SIGACTION(2) — Unix Programmer’s Manual

NAME

sigaction − software signal facilities

SYNOPSIS

#include <signal.h>

struct sigaction {
int (∗sa_handler)();
sigset_t sa_mask;
int sa_flags;
};

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

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 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 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.

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.

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 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 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. Further, if the SA_ONSTACK bit is set in sa_flags, the system will deliver the signal to the process on a signal stack, specified with sigstack(2).

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(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.

Execve(2) 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>:

NAMEActionDescription
SIGHUPterminateterminal line hangup
SIGINTterminateinterrupt program
SIGQUITcorequit program
SIGILLcoreillegal instruction
SIGTRAPcoretrace trap
SIGIOTcoreabort(2) call (same as SIGABRT)
SIGEMTcoreemulate instruction executed
SIGFPEcorefloating-point exception
SIGKILLterminatekill program
SIGBUScorebus error
SIGSEGVcoresegmentation violation
SIGSYScoresystem call given invalid argument
SIGPIPEterminatewrite on a pipe with no reader
SIGALRMterminatereal-time timer expired
SIGTERMterminatesoftware termination signal
SIGURGdiscardurgent condition present on socket
SIGSTOPstopstop (cannot be caught or ignored)
SIGTSTPstopstop generated from keyboard
SIGCONTdiscardcontinue after stop
SIGCHLDdiscardchild status has changed
SIGTTINstopbackground read attempted on control terminal
SIGTTOUstopbackground write attemped to control terminal
SIGIOdiscardI/O is possible on a descriptor (see fcntl(2))
SIGXCPUterminatecpu time limit exceeded (see setrlimit(2))
SIGXFSZterminatefile size limit exceeded (see setrlimit(2))
SIGVTALRMterminatevirtual time alarm (see setitimer(2))
SIGPROFterminateprofiling timer alarm (see setitimer(2))
SIGWINCHdiscardWindow size change
SIGINFOdiscardstatus request from keyboard
SIGUSR1terminateUser defined signal 1
SIGUSR2terminateUser defined signal 2

NOTE

The mask specified in act is not allowed to block SIGKILL or SIGSTOP. This is done silently by the system.

RETURN VALUES

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

EXAMPLE

The handler routine can be declared:

int handler(sig, code, scp)
int sig, code;
struct sigcontext ∗scp;

Here sig is the signal number, into which the hardware faults and traps are mapped. Code is a parameter that is either a constant or the code provided by the hardware. Scp is a pointer to the sigcontext structure (defined in <signal.h>, used to restore the context from before the signal.

ERRORS

Sigaction will fail and no new signal 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 is made to ignore or supply a handler for SIGKILL or SIGSTOP.

STANDARDS

The sigaction function is defined by IEEE Std1003.1-1988 (“POSIX”). The SA_ONSTACK and SA_RESTART flags are Berkeley extensions, as are the signals, SIGTRAP, SIGEMT, SIGBUS, SIGSYS, SIGURG, SIGIO, SIGXCPU, SIGXFSZ, SIGVTALRM, SIGPROF, SIGWINCH, and SIGINFO. Those signals are available on most BSD−derived systems.

BUGS

The networking related syscalls are not properly restarted in 2.11BSD. The SIGINFO signal is not implemented in 2.11BSD.

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