void
(*signal(int sigcatch, void (*func)(int sigraised))) (int);
void
(*bsd_signal(int sigcatch, void (*func)(int sigraised)))
(int);
The signal() and bsd_signal() facilities are simplified interfaces to the
more general sigaction(2) facility. The bsd_signal() interface is provided
for source compatibility only. It is mainly used on
systems where
the standard signal() does not have BSD semantics. On
OpenBSD the two
interfaces are identical.
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 any 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
SIGHUP terminate process terminal line hangup
SIGINT terminate process interrupt program
SIGQUIT create core image quit program
SIGILL create core image illegal instruction
SIGTRAP create core image trace trap
SIGABRT create core image abort(3) call (formerly SIGIOT)
SIGEMT create core image emulate instruction
executed
SIGFPE create core image floating-point exception
SIGKILL terminate process kill program
SIGBUS create core image bus error
SIGSEGV create core image segmentation violation
SIGSYS create core image system call given
invalid
argument
SIGPIPE terminate process write on a pipe with
no reader
SIGALRM terminate process real-time timer expired
SIGTERM terminate process software termination
signal
SIGURG discard signal urgent condition
present on
socket
SIGSTOP stop process stop (cannot be
caught or
ignored)
SIGTSTP stop process stop signal generated from
keyboard
SIGCONT discard signal continue after stop
SIGCHLD discard signal child status has
changed
SIGTTIN stop process background read attempted from
control terminal
SIGTTOU stop process background write attempted to
control terminal
SIGIO discard signal I/O is possible on a
descriptor
(see fcntl(2))
SIGXCPU terminate process CPU time limit exceeded (see
setrlimit(2))
SIGXFSZ terminate process file size limit exceeded (see
setrlimit(2))
SIGVTALRM terminate process virtual time alarm
(see
setitimer(2))
SIGPROF terminate process profiling timer
alarm (see
setitimer(2))
SIGWINCH discard signal window size change
SIGINFO discard signal status request from
keyboard
SIGUSR1 terminate process user-defined signal
1
SIGUSR2 terminate process user-defined signal
2
The func argument is a function to be called as the action
upon receipt
of the signal sigcatch. The function will be called with
one argument,
sigraised, which is the signal raised (thus the same function, func, can
be used by more than one 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.
If the func is set to SIG_IGN for the SIGCHLD signal, the
system will not
create zombie processes when children of the calling process
exit. If
the calling process subsequently issues a wait(2) (or equivalent), it
blocks until all of the calling process's child processes
terminate, and
then returns a value of -1 with errno set to ECHILD. This
differs from
historical BSD behavior but is consistent with AT&T System V as the X/Open Portability Guide Issue 4.2 (``XPG4.2'').
The handled signal is unblocked when func 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). The siginterrupt(3)
function can be used to change the system call restart behavior for a
specific signal.
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.
The following functions are either reentrant or not interruptible by signals
and are async-signal safe. Therefore applications may
invoke them,
without restriction, from signal-catching functions:
Base Interfaces:
_exit(), access(), alarm(), cfgetispeed(), cfgetospeed(),
cfsetispeed(),
cfsetospeed(), chdir(), chmod(), chown(), close(), creat(),
dup(),
dup2(), execle(), execve(), fcntl(), fork(), fpathconf(),
fstat(),
fsync(), getegid(), geteuid(), getgid(), getgroups(),
getpgrp(),
getpid(), getppid(), getuid(), kill(), link(), lseek(),
mkdir(),
mkfifo(), open(), pathconf(), pause(), pipe(), raise(),
read(), rename(),
rmdir(), setgid(), setpgid(), setsid(), setuid(),
sigaction(),
sigaddset(), sigdelset(), sigemptyset(), sigfillset(),
sigismember(),
signal(), sigpending(), sigprocmask(), sigsuspend(),
sleep(), stat(),
sysconf(), tcdrain(), tcflow(), tcflush(), tcgetattr(),
tcgetpgrp(),
tcsendbreak(), tcsetattr(), tcsetpgrp(), time(), times(),
umask(),
uname(), unlink(), utime(), wait(), waitpid(), write().
Realtime Interfaces:
aio_error(), clock_gettime(), sigpause(),
timer_getoverrun(),
aio_return(), fdatasync(), sigqueue(), timer_gettime(),
aio_suspend(),
sem_post(), sigset(), timer_settime().
ANSI C Interfaces:
strcpy(), strcat(), strncpy(), strncat(), and perhaps some
others.
Extension Interfaces:
strlcpy(), strlcat(), syslog_r().
Most functions not in the above lists are considered to be
unsafe with
respect to signals. That is to say, the behaviour of such
functions when
called from a signal handler is undefined. In general
though, signal
handlers should do little more than set a flag; most other
actions are
not safe.
Additionally, inside the signal handler it is also considered more safe
to make a copy of the global variable errno and restore it
before returning
from the signal handler.
A few other functions are signal race safe in OpenBSD but
probably not on
other systems:
snprintf() Safe.
vsnprintf() Safe.
syslog_r() Safe if the syslog_data struct is initialized as a
local variable.
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.
signal() will fail and no action will take place if one of
the following
occur:
[EINVAL] A specified signal is not a valid signal number.
[EINVAL] An attempt is made to ignore or supply a handler for
SIGKILL or SIGSTOP.
kill(1), kill(2), ptrace(2), sigaction(2), sigaltstack(2),
sigprocmask(2), sigsuspend(2), setjmp(3), siginterrupt(3),
tty(4)
This signal() facility appeared in 4.0BSD.
OpenBSD 3.6 April 19, 1994
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