signal(3HEAD)

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | NOTES

NAME

signal– base signals

SYNOPSIS

#include <signal.h>

DESCRIPTION

A signal is an asynchronous notification of an event. A signal is said to be generated for (or sent to) a process when the event associated with that signal first occurs. Examples of such events include hardware faults, timer expiration and terminal activity, as well as the invocation of the kill(2) or sigsend(2) functions. In some circumstances, the same event generates signals for multiple processes. A process may request a detailed notification of the source of the signal and the reason why it was generated. See siginfo(3HEAD).

Signals can be generated synchronously or asynchronously. Events directly caused by the execution of code by a thread, such as a reference to an unmapped, protected, or bad memory can generate SIGSEGV or SIGBUS; a floating point exception can generate SIGFPE; and the execution of an illegal instruction can generate SIGILL. Such events are referred to as traps; signals generated by traps are said to be synchronously generated. Synchronously generated signals are initiated by a specific thread and are delivered to and handled by that thread.

Signals may also be generated by calling kill(), sigqueue(), or sigsend(). Events such as keyboard interrupts generate signals, such as SIGINT, which are sent to the target process. Such events are referred to as interrupts; signals generated by interrupts are said to be asynchronously generated. Asynchronously generated signals are not directed to a particular thread but are handled by an arbitrary thread that meets either of the following conditions:

• The thread is blocked in a call to sigwait(2) whose argument includes the type of signal generated.

• The thread has a signal mask that does not include the type of signal generated. A process responds to signals in similar ways whether it is using threads or it is using lightweight processes (LWPs). See thr_create(3THR). Each process may specify a system action to be taken in response to each signal sent to it, called the signal's disposition. All threads or LWPs in the process share the disposition. The set of system signal actions for a process is initialized from that of its parent. Once an action is installed for a specific signal, it usually remains installed until another disposition is explicitly requested by a call to either sigaction(), signal() or sigset(), or until the process execs(). See sigaction(2) and signal(3C). When a process execs, all signals whose disposition has been set to catch the signal will be set to SIG_DFL. Alternatively, a process may request that the system automatically reset the disposition of a signal to SIG_DFL after it has been caught. See sigaction(2) and signal(3C).

SIGNAL DELIVERY

A signal is said to be delivered to a process when a thread or LWP within the process takes the appropriate action for the disposition of the signal. Delivery of a signal can be blocked. There are two methods for handling delivery of a signal in a multithreaded application. The first method specifies a signal handler function to execute when the signal is received by the process. See sigaction(2). The second method creates a thread to handle the receipt of the signal sigaction() can be used for both synchronously and asynchronously generated signals. sigwait() will only work for asynchronously generated signals, as synchronously generated signals are sent to the thread that caused the event. sigwait() is the recommended interface for use with a multithreaded application. See sigwait(2).

SIGNAL MASK

Each thread or LWP has a signal mask that defines the set of signals currently blocked from delivery to it. The signal mask of the main thread or LWP is inherited from the signal mask of the thread or LWP that created it in the parent process. The selection of the thread or LWP within the process that is to take the appropriate action for the signal is based on the method of signal generation and the signal masks of the threads or LWPs in the receiving process. Signals that are generated by action of a particular thread or LWP such as hardware faults are delivered to the thread or LWP that caused the signal. See thr_sigsetmask(3THR) or sigprocmask(2). See alarm(2) for current semantics of delivery of SIGALRM. Signals that are directed to a particular thread or LWP are delivered to the targeted thread or LWP. See thr_kill(3THR) or _lwp_kill(2). If the selected thread or LWP has blocked the signal, it remains pending on the thread or LWP until it is unblocked. For all other types of signal generation (for example, kill(2), sigsend(2), terminal activity, and other external events not ascribable to a particular thread or LWP) one of the threads or LWPs that does not have the signal blocked is selected to process the signal. If all the threads or LWPs within the process block the signal, it remains pending on the process until a thread or LWP in the process unblocks it. If the action associated with a signal is set to ignore the signal then both currently pending and subsequently generated signals of this type are discarded immediately for this process.

The determination of which action is taken in response to a signal is made at the time the signal is delivered to a thread or LWP within the process, allowing for any changes since the time of generation. This determination is independent of the means by which the signal was originally generated.

The signals currently defined by <signal.h> are as follows:

Name	Value	Default	Event
SIGHUP	1	Exit	Hangup (see termio(7I))
SIGINT	2	Exit	Interrupt (see termio(7I))
SIGQUIT	3	Core	Quit (see termio(7I))
SIGILL	4	Core	Illegal Instruction
SIGTRAP	5	Core	Trace or Breakpoint Trap
SIGABRT	6	Core	Abort
SIGEMT	7	Core	Emulation Trap
SIGFPE	8	Core	Arithmetic Exception
SIGKILL	9	Exit	Killed
SIGBUS	10	Core	Bus Error
SIGSEGV	11	Core	Segmentation Fault
SIGSYS	12	Core	Bad System Call
SIGPIPE	13	Exit	Broken Pipe
SIGALRM	14	Exit	Alarm Clock
SIGTERM	15	Exit	Terminated
SIGUSR1	16	Exit	User Signal 1
SIGUSR2	17	Exit	User Signal 2
SIGCHLD	18	Ignore	Child Status Changed
SIGPWR	19	Ignore	Power Fail or Restart
SIGWINCH	20	Ignore	Window Size Change
SIGURG	21	Ignore	Urgent Socket Condition
SIGPOLL	22	Exit	Pollable Event (see streamio(7I))
SIGSTOP	23	Stop	Stopped (signal)
SIGTSTP	24	Stop	Stopped (user) (see termio(7I))
SIGCONT	25	Ignore	Continued
SIGTTIN	26	Stop	Stopped (tty input) (see termio(7I))
SIGTTOU	27	Stop	Stopped (tty output) (see termio(7I))
SIGVTALRM	28	Exit	Virtual Timer Expired
SIGPROF	29	Exit	Profiling Timer Expired
SIGXCPU	30	Core	CPU time limit exceeded (see getrlimit(2))
SIGXFSZ	31	Core	File size limit exceeded (see getrlimit(2))
SIGWAITING	32	Ignore	Concurrency signal reserved by threads library
SIGLWP	33	Ignore	Inter-LWP signal reserved by threads library
SIGFREEZE	34	Ignore	Check point Freeze
SIGTHAW	35	Ignore	Check point Thaw
SIGCANCEL	36	Ignore	Cancellation signal reserved by threads library
SIGRTMIN	*	Exit	First real time signal
(SIGRTMIN+1)	*	Exit	Second real time signal
. . .
(SIGRTMAX-1)	*	Exit	Second-to-last real time signal
SIGRTMAX	*	Exit	Last real time signal

The symbols SIGRTMIN through SIGRTMAX are evaluated dynamically in order to permit future configurability.

SIGNAL DISPOSITION

A process, using a signal(3C), sigset(3C) or sigaction(2) system call, may specify one of three dispositions for a signal: take the default action for the signal, ignore the signal, or catch the signal.

Default Action: SIG_DFL

A disposition of SIG_DFL specifies the default action. The default action for each signal is listed in the table above and is selected from the following:

Exit

When it gets the signal, the receiving process is to be terminated with all the consequences outlined in exit(2).

Core

When it gets the signal, the receiving process is to be terminated with all the consequences outlined in exit(2). In addition, a ``core image'' of the process is constructed in the current working directory.

Stop

When it gets the signal, the receiving process is to stop. When a process is stopped, all the threads and LWPs within the process also stop executing.

Ignore

When it gets the signal, the receiving process is to ignore it. This is identical to setting the disposition to SIG_IGN.

Ignore Signal: SIG_IGN

A disposition of SIG_IGN specifies that the signal is to be ignored. Setting a signal action to SIG_IGN for a signal that is pending causes the pending signal to be discarded, whether or not it is blocked. Any queued values pending are also discarded, and the resources used to queue them are released and made available to queue other signals.

Catch Signal: function address

A disposition that is a function address specifies that, when it gets the signal, the thread or LWP within the process that is selected to process the signal will execute the signal handler at the specified address. Normally, the signal handler is passed the signal number as its only argument; if the disposition was set with the sigaction() however, additional arguments may be requested (see sigaction(2)). When the signal handler returns, the receiving process resumes execution at the point it was interrupted, unless the signal handler makes other arrangements. If an invalid function address is specified, results are undefined.

If the disposition has been set with the sigset() or sigaction(), the signal is automatically blocked in the thread or LWP while it is executing the signal catcher. If a longjmp() is used to leave the signal catcher, then the signal must be explicitly unblocked by the user. See setjmp(3C), signal(3C) and sigprocmask(2).

If execution of the signal handler interrupts a blocked function call, the handler is executed and the interrupted function call returns -1 to the calling process with errno set to EINTR. However, if the SA_RESTART flag is set, the function call will be transparently restarted.

Some signal-generating functions, such as high resolution timer expiration, asynchronous I/O completion, inter-process message arrival, and the sigqueue(3RT) function, support the specification of an application defined value, either explicitly as a parameter to the function, or in a sigevent structure parameter. The sigevent structure is defined by <signal.h> and contains at least the following members:

Member	Member
Type	Name	Description
int	sigev_notify	Notification type
int	sigev_signo	Signal number
union sigval	sigev_value	Signal value

The sigval union is defined by <signal.h> and contains at least the following members:

Member	Member
Type	Name	Description
int	sival_int	Integer signal value
void *	sival_ptr	Pointer signal value

The sigev_notify member specifies the notification mechanism to use when an asynchronous event occurs. The sigev_notify member may be defined with the following values:

SIGEV_NONE

No asynchronous notification is delivered when the event of interest occurs.

SIGEV_SIGNAL

A queued signal, with its value application-defined, is generated when the event of interest occurs.

Your implementation may define additional notification mechanisms.

The sigev_signo member specifies the signal to be generated.

The sigev_value member references the application defined value to be passed to the signal-catching function at the time of the signal delivery as the si_value member of the siginfo_t structure.

The sival_int member is used when the application defined value is of type int, and the sival_ptr member is used when the application defined value is a pointer.

When a signal is generated by sigqueue(3RT) or any signal-generating function which supports the specification of an application defined value, the signal is marked pending and, if the SA_SIGINFO flag is set for that signal, the signal is queued to the process along with the application specified signal value. Multiple occurrences of signals so generated are queued in FIFO order. If the SA_SIGINFO flag is not set for that signal, later occurrences of that signal's generation, when a signal is already queued, are silently discarded.

SEE ALSO

intro(2), _lwp_kill(2), _lwp_sigredirect(2), _signotifywait(2), alarm(2), exit(2), getrlimit(2), ioctl(2), kill(2), pause(2), sigaction(2), sigaltstack(2), sigprocmask(2), sigsend(2), sigsuspend(2), sigwait(2), wait(2), setjmp(3C), signal(3C), sigqueue(3RT), sigsetops(3C), thr_create(3THR), thr_kill(3THR), thr_sigsetmask(3THR), siginfo(3HEAD), ucontext(3HEAD)

NOTES

The dispositions of the SIGKILL and SIGSTOP signals cannot be altered from their default values. The system generates an error if this is attempted.

The SIGKILL and SIGSTOP signals cannot be blocked. The system silently enforces this restriction.

Whenever a process receives a SIGSTOP, SIGTSTP, SIGTTIN, or SIGTTOU signal, regardless of its disposition, any pending SIGCONT signal are discarded.

Whenever a process receives a SIGCONT signal, regardless of its disposition, any pending SIGSTOP, SIGTSTP, SIGTTIN, and SIGTTOU signals is discarded. In addition, if the process was stopped, it is continued.

SIGPOLL is issued when a file descriptor corresponding to a STREAMS file has a “selectable” event pending. See intro(2). A process must specifically request that this signal be sent using the I_SETSIG ioctl call. Otherwise, the process will never receive SIGPOLL.

If the disposition of the SIGCHLD signal has been set with signal or sigset, or with sigaction and the SA_NOCLDSTOP flag has been specified, it will only be sent to the calling process when its children exit; otherwise, it will also be sent when the calling process's children are stopped or continued due to job control.

The name SIGCLD is also defined in this header and identifies the same signal as SIGCHLD. SIGCLD is provided for backward compatibility, new applications should use SIGCHLD.

The disposition of signals that are inherited as SIG_IGN should not be changed.

A signal directed by kill(2), sigqueue(3RT), sigsend(2), terminal activity, and other external events not ascribable to a particular thread or LWP, such as the SIGXFSZ or SIGPIPE signal, to a multithreaded process, that is, a process linked with -lthread or -lpthread, is routed to this process through a special, designated LWP within this process, called the Asynchronous Signal LWP (ASLWP). The ASLWP within the multi-threaded process receives notification of any signal directed to this process. Upon receiving this notification, the ASLWP forwards it to a thread within the process that has the signal unmasked. Actual signal delivery to the thread occurs only when the thread is running on an LWP. If no threads exist having that signal number unblocked, the signal remains pending. The ASLWP is usually blocked in a call to _signotifywait(2), waiting for such notifications. The eventual target thread receives the signal by way of a call to _lwp_sigredirect(2), made either by the ASLWP or the thread itself, redirecting the signal to the LWP that the target thread is running on.

Signals which are generated synchronously should not be masked. If such a signal is blocked and delivered, the receiving process is killed.

NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | NOTES