SIGACTION(2) | System Calls Manual | SIGACTION(2) |
sigaction
—
#include <signal.h>
int
sigaction
(int
sig, const struct
sigaction * restrict act,
struct sigaction * restrict
oact);
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
SA_NODEFER
, so that if the specified signal is
blocked in sa_mask, then
SA_NODEFER
will have no effect.SA_NOCLDSTOP
SIGCHLD
signal, the
SIGCHLD
signal will be generated only when a child
process exits, not when a child process stops or continues.SA_NOCLDWAIT
SIGCHLD
to SIG_IGN
.SA_ONSTACK
SA_RESETHAND
SA_RESTART
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
SA_NOKERNINFO
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.
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), 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).
SIGKILL
or SIGSTOP
. This is
enforced silently by the system.
sigaction
() will fail and no new signal handler will be
installed if one of the following occurs:
EFAULT
]EINVAL
]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
.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.
May 22, 2018 | NetBSD 9.2 |