NAME
signal
—
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 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 (cannot be caught or ignored) |
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 |
SIGTHR |
discard signal | thread AST |
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
.
The handled signal is unblocked when func 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. (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:
Standard Interfaces:
_exit
(),
_Exit
(),
abort
(),
accept
(),
access
(),
alarm
(),
bind
(),
cfgetispeed
(),
cfgetospeed
(),
cfsetispeed
(),
cfsetospeed
(),
chdir
(),
chmod
(),
chown
(),
clock_gettime
(),
close
(),
connect
(),
creat
(),
dup
(),
dup2
(),
execl
(),
execle
(),
execv
(),
execve
(),
faccessat
(),
fchdir
(),
fchmod
(),
fchmodat
(),
fchown
(),
fchownat
(),
fcntl
(),
fdatasync
(),
fork
(),
fpathconf
(),
fstat
(),
fstatat
(),
fsync
(),
ftruncate
(),
futimens
(),
futimes
(),
getegid
(),
geteuid
(),
getgid
(),
getgroups
(),
getpeername
(),
getpgrp
(),
getpid
(),
getppid
(),
getsockname
(),
getsockopt
(),
getuid
(),
kill
(),
link
(),
linkat
(),
listen
(),
lseek
(),
lstat
(),
mkdir
(),
mkdirat
(),
mkfifo
(),
mkfifoat
(),
mknod
(),
mknodat
(),
open
(),
openat
(),
pathconf
(),
pause
(),
pipe
(),
poll
(),
pselect
(),
pthread_sigmask
(),
raise
(),
read
(),
readlink
(),
readlinkat
(),
recv
(),
recvfrom
(),
recvmsg
(),
rename
(),
renameat
(),
rmdir
(),
select
(),
send
(),
sendmsg
(),
sendto
(),
setgid
(),
setpgid
(),
setsid
(),
setsockopt
(),
setuid
(),
shutdown
(),
sigaction
(),
sigaddset
(),
sigdelset
(),
sigemptyset
(),
sigfillset
(),
sigismember
(),
signal
(),
sigpause
(),
sigpending
(),
sigprocmask
(),
sigsuspend
(),
sleep
(),
sockatmark
(),
socket
(),
socketpair
(),
stat
(),
strcat
(),
strcpy
(),
strncat
(),
strncpy
(),
symlink
(),
symlinkat
(),
sysconf
(),
tcdrain
(),
tcflow
(),
tcflush
(),
tcgetattr
(),
tcgetpgrp
(),
tcsendbreak
(),
tcsetattr
(),
tcsetpgrp
(),
time
(),
times
(),
umask
(),
uname
(),
unlink
(),
unlinkat
(),
utime
(),
utimensat
(),
utimes
(),
wait
(),
waitpid
(),
write
(),
and perhaps some others.
Extension Interfaces:
accept4
(),
chflags
(),
chflagsat
(),
dup3
(),
fchflags
(),
getentropy
(),
getresgid
(),
getresuid
(),
pipe2
(),
ppoll
(),
sendsyslog
(),
setresgid
(),
setresuid
(),
strlcat
(),
strlcpy
(),
wait3
(),
wait4
().
In addition, access and updates to errno are guaranteed to be safe. 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, ideally of type volatile sig_atomic_t; most other actions are not safe.
Additionally, it is advised that signal handlers guard against modification of the external symbol errno by the above functions, saving it at entry and restoring it on return, thus:
void handler(int sig) { int save_errno = errno; ... errno = save_errno; }
The functions below are async-signal-safe in OpenBSD except when used with floating-point arguments or directives, but are probably unsafe on other systems:
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 occurs:
- [
EINVAL
] - A specified signal is not a valid signal number.
- [
EINVAL
] - An attempt is made to ignore or supply a handler for
SIGKILL
orSIGSTOP
.
SEE ALSO
kill(1), kill(2), ptrace(2), sigaction(2), sigaltstack(2), sigprocmask(2), sigsuspend(2), setjmp(3), siginterrupt(3), tty(4)
HISTORY
A signal
() system call first appeared in
Version 4 AT&T UNIX. In
4.2BSD, it was reimplemented as a wrapper around the
former sigvec
() system call, and for
4.3BSD-Reno, it was rewritten to use
sigaction(2) instead.