sigaction - detailed signal management
int sigaction(int sig, const struct sigaction *restrict act, struct sigaction *restrict oact);
The sigaction() function allows the calling process to examine or specify the action to be taken on delivery of a specific signal. See sig-
nal.h(3HEAD) for an explanation of general signal concepts.
The sig argument specifies the signal and can be assigned any of the signals specified in signal.h(3HEAD) except SIGKILL and SIGSTOP.
If the argument act is not NULL, it points to a structure specifying the new action to be taken when delivering sig. If the argument oact
is not NULL, it points to a structure where the action previously associated with sig is to be stored on return from sigaction().
The sigaction structure includes the following members:
void (*sa_sigaction)(int, siginfo_t *, void *);
The storage occupied by sa_handler and sa_sigaction may overlap, and a standard-conforming application (see standards(5)) must not use both
The sa_handler member identifies the action to be associated with the specified signal, if the SA_SIGINFO flag (see below) is cleared in
the sa_flags field of the sigaction structure. It may take any of the values specified in signal.h(3HEAD) or that of a user specified sig-
nal handler. If the SA_SIGINFO flag is set in the sa_flags field, the sa_sigaction field specifies a signal-catching function.
The sa_mask member specifies a set of signals to be blocked while the signal handler is active. On entry to the signal handler, that set of
signals is added to the set of signals already being blocked when the signal is delivered. In addition, the signal that caused the handler
to be executed will also be blocked, unless the SA_NODEFER flag has been specified. SIGSTOP and SIGKILL cannot be blocked (the system
silently enforces this restriction).
The sa_flags member specifies a set of flags used to modify the delivery of the signal. It is formed by a logical OR of any of the follow-
SA_ONSTACK If set and the signal is caught, and if the thread that is chosen to processes a delivered signal has an alternate signal
stack declared with sigaltstack(2), then it will process the signal on that stack. Otherwise, the signal is delivered on
the thread's normal stack.
SA_RESETHAND If set and the signal is caught, the disposition of the signal is reset to SIG_DFL and the signal will not be blocked on
entry to the signal handler (SIGILL, SIGTRAP, and SIGPWR cannot be automatically reset when delivered; the system silently
enforces this restriction).
SA_NODEFER If set and the signal is caught, the signal will not be automatically blocked by the kernel while it is being caught.
SA_RESTART If set and the signal is caught, functions that are interrupted by the execution of this signal's handler are transparently
restarted by the system, namely fcntl(2), ioctl(2), wait(3C), waitid(2), and the following functions on slow devices like
terminals: getmsg() and getpmsg() (see getmsg(2)); putmsg() and putpmsg() (see putmsg(2)); pread(), read(), and readv()
(see read(2)); pwrite(), write(), and writev() (see write(2)); recv(), recvfrom(), and recvmsg() (see recv(3SOCKET)); and
send(), sendto(), and sendmsg() (see send(3SOCKET). Otherwise, the function returns an EINTR error.
SA_SIGINFO If cleared and the signal is caught, sig is passed as the only argument to the signal-catching function. If set and the
signal is caught, two additional arguments are passed to the signal-catching function. If the second argument is not
equal to NULL, it points to a siginfo_t structure containing the reason why the signal was generated (see sig-
info.h(3HEAD)); the third argument points to a ucontext_t structure containing the receiving process's context when the
signal was delivered (see ucontext.h(3HEAD)).
SA_NOCLDWAIT If set and sig equals SIGCHLD, the system will not create zombie processes when children of the calling process exit. If
the calling process subsequently issues a wait(3C), it blocks until all of the calling process's child processes terminate,
and then returns -1 with errno set to ECHILD.
SA_NOCLDSTOP If set and sig equals SIGCHLD, SIGCHLD will not be sent to the calling process when its child processes stop or continue.
Upon successful completion, 0 is returned. Otherwise, -1 is returned, errno is set to indicate the error, and no new signal handler is
The sigaction() function will fail if:
EINVAL The value of the sig argument is not a valid signal number or is equal to SIGKILL or SIGSTOP. In addition, if in a multi-
threaded process, it is equal to SIGWAITING, SIGCANCEL, or SIGLWP.
See attributes(5) for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|Interface Stability |Standard |
|MT-Level |Async-Signal-Safe |
kill(1), intro(2), exit(2), fcntl(2), getmsg(2), ioctl(2), kill(2), pause(2), putmsg(2), read(2), sigaltstack(2), sigprocmask(2),
sigsend(2), sigsuspend(2), waitid(2), write(2), recv(3SOCKET), send(3SOCKET), siginfo.h(3HEAD), signal(3C), signal.h(3HEAD), sigsetops(3C),
ucontext.h(3HEAD), wait(3C), attributes(5), standards(5)
The handler routine can be declared:
void handler (int sig, siginfo_t *sip, ucontext_t *ucp);
The sig argument is the signal number. The sip argument is a pointer (to space on the stack) to a siginfo_t structure, which provides addi-
tional detail about the delivery of the signal. The ucp argument is a pointer (again to space on the stack) to a ucontext_t structure
(defined in <sys/ucontext.h>) which contains the context from before the signal. It is not recommended that ucp be used by the handler to
restore the context from before the signal delivery.
23 Mar 2005 sigaction(2)