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 invoca‐
tion 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 refer‐
ence to an unmapped, protected, or bad memory can generate SIGSEGV or
SIGBUS; a floating point exception can generate SIGFPE; and the execu‐
tion 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 asyn‐
chronously 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 disposi‐
tion is explicitly requested by a call to either sigac‐
tion(), 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 sigac‐
tion(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 sig‐
nal. 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 asyn‐
chronously 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 cur‐
rently 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), terminal
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 gen‐
erated 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 orig‐
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
SIGINFO 41 Ignore Status request
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 per‐
mit 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 the
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:
When it gets the signal, the receiving process is to be ter‐
minated with all the consequences outlined in exit(2).
When it gets the signal, the receiving process is to be ter‐
minated with all the consequences outlined in exit(2). In
addition, a ``core image'' of the process is constructed in
the current working directory.
When it gets the signal, the receiving process is to stop.
When a process is stopped, all the threads within the process
also stop executing.
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 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 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 expira‐
tion, asynchronous I/O completion, inter-process message arrival, and
the sigqueue(3C) 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
void(*)(union sigval) sigev_notify_function Notification function
(pthread_attr_t *) sigev_notify_attributes Notification attributes
The sigval union is defined by <signal.h>and contains at least the fol‐
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:
No asynchronous notification is delivered when the
event of interest occurs.
A queued signal, with its value equal to sigev_signo,
is generated when the event of interest occurs.
The sigev_notify_function is called, with sigev_value
as its argument, to perform notification when the asyn‐
chronous event occurs. The function is executed in an
environment as if it were the start routine for a newly
created thread with thread attributes
sigev_notify_attributes. If sigev_notify_attributes is
NULL, the thread runs as a detached thread with default
attributes. Otherwise, the thread runs with the speci‐
fied attributes, but as a detached thread regardless.
The thread runs with all blockable signals blocked.
An asynchronous notification is delivered to an event
port when the event of interest occurs. The
sigev_value.sival_ptr member points to a port_notify_t
structure defined in <port.h> (see port_associate(3C)).
The event port identifier as well as an application-
defined cookie are part of the port_notify_t structure.
The sigev_signo member contains the application-defined value to be
passed to the signal-catching function (for notification type
SIGEV_SIGNAL) at the time of the signal delivery as the si_value member
of the siginfo_t structure, or as the argument to the notification
function (for notification type SIGEV_THREAD) that is called when the
asynchronous event occurs. For notification type SIGEV_PORT,
sigev_value.sival_ptr points to a port_notify_t structure that speci‐
fies the port and an application-defined cookie.
The sigev_value member references the application defined value to be
passed to the signal-catching function at the time of the signal deliv‐
ery 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(3C) 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 │ Committed │
│Standard │ See standards(5). │
SEE ALSOlockd(1M), Intro(2), alarm(2), exit(2), fcntl(2), getrlimit(2),
ioctl(2), kill(2), pause(2), setrctl(2), sigaction(2), sigaltstack(2),
sigprocmask(2), sigsend(2), sigsuspend(2), sigwait(2), port_asso‐
ciate(3C), pthread_create(3C), pthread_kill(3C), pthread_sigmask(3C),
setjmp(3C), siginfo.h(3HEAD), signal(3C), sigqueue(3C), sigsetops(3C),
ucontext.h(3HEAD), wait(3C), attributes(5), standards(5)NOTES
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 sys‐
tem 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(3C).
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 dispo‐
sition, 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
Signals which are generated synchronously should not be masked. If such
a signal is blocked and delivered, the receiving process is killed.
Jan 4, 2014 SIGNAL.H(3HEAD)