sigaction(2POSIX)

NAME | SYNOPSIS | API RESTRICTIONS | DESCRIPTION | RETURN VALUES | ERRORS | ATTRIBUTES | SEE ALSO | NOTES

NAME

sigaction- software signal facilities

SYNOPSIS

#include <signal.h>
struct sigaction {
        void     (*sa_handler)();  /* signal handler */
        sigset_t sa_mask;          /* signal mask to apply */
        int      sa_flags;         /* see signal options below */
};

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

API RESTRICTIONS

The function or functions documented here may not be used safely in all application contexts with all APIs provided in the ChorusOS 5.0 product.

See API(5FEA) for details.

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 normally 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 normally execute with the signal that caused their invocation blocked, but other signals may still 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(2POSIX) call, or when a signal is delivered to the process.

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 (for example, 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(2POSIX) 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.

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() call is made). This mask is formed by taking the union of the current signal mask set, the signal to be delivered, and the signal mask associated with the handler to be invoked.

sigaction() assigns an action for a signal specified by sig. 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 normally remains installed until another sigaction() call is made, or an execve(2POSIX) 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 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. If the SA_NOCLDSTOP bit is 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. If the SA_NODEFER bit is set, further occurrences of the delivered signal are not masked during the execution of the handler. If the SA_RESETHAND bit is set, the handler is reset back to SIG_DFL at the moment the signal is delivered. If the SA_SIGINFO bit is set the handler function is assumed to be pointed to by the sa_sigaction member of struct sigaction. This bit should not be set when assigning SIG_DFL or SIG_IGN.

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. Restart of pending calls is requested by setting the SA_RESTART bit in sa_flags. The affected system calls include open(2POSIX), read(2POSIX), write(2POSIX), sendto(2POSIX), recvfrom(2POSIX), sendmsg(2POSIX) and recvmsg(2POSIX) on a communications channel or a slow device (such as a terminal, but not a regular file) and during a wait(2POSIX) or ioctl(2POSIX). However, calls that have already committed are not restarted, but instead return a partial success (for example, a short read count).

If act or oact points to memory that is not a valid part of the process address space, SIGSEGV is delivered to the process.

After a fork(2POSIX), all signals, the signal mask, the signal stack, and the restart/interrupt flags are inherited by the child.

Note: After a spawn(2POSIX), the signal mask is not inherited from the current thread. It is zeroed.

execve(2POSIX) 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.

The following is a list of all signals with names as in the include file <signal.h>:

 

Name	Default Action	Description
SIGHUP	terminate process	terminal line hangup
SIGINT	terminate process	interrupt program
SIGQUIT	create core image	quit program
SIGILL	create core image	illegal instruction
SIGTRAP	create core image	trace trap
SIGABRT	create core image	abort(3STDC) call (formerly SIGIOT)
SIGEMT	create core image	emulate instruction executed
SIGFPE	create core image	floating-point exception
SIGKILL	terminate process	kill program
SIGBUS	create core image	bus error
SIGSEGV	create core image	segmentation violation
SIGSYS	create core image	non-existent system call invoked
SIGPIPE	terminate process	write on a pipe with no reader
SIGALRM	terminate process	real-time timer expired
SIGTERM	terminate process	software termination signal
SIGURG	discard signal	urgent condition present on socket
SIGCHLD	discard signal	child status has changed
SIGTTIN	stop process	background read attempted from control terminal
SIGTTOU	stop process	background write attempted to control terminal
SIGIO	discard signal	I/O is possible on a descriptor (see fcntl(2POSIX))
SIGXFSZ	terminate process	file size limit exceeded (see setrlimit(2POSIX))
SIGVTALRM	terminate process	virtual time alarm
SIGPROF	terminate process	profiling timer alarm
SIGINFO	discard signal	status request from keyboard
SIGUSR1	terminate process	User defined signal 1
SIGUSR2	terminate process	User defined signal 2

RETURN VALUES

A 0 value indicates that the call succeeded. A -1 return value indicates that an error occurred and errno is set to indicate the reason.

ERRORS

The sigaction() function will fail and no new 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.

EINVAL

An attempt has been made to ignore or supply a handler for SIGKILL.

ATTRIBUTES

See attributes(5) for descriptions of the following attributes:

ATTRIBUTE TYPE	ATTRIBUTE VALUE
Interface Stability	Evolving
MT-Level	Async-Signal-Safe

SEE ALSO

kill(2POSIX), sigsuspend(2POSIX), sigsetops(3POSIX), setjmp(3STDC)

NOTES

The sa_mask field specified in act is not allowed to block SIGKILL. Any attempt to do so will be silently ignored.

ChorusOS 5.0  Last Revised December 2001

NAME | SYNOPSIS | API RESTRICTIONS | DESCRIPTION | RETURN VALUES | ERRORS | ATTRIBUTES | SEE ALSO | NOTES