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Manual page for sigaction(2)

sigaction - detailed signal management

SYNOPSIS

#include <signal.h>

int sigaction(int sig, const struct sigaction *act, struct sigaction *oact);

MT-LEVEL

Async-Signal-Safe

DESCRIPTION

sigaction() allows the calling process to examine or specify the action to be taken on delivery of a specific signal. (See signal.5 for an explanation of general signal concepts.)

sig specifies the signal and can be assigned any of the signals specified in signal.5 except SIGKILL and SIGSTOP. In a multi-threaded process, sig cannot be
SIGWAITING, SIGCANCEL, or SIGLWP.

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_handler)();
void	(*sa_sigaction)(int, siginfo_t *, void *);
sigset_t	sa_mask;
int	sa_flags;

sa_handler 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.5 or that of a user specified signal handler. If the SA_SIGINFO flag is set in the sa_flags field, the sa_sigaction field specifies a signal-catching function.

sa_mask 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).

sa_flags specifies a set of flags used to modify the delivery of the signal. It is formed by a logical OR of any of the following values:

SA_ONSTACK: If set and the signal is caught, and if the LWP 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 LWP main stack. Unbound threads (see thr_create.3t may not have alternate signal stacks.
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, certain functions that are interrupted by the execution of this signal's handler are transparently restarted by the system; namely, read.2 or write.2 on slow devices like terminals, ioctl.2 fcntl.2 wait.2 and waitid.2 Otherwise, that 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, pending signals of type sig are reliably queued to the calling process and 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 siginfo.5 the third argument points to a ucontext_t structure containing the receiving process's context when the signal was delivered (see ucontext.5
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.2 it blocks until all of the calling process's child processes terminate, and then returns a value of -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.
SA_WAITSIG: If set and sig equals SIGWAITING, then the system will send SIGWAITING to the process when all the LWPs in the process are blocked.

RETURN VALUES

On success, sigaction() returns zero. On failure, it returns -1 and sets errno to indicate the error. If sigaction() fails, no new signal handler is installed.

ERRORS

sigaction() fails if any of the following is true:

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.
EFAULT: act or oact points to an illegal address.

NOTES

The handler routine can be declared:
void handler (int sig, siginfo_t *sip, ucontext_t *uap);

Here, sig is the signal number. sip is a pointer (to space on the stack) to a siginfo_t structure, which provides additional detail about the delivery of the signal. uap 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 uap be used by the handler to restore the context from before the signal delivery.

Last modified 21/April/97