NAME
sigaction —
software signal
facilities
LIBRARY
Standard C Library (libc, -lc)
SYNOPSIS
#include <signal.h>
int
sigaction(
int
sig,
const struct sigaction
* restrict act,
struct
sigaction * restrict oact);
DESCRIPTION
The system defines a set of signals that may be delivered to a process. Signal
delivery resembles the occurrence of a hardware interrupt: the signal is
blocked from further occurrence, the current process context is saved, and a
new one is built. A process may specify a
handler to which a
signal is delivered, or specify that a signal is to be
ignored. A process may also specify that a default action is
to be taken by the system when a signal occurs. A signal may also be
blocked, in which case its delivery is postponed until it is
unblocked. The action to be taken on delivery is determined
at the time of delivery. Normally, signal handlers execute on the current
stack of the process. This may be changed, on a per-handler basis, so that
signals are taken on a special
signal stack.
Signal routines execute with the signal that caused their invocation
blocked, but other signals may yet occur. A global
signal mask defines the set of signals currently blocked
from delivery to a process. The signal mask for a process is initialized from
that of its parent (normally empty). It may be changed with a
sigprocmask(2) call, or
when a signal is delivered to the process. Signal masks are represented using
the
sigset_t type; the
sigsetops(3) interface is
used to modify such data.
When a signal condition arises for a process, the signal is added to a set of
signals pending for the process. If the signal is not currently
blocked by the process then it is delivered to the process.
Signals may be delivered any time a process enters the operating system (e.g.,
during a system call, page fault or trap, or clock interrupt). If multiple
signals are ready to be delivered at the same time, any signals that could be
caused by traps are delivered first. Additional signals may be processed at
the same time, with each appearing to interrupt the handlers for the previous
signals before their first instructions. The set of pending signals is
returned by the
sigpending(2) function. When
a caught signal is delivered, the current state of the process is saved, a new
signal mask is calculated (as described below), and the signal handler is
invoked. The call to the handler is arranged so that if the signal handling
routine returns normally the process will resume execution in the context from
before the signal's delivery. If the process wishes to resume in a different
context, then it must arrange to restore the previous context itself.
struct sigaction includes the following members:
void (*sa_sigaction)(int sig, siginfo_t *info, void *ctx);
void (*sa_handler)(int sig);
sigset_t sa_mask;
int sa_flags;
When a signal is delivered to a process a new signal mask is installed for the
duration of the process' signal handler (or until a
sigprocmask(2) call is
made). This mask is formed by taking the union of the current signal mask, the
signal to be delivered, and the signal mask associated with the handler to be
invoked,
sa_mask.
sigaction() assigns an action for a specific signal. If
act is non-zero, it specifies an action
(
SIG_DFL
,
SIG_IGN
, or a
handler routine) and mask to be used when delivering the specified signal. If
oact is non-zero, the previous handling information for
the signal is returned to the user.
Once a signal handler is installed, it remains installed until another
sigaction() call is made, or an
execve(2) is performed. A
signal-specific default action may be reset by setting
sa_handler to
SIG_DFL
. The
defaults are process termination, possibly with core dump; no action; stopping
the process; or continuing the process. See the signal list below for each
signal's default action. If
sa_handler is set to
SIG_DFL
, the default action for the signal is to
discard the signal, and if a signal is pending, the pending signal is
discarded even if the signal is masked. If
sa_handler is
set to
SIG_IGN
, current and pending instances of the
signal are ignored and discarded.
Options may be specified by setting
sa_flags.
-
-
SA_NODEFER
- If set, then the signal that caused the handler to be
executed is not added to the list of block signals. Please note that
sa_mask takes precedence over
SA_NODEFER
, so that if the specified signal is
blocked in sa_mask, then
SA_NODEFER
will have no effect.
-
-
SA_NOCLDSTOP
- If set when installing a catching function for the
SIGCHLD
signal, the
SIGCHLD
signal will be generated only when a child
process exits, not when a child process stops.
-
-
SA_NOCLDWAIT
- If set, the system will not create a zombie when the child
exits, but the child process will be automatically waited for. The same
effect can be achieved by setting the signal handler for
SIGCHLD
to SIG_IGN
.
-
-
SA_ONSTACK
- If set, the system will deliver the signal to the process
on a signal stack, specified with
sigaltstack(2).
-
-
SA_RESETHAND
- If set, the default action will be reinstated when the
signal is first posted.
-
-
SA_RESTART
- Normally, if a signal is caught during the system calls
listed below, the call may be forced to terminate with the error
EINTR
, the call may return with a data transfer
shorter than requested, or the call may be restarted. Restarting of
pending calls is requested by setting the
SA_RESTART
bit in sa_flags.
The affected system calls include
open(2),
read(2),
write(2),
sendto(2),
recvfrom(2),
sendmsg(2) and
recvmsg(2) on a
communications channel or a slow device (such as a terminal, but not a
regular file) and during a
wait(2) or
ioctl(2). However, calls that
have already committed are not restarted, but instead return a partial
success (for example, a short read count).
After a fork(2) or
vfork(2) all signals, the
signal mask, the signal stack, and the restart/interrupt flags are
inherited by the child.
The execve(2) system call
reinstates the default action for all signals which were caught and resets
all signals to be caught on the user stack. Ignored signals remain
ignored; the signal mask remains the same; signals that restart pending
system calls continue to do so.
See signal(7) for
comprehensive list of supported signals.
-
-
SA_SIGINFO
- If set, the signal handler function will receive additional
information about the caught signal. An alternative handler that gets
passed additional arguments will be called which is named
sa_sigaction. The sig argument
of this handler contains the signal number that was caught. The
info argument contains additional signal specific
information which is listed in
siginfo(2). The
ctx argument is a pointer to the
ucontext(2) context where
the signal handler will return to.
-
-
SA_NOKERNINFO
- This flag is relevant only to
SIGINFO
, and turns off printing kernel messages on
the tty. It is similar to the NOKERNINFO
flag in
termios(4).
If the signal handler is called due to signal delively resulting from reasons
other than direct calls to
kill(2)
or
_lwp_kill(2) or indirect
calls to
_lwp_kill(2) via
abort(3) or
raise(3) any activity (such as
calling functions or assigning variables in the global or static scopes) other
than setting a variable of the type
volatile
sig_atomic_t is undefined.
Signal-safe functions
Only functions that are guaranteed to be async-signal-safe can safely be used in
signal handlers. These are functions that are either reentrant or
non-interruptible. (These functions are also the only functions that may be
used in a child process after doing
fork(2) in a threaded program.)
The following functions are async-signal-safe. Any function not listed below is
unsafe to use in signal handlers.
_Exit(2),
_exit(2),
abort(3),
accept(2),
access(2),
alarm(3),
bind(2),
cfgetispeed(3),
cfgetospeed(3),
cfsetispeed(3),
cfsetospeed(3),
chdir(2),
chmod(2),
chown(2),
clock_gettime(2),
close(2),
connect(2),
creat(3),
dup(2),
dup2(2),
execle(3),
execve(2),
fchmod(2),
fchown(2),
fcntl(2),
fdatasync(2),
fork(2),
fpathconf(2),
fstat(2),
fsync(2),
ftruncate(2),
getegid(2),
geteuid(2),
getgid(2),
getgroups(2),
getpeername(2),
getpgrp(2),
getpid(2),
getppid(2),
getsockname(2),
getsockopt(2),
getuid(2),
kill(2),
link(2),
listen(2),
lseek(2),
lstat(2),
mkdir(2),
mkfifo(2),
open(2),
pathconf(2),
pause(3),
pipe(2),
poll(2),
pthread_mutex_unlock(3),
raise(3),
read(2),
readlink(2),
recv(2),
recvfrom(2),
recvmsg(2),
rename(2),
rmdir(2),
select(2),
sem_post(3),
send(2),
sendmsg(2),
sendto(2),
setgid(2),
setpgid(2),
setsid(2),
setsockopt(2),
setuid(2),
shutdown(2),
sigaction(2),
sigaddset(3),
sigdelset(3),
sigemptyset(3),
sigfillset(3),
sigismember(3),
sleep(3),
signal(3),
sigpause(3),
sigpending(2),
sigprocmask(2),
sigset(3),
sigsuspend(2),
sockatmark(3),
socket(2),
socketpair(2),
stat(2),
symlink(2),
sysconf(3),
tcdrain(3),
tcflow(3),
tcflush(3),
tcgetattr(3),
tcgetpgrp(3),
tcsendbreak(3),
tcsetattr(3),
tcsetpgrp(3),
time(3),
timer_getoverrun(2),
timer_gettime(2),
timer_settime(2),
times(3),
umask(2),
uname(3),
unlink(2),
utime(3),
wait(2),
waitpid(2),
write(2).
NOTES
The mask specified in
act is not allowed to block
SIGKILL
or
SIGSTOP
. This is
enforced silently by the system.
RETURN VALUES
A 0 value indicates that the call succeeded. A -1 return value indicates an
error occurred and
errno is set to indicate the reason.
ERRORS
sigaction() will fail and no new signal handler will be
installed if one of the following occurs:
-
-
- [
EFAULT
]
- Either act or
oact points to memory that is not a valid part of
the process address space.
-
-
- [
EINVAL
]
- sig is not a valid signal number; or
an attempt is made to ignore or supply a handler for
SIGKILL
or SIGSTOP
; or the
sa_flags word contains bits other than
SA_NOCLDSTOP
,
SA_NOCLDWAIT
, SA_NODEFER
,
SA_ONSTACK
, SA_RESETHAND
,
SA_RESTART
, and
SA_SIGINFO
.
SEE ALSO
kill(1),
kill(2),
ptrace(2),
sigaltstack(2),
sigprocmask(2),
sigstack(2),
sigsuspend(2),
fpgetmask(3),
fpsetmask(3),
setjmp(3),
sigblock(3),
siginterrupt(3),
signal(3),
sigpause(3),
sigsetmask(3),
sigsetops(3),
tty(4)
STANDARDS
The
sigaction() function conforms to
IEEE Std
1003.1-1990 (“POSIX.1”). The
SA_ONSTACK
and
SA_RESTART
flags are Berkeley extensions, available on most
BSD-derived systems.