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signal(5)

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 or sigsend system calls. In some circumstances, the same event generates signals for multiple processes. The receiver may request a detailed notification of the source of the signal and the reason why it was generated [see siginfo(5)].

Each process may specify a system action to be taken in response to each signal sent to it, called the signal's 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(2)]. When a process execs, all signals whose disposition has been set to catch the signal will be set to SIG_DFL. Alternatively, on request, the system will automatically reset the disposition of a signal to SIG_DFL after it has been caught [see sigaction(2) and signal(2)].

A signal is said to be delivered to a process when the appropriate action for the process and signal is taken. During the time between the generation of a signal and its delivery, the signal is said to be pending [see sigpending(2)]. Ordinarily, this interval cannot be detected by an application. However, a signal can be blocked from delivery [see signal(2) and sigprocmask(2)]. If the action associated with a blocked signal is anything other than to ignore the signal, and if that signal is generated for the process, the signal remains pending until either it is unblocked or the signal's disposition requests that the signal be ignored. If the signal disposition of a blocked signal requests that the signal be ignored, and if that signal is generated for the process, the signal is discarded immediately upon generation.

Each process has a signal mask that defines the set of signals currently blocked from delivery to it [see sigprocmask(2)]. The signal mask for a process is initialized from that of its creator.

The determination of which action is taken in response to a signal is made at the time the signal is delivered, 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 in sys/signal.h are as follows:

Name Value Default Event
SIGHUP 1 Exit Hangup [see termio(7)]
SIGINT 2 Exit Interrupt [see termio(7)]
SIGQUIT 3 Core Quit [see termio(7)]
SIGILL 4 Core Illegal Instruction
SIGTRAP 5 Core Trace/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
SIGPWR 19 Ignore Power Fail/Restart
SIGWINCH 20 Ignore Window Size Change
SIGURG 21 Ignore Urgent Socket Condition
SIGPOLL 22 Exit Pollable event
SIGSTOP 23 Stop Stopped (signal)
SIGTSTP 24 Stop Stopped (user) [see termio(7)]
SIGCONT 25 Ignore Continued
SIGTTIN 26 Stop Stopped (tty input) [see termio(7)]
SIGTTOU 27 Stop Stopped (tty output) [see termio(7)]
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 All LWPs blocked
SIGLWP 33 Ignore Virtual Interprocessor Interrupt
for Threads Library
SIGAIO 34 Ignore Asynchronous I/O 

           +-----------+-------+---------+-------------------------+
           |Name       | Value | Default | Event                   |
           +-----------+-------+---------+-------------------------+
           |SIGHUP     | 1     | Exit    | Hangup [see termio(7)   |
           +-----------+-------+---------+-------------------------+
           |SIGINT     | 2     | Exit    | Interrupt [see          |
           |           |       |         | termio(7)               |
           +-----------+-------+---------+-------------------------+
           |SIGQUIT    | 3     | Core    | Quit [see termio(7)     |
           +-----------+-------+---------+-------------------------+
           |SIGILL     | 4     | Core    | Illegal Instruction     |
           +-----------+-------+---------+-------------------------+
           |SIGTRAP    | 5     | Core    | Trace/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            |
           +-----------+-------+---------+-------------------------+
           |SIGPWR     | 19    | Ignore  | Power Fail/Restart      |
           +-----------+-------+---------+-------------------------+
           |SIGWINCH   | 20    | Ignore  | Window Size Change      |
           +-----------+-------+---------+-------------------------+
           |SIGURG     | 21    | Ignore  | Urgent Socket Condition |
           +-----------+-------+---------+-------------------------+
           |SIGPOLL    | 22    | Exit    | Pollable event          |
           +-----------+-------+---------+-------------------------+
           |SIGSTOP    | 23    | Stop    | Stopped (signal)        |
           +-----------+-------+---------+-------------------------+
           |SIGTSTP    | 24    | Stop    | Stopped (user) [see     |
           |           |       |         | termio(7)               |
           +-----------+-------+---------+-------------------------+
           |SIGCONT    | 25    | Ignore  | Continued               |
           +-----------+-------+---------+-------------------------+
           |SIGTTIN    | 26    | Stop    | Stopped (tty input)     |
           |           |       |         | [see termio(7)          |
           +-----------+-------+---------+-------------------------+
           |SIGTTOU    | 27    | Stop    | Stopped (tty output)    |
           |           |       |         | [see termio(7)          |
           +-----------+-------+---------+-------------------------+
           |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  | All LWPs blocked        |
           +-----------+-------+---------+-------------------------+
           |SIGLWP     | 33    | Ignore  | Virtual Interprocessor  |
           |           |       |         | Interrupt               |
           |           |       |         | for Threads Library     |
           +-----------+-------+---------+-------------------------+
           |SIGAIO     | 34    | Ignore  | Asynchronous I/O        |
           +-----------+-------+---------+-------------------------+

The signal, sigset or sigaction system calls, can be used to 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.

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.

Catch signal: function address

A disposition that is a function address specifies that, when it gets the signal, the receiving process is to 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 function 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 function, the signal is automatically blocked by the system while the signal catcher is executing. If a longjmp [see setjmp(3C)] is used to leave the signal catcher, then the signal must be explicitly unblocked by the user [see signal(2) and sigprocmask(2)].

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

Notices

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 is discarded. A process stopped by the above four signals is said to be in a job control stop.

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

SIGPOLL is issued when a file descriptor corresponding to a STREAMS [see intro(2)] file has a ``selectable'' event pending. 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.

For backward compatibility, the names SIGCLD, SIGIOT, and SIGIO are defined in this header file. SIGCLD identifies the same signal as SIGCHLD. SIGIOT identifies the same signal as SIGABRT, and SIGIO identifies the same signal as SIGPOLL. However, new applications should use SIGCHLD, SIGABRT, and SIGPOLL.

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

Considerations for threads programming

Signal disposition (that is, to default or to ignore or to trap by function a given signal type) is maintained at the process level and is shared by all threads. Signal masks, on the other hand, are maintained per thread.

Depending on circumstances (outlined below), caught signals are handled either by a specific thread or an arbitrary thread.


Synchronously Generated Signals
Signals that are initiated by a specific thread (for example, division by zero, a request for a SIGALRM signal, a reference to an invalid address) are delivered to and handled by that thread. (Note: that thread will use the common handler function currently defined for the containing process.)

Asynchronously Generated Signals
Signals that are not initiated by a specific thread (for example, a SIGINT signal from a terminal, a signal from another process via kill(2)) are handled by an arbitrary thread of the process that meets either of the following conditions.

  1. The thread is blocked in a sigwait(2)
    system call whose argument does include the type of the caught signal.

  2. The thread has a signal mask that does not include the type of the caught signal.

A caught signal will be delivered to only one thread of a process. Applications cannot predict which of several eligible threads will receive a caught signal. If this behavior is undesirable, applications should maintain only a single eligible thread per signal type.

Signal handling occurs only when a thread is scheduled to run. That latency can be reduced by having signals caught by (permanently) ``bound'' threads.

The Threads Library disallows changes to the disposition of SIGLWP and SIGWAITING signal types. Those signal types are important to the functioning of the Threads Library.

Considerations for lightweight processes

Internally, signal masks are maintained per LWP. When a thread is bound to an LWP for execution the Threads Library synchronizes the signal mask of the thread to that of the LWP. Thus, there is some additional overhead (a sigprocmask(2) system call) when switching in a multiplexed thread with a mask that differs from that of the preempted thread.

In addition to signal masks, the operating system can also define an alternate signal handling stack per LWP. The Threads Library does not support alternate signal handling stacks for threads.

References

exit(2), getrlimit(2), intro(2), kill(2), pause(2), sigaction(2), sigaltstack(2), siginfo(5), signal(2), sigprocmask(2), sigsend(2), sigsetops(3C), sigsuspend(2), ucontext(5), wait(2)
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