signal(2) System Calls Manual signal(2)
NAME
signal(), sigset(), sighold(), sigrelse(), sigignore(), sigpause() - signal management
SYNOPSIS
DESCRIPTION
The functions described in this reference page provide simplified signal management:
o The function chooses one of three ways in which receipt of the signal number sig is to be subsequently handled.
o The function is used to modify signal dispositions.
o The function adds sig to the calling process' signal mask.
o The function removes sig from the calling process' signal mask.
o The function sets the disposition of sig to
o The function removes sig from the calling process' signal mask and suspends the calling process until a signal is received.
The system defines a set of signals that can be delivered to a process. The set of signals is defined in signal(5), along with the meaning
and side effects of each signal. An alternate mechanism for handling these signals is defined here. The facilities described here should
not be used in conjunction with the other facilities described under signal(2), sigvector(2), sigblock(2), sigsetmask(2), sigpause(3C), and
sigspace(2).
The function chooses one of three ways in which receipt of the signal number sig is to be subsequently handled. If the value of func is
default handling for that signal will occur. If the value of func is the signal will be ignored. Otherwise, func must point to a function
to be called when that signal occurs. Such a function is called a signal handler.
When a signal occurs, if func points to a function, first the equivalent of:
is executed or an implementation-dependent blocking of the signal is performed. (If the value of sig is whether the reset to occurs is
implementation-dependent.) Next the equivalent of:
is executed. The func function may terminate by executing a return statement or by calling or If executes a return statement and the value
of sig was or any other implementation-dependent value corresponding to a computational exception, the behavior is undefined. Otherwise,
the program will resume execution at the point it was interrupted.
If the signal occurs other than as the result of calling or the behavior is undefined if the signal handler calls any function in the stan-
dard library other than one of the functions listed on the thread_safety(5) manpage or refers to any object with static storage duration
other than by assigning a value to a static storage duration variable of type volatile sig_atomic_t. Furthermore, if such a call fails,
the value of is indeterminate.
At program startup, the equivalent of:
is executed for some signals, and the equivalent of:
is executed for all other signals (see
Acceptable values for sig are described in signal(5).
Acceptable values for func are:
Execute the default action, which varies depending on the signal.
The default action for most signals is to terminate the process (see signal(5)).
A pending signal is discarded (whether or not it is blocked) if action is set to but the default action of the pending
signal is to ignore the signal (as in the case of
Ignore the signal.
When is called with action set to and an instance of the signal sig is pending, the pending signal is discarded, whether
or not it is blocked.
and signals cannot be ignored.
address Catch the signal.
Upon receipt of signal sig, reset the value of action for the caught signal to (except signals marked with "not reset
when caught"; see signal(5)), call the signal-catching function to which address points, and resume executing the
receiving process at the point where it was interrupted.
In HP-UX, the signal-catching function is called with the following three parameters:
sig The signal number.
code A word of information usually provided by the hardware.
scp A pointer to the machine-dependent structure sigcontext defined in
The pointer scp is valid only during the context of the signal-catching function. The structure pointer scp is always
defined.
The code word is always zero for all signals except and For code has the following values:
illegal instruction trap;
break instruction trap;
privileged operation trap;
privileged register trap.
For code has the following values:
overflow trap;
conditional trap;
assist exception trap;
assist emulation trap.
As defined by the IEEE POSIX Standard, HP-UX does not raise an exception on floating-point divide by zero. The result
of floating-point divide by zero is infinity which can be checked by
The signals and cannot be caught.
The function is used to modify signal dispositions.
The sig argument specifies the signal, which may be any signal except and
The disp argument specifies the signal's disposition, which may be or the address of a signal handler.
If disp is the address of a signal handler, the system will add sig to the calling process' signal mask before executing the signal han-
dler. When the signal handler returns, the system will restore the calling process' signal mask to its state prior to the delivery of the
signal.
If disp is equal to sig will be added to the calling process' signal mask and sig's disposition will remain unchanged.
If disp is not equal to sig will be removed from the calling process' signal mask.
If disp is not or it must be a pointer to a function, the signal-catching handler, that is called when signal sig occurs. Before calling
the signal-catching handler, the system signal action of sig is set to Any pending signal of this type is released. This handler address
is retained across calls to the other signal management functions listed here. Upon receipt of signal sig, the receiving process executes
the signal-catching handler pointed to by disp.
During a normal return from the signal-catching handler, the system signal action is restored to disp and any held signal of this type is
released. If a non-local goto is taken, must be called to restore the system signal action to disp and release any held signal of this
type.
For either or if the action for the signal is set to child processes of the calling processes will not be transformed into zombie processes
when they terminate. If the calling process subsequently waits for its children, and the process has no unwaited for children that were
transformed into zombie processes, it will block until all of its children terminate, and and will fail and set to
RETURN VALUE
If the request can be honored, returns the value of for the most recent call to for the specified signal sig. Otherwise, is returned and a
positive value is stored in
Upon successful completion, returns if the signal had been blocked and the signal's previous disposition if it had not been blocked. Oth-
erwise, is returned and is set to indicate the error.
For all other functions, upon successful completion, 0 is returned. Otherwise, -1 is returned and is set to indicate the error.
ERRORS
The function will fail if:
The sig argument is not a valid signal number or an attempt is made to catch a signal that cannot be caught or ignore a
signal that cannot be ignored.
The function may fail if:
An attempt was made to set the action to
for a signal that cannot be caught or ignored (or both).
The function will fail if:
The func argument points to memory that is not a valid part of the process address space. Reliable detection of this
error is implementation-dependent.
The sig argument is an illegal signal number.
An attempt is made to ignore, hold, or supply a handler
for a signal that cannot be ignored, held, or caught; see signal(5).
The and functions will fail if:
The sig argument is an illegal signal number.
An attempt is made to ignore, hold, or supply a handler
for a signal that cannot be ignored, held, or caught; see signal(5).
The returns when the following occurs:
A signal was caught.
APPLICATION USAGE
The function provides a more comprehensive and reliable mechanism for controlling signals; new applications should use rather than
The function, in conjunction with or may be used to establish critical regions of code that require the delivery of a signal to be tempo-
rarily deferred.
The function should be used in preference to for broader portability.
The function suspends the calling process until it receives an unblocked signal. If the signal sig is held, it is released before the
process pauses. is useful for testing variables that are changed when a signal occurs. For example, should be used to block the signal
first, then test the variables. If they have not changed, call to wait for the signal.
Threads Considerations
The signal disposition (such as catch/ignore/default) established by is shared by all threads in the process. Blocked signal masks are
maintained by each thread.
The use of this function is unspecified in a multithreaded process.
For more information regarding signals and threads, refer to signal(5).
EXAMPLES
The following call to sets up a signal-catching function for the signal:
WARNINGS
should not be used in conjunction with the facilities described under bsdproc(3C), sigaction(2), sigset(3C), or sigvector(2).
does not detect an invalid value for action, and if it does not equal or or point to a valid function address, subsequent receipt of the
signal sig causes undefined results.
AUTHOR
was developed by HP, AT&T, and the University of California, Berkeley.
SEE ALSO
kill(1), init(1M), exec(2), exit(2), kill(2), lseek(2), pause(2), sigaction(2), sigvector(2), wait(2), waitid(2), abort(3C), setjmp(3C),
signal(5).
STANDARDS CONFORMANCE
signal(2)