signal.h, signal - base signals
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.h(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 syn-
chronously 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:
o The thread is blocked in a call to sigwait(2) whose argument includes the type of signal generated.
o The thread has a signal mask that does not include the type of signal generated. See pthread_sigmask(3C). Each process can specify a
system action to be taken in response to each signal sent to it, called the signal's disposition. All threads 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(), sig-
nal() or sigset(), or until the process execs(). See sigaction(2) and signal(3C). When a process execs, all signals whose disposi-
tion 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).
A signal is said to be delivered to a process when a thread 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
uses sigwait(2) to create a thread to handle the receipt of the signal. The sigaction() function can be used for both synchronously and
asynchronously generated signals. The sigwait() function will work only for asynchronously generated signals, as synchronously generated
signals are sent to the thread that caused the event. The sigwait() function is the recommended for use with a multithreaded application.
Each thread has a signal mask that defines the set of signals currently blocked from delivery to it. The signal mask of the main thread is
inherited from the signal mask of the thread that created it in the parent process. The selection of the thread 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 in the
receiving process. Signals that are generated by action of a particular thread such as hardware faults are delivered to the thread that
caused the signal. See pthread_sigmask(3C) or sigprocmask(2). See alarm(2) for current semantics of delivery of SIGALRM. Signals that are
directed to a particular thread are delivered to the targeted thread. See pthread_kill(3C). If the selected thread has blocked the signal,
it remains pending on the thread until it is unblocked. For all other types of signal generation (for example, kill(2), sigsend(2), ter-
minal activity, and other external events not ascribable to a particular thread) one of the threads that does not have the signal blocked
is selected to process the signal. If all the threads within the process block the signal, it remains pending on the process until a thread
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 within the
process, allowing for any changes since the time of generation. This determination is independent of the means by which the signal was
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 getr-
SIGXFSZ 31 Core File size limit exceeded (see getr-
SIGWAITING 32 Ignore Reserved
SIGLWP 33 Ignore Reserved
SIGFREEZE 34 Ignore Check point Freeze
SIGTHAW 35 Ignore Check point Thaw
SIGCANCEL 36 Ignore Reserved for threading support
SIGLOST 37 Exit Resource lost (for example, record-
SIGXRES 38 Ignore Resource control exceeded (see
SIGJVM1 39 Ignore Reserved for Java Virtual Machine 1
SIGJVM2 40 Ignore Reserved for Java Virtual Machine 2
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 to permit future configurability.
Applications should not use any of the signals marked "reserved" in the above table for any purpose, to avoid interfering with their use by
A process using a signal(3C), sigset(3C) or sigaction(2) system call can 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 within the process also 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. 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 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(2) function, however, additional arguments can be requested. When the signal han-
dler 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 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. If the SA_RESTART flag is set, however, certain function calls will be transparently
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:
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:
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.
SIGEV_PORT An asynchronous notification is delivered to an event port when the event of interest occurs. The sival_ptr member points
to a port_notify_t structure (see port_associate(3C)). The event port identifier as well as an application-defined cookie
are part of the port_notify_t structure.
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 attributes(5) for descriptions of the following attributes:
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
|Interface Stability |Standard |
lockd(1M), intro(2), alarm(2), exit(2), fcntl(2), getrlimit(2), ioctl(2), kill(2), pause(2), setrctl(2), sigaction(2), sigaltstack(2), sig-
procmask(2), sigsend(2), sigsuspend(2), sigwait(2), port_associate(3C), pthread_create(3C), pthread_kill(3C), pthread_sigmask(3C),
setjmp(3C), siginfo.h(3HEAD), signal(3C), sigqueue(3RT), sigsetops(3C), thr_create(3C), thr_kill(3C), thr_sigsetmask(3C), ucon-
text.h(3HEAD), wait(3C), attributes(5), standards(5)
The dispositions of the SIGKILL and SIGSTOP signals cannot be altered from their default values. The system generates an error if this is
The SIGKILL, SIGSTOP, and SIGCANCEL signals cannot be blocked. The system silently enforces this restriction.
The SIGCANCEL signal cannot be directed to an individual thread using pthread_kill(3C), but it can be sent to a process using kill(2),
sigsend(2), or sigqueue(3RT).
Whenever a process receives a SIGSTOP, SIGTSTP, SIGTTIN, or SIGTTOU signal, regardless of its disposition, any pending SIGCONT signal are
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 speci-
fied, it will only be sent to the calling process when its children exit; otherwise, it will also be sent when the calling process's chil-
dren 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.
Signals which are generated synchronously should not be masked. If such a signal is blocked and delivered, the receiving process is killed.
20 Oct 2003 signal.h(3HEAD)