SIGNAL(3) BSD Programmer's Manual SIGNAL(3)NAMEsignal - simplified software signal facilities
SYNOPSIS
#include <signal.h>
void
(*signal(int sigcatch, void (*func)(int sigraised)))(int);
void
(*bsd_signal(int sigcatch, void (*func)(int sigraised)))(int);
DESCRIPTION
The signal() and bsd_signal() facilities are simplified interfaces to the
more general sigaction(2) facility. The bsd_signal() interface is provid-
ed for source compatibility only. It is mainly used on systems where the
standard signal() does not have BSD semantics. On OpenBSD the two inter-
faces 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 ter-
mination of a process and those that do not. Signals which cause termina-
tion 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 re-
ceiving 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 in-
stances 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 histor-
ical BSD behavior but is consistent with AT&T System V UNIX as well as
the X/Open Portability Guide Issue 4.2 ("XPG4.2").
The handled signal is unblocked when func returns and the process contin-
ues from where it left off when the signal occurred. Unlike previous sig-
nal 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 res-
tarted. (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 pro-
cess inherits the signals. All caught signals may be reset to their de-
fault action by a call to the execve(2) function; ignored signals remain
ignored.
The following functions are either reentrant or not interruptible by sig-
nals and are asyncronous-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(), get-
pid(), getppid(), getuid(), kill(), link(), lseek(), mkdir(), mkfifo(),
open(), pathconf(), pause(), pipe(), raise(), read(), rename(), rmdir(),
setgid(), setpgid(), setsid(), setuid(), sigaction(), sigaddset(), sig-
delset(), sigemptyset(), sigfillset(), sigismember(), signal(), sigpend-
ing(), 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 return-
ing 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.
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.
ERRORSsignal() will fail and no action will take place if one of the following
occurs:
[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.
SEE ALSOkill(1), kill(2), ptrace(2), sigaction(2), sigaltstack(2),
sigprocmask(2), sigsuspend(2), setjmp(3), siginterrupt(3), tty(4)HISTORY
This signal() facility appeared in 4.0BSD.
MirOS BSD #10-current April 19, 1994 2
