sigaction(2)

NAME

sigaction − examine and change signal action

SYNOPSIS

#include <signal.h>

struct sigaction {
void(∗sa_handler)();
sigset_t sa_mask;
intsa_flags;
};

int sigaction (sig, act, oact)
int sig;
struct sigaction ∗act, ∗oact;

DESCRIPTION

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

The argument sig specifies the signal and can be assigned any of the signals specified in signal(5) except SIGKILL or 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. If the argument act is NULL, signal handling is unchanged by this function call; thus, sigaction can be used to enquire about the current handling of a given signal.

The sa_handler field of the sigaction structure specifies the disposition of the signal and may take any of the values specified in signal(5).

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 an alternate signal stack has been declared with sigaltstack(2), the signal is delivered to the calling process on that stack. Otherwise, the signal is delivered on the same stack as the main program.

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, a system call that is interrupted by the execution of this signal’s handler is transparently restarted by the system. Otherwise, that system call returns an EINTR error.

SA_SIGINFO If cleared and the signal is caught, sig is passed as the first argument to the signal-catching function, followed by additional implementation-dependent arguments. 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, sig will not be sent to the calling process when its child processes stop or continue.

RETURN VALUE

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

If any of the following conditions occur, sigaction will return −1 and set errno to the corresponding value:

[EINVAL] The value of the sig argument is an invalid or unsupported signal number, or an attempt was made to catch a signal that cannot be caught or to ignore a signal that cannot be ignored.

[EFAULT] act and/or oact is an invalid address.

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