signal(2) signal(2)
NAME
signal, sigset, sighold, sigrelse, sigignore, sigpause -signal
management
SYNOPSIS
#include <signal.h>
void (*signal(int sig, void (*func)(int)))(int);
int sighold(int sig);
int sigignore(int sig);
int sigpause(int sig);
int sigrelse(int sig);
void (*sigset(int sig, void (*disp)(int)))(int);
DESCRIPTION
The signal() 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 SIG_DFL, default handling for that signal will occur. If the
value of func is SIG_IGN, 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 a:
signal(sig, SIG_DFL);
is executed or an implementation-dependent blocking of the signal is
performed. (If the value of sig is SIGILL, whether the reset to
SIG_DFL occurs is implementation-dependent.) Next the equivalent of:
(*func)(sig);
is executed. The func function may terminate by executing a return
statement or by calling abort(), exit(), or longjmp(). If func()
executes a return statement and the value of sig was SIGFPE or any
other implementation-dependent value corresponding to a computational
exception, the behaviour 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 abort(),
kill() or raise(), the behaviour is undefined if the signal handler
calls any function in the standard library other than one of the
functions listed on the sigaction() page 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,
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if such a call fails, the value of errno is indeterminate.
At program startup, the equivalent of:
signal(sig, SIG_IGN);
is executed for some signals, and the equivalent of:
signal(sig, SIG_DFL);
is executed for all other signals (see exec).
The sigset(), sighold(), sigignore(), sigpause() and segrelse()
functions provide simplified signal management.
The sigset() function is used to modify signal dispositions. The sig
argument specifies the signal, which may be any signal except SIGKILL
and SIGSTOP. The disp argument specifies the signal's disposition,
which may be SIG_DFL, SIG_IGN or the address of a signal handler. If
sigset() is used, and disp is the address of a signal handler, the
system will add sig to the calling process' signal mask before
executing the signal handler; when the signal handler returns, the
system will restore the calling process' signal mask to its state
prior the delivery of the signal. In addition, if sigset() is used,
and disp is equal to SIG_HOLD, sig will be added to the calling
process' signal mask and sig's disposition will remain unchanged. If
sigset() is used, and disp is not equal to SIG_HOLD, sig will be
removed from the calling process' signal mask.
The sighold() function adds sig to the calling process' signal mask.
The sigrelse() function removes sig from the calling process' signal
mask.
The sigignore() function sets the disposition of sig to SIG_IGN.
The sigpause() function removes sig from the calling process' signal
mask and suspends the calling process until a signal is received.
If the action for the SIGCHLD signal is set to SIG_IGN, 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 wait(), wait3(), waitid() and waitpid()
will fail and set errno to ECHILD.
RETURN VALUE
If the request can be honoured, signal() returns the value of func()
for the most recent call to signal() for the specified signal sig.
Otherwise, SIG_ERR is returned and a positive value is stored in
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errno.
Upon successful completion, sigset() returns SIG_HOLD if the signal
had been blocked and the signal's previous disposition if it had not
been blocked. Otherwise, SIG_ERR is returned and errno is set to
indicate the error.
For all other functions, upon successful completion, 0 is returned.
Otherwise, -1 is returned and errno is set to indicate the error.
ERRORS
The signal() function will fail if:
[EINVAL] 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 signal() function may fail if:
[EINVAL] An attempt was made to set the action to SIG_DFL
for a signal that cannot be caught or ignored (or
both).
The sigset(), sighold(), sigrelse(), sigignore(), and sigpause()
functions will fail if:
[EINVAL] The sig argument is an illegal signal number.
The sigset(), and sigignore() functions will fail if:
[EINVAL] An attempt is made to catch a signal that cannot
be caught, or to ignore a signal that cannot be
ignored.
APPLICATION USAGE
The sigaction() function provides a more comprehensive and reliable
mechanism for controlling signals; new applications should use
sigaction() rather than signal().
The sighold() function, in conjunction with sigrelse() or sigpause(),
may be used to establish critical regions of code that require the
delivery of a signal to be temporarily deferred.
The sigsuspend() function should be used in preference to sigpause()
for broader portability.
SEE ALSO
exec, pause(), sigaction(), waitid(), .
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signal(2) signal(2)
CHANGE HISTORY
First released in Issue 1.
Derived from Issue 1 of the SVID.
Issue 4
The following changes are incorporated for alignment with the ISO C
standard:
- The function is no longer marked as an extension.
- The argument int is added to the definition of func()
in the SYNOPSIS section.
- In Issue 3, this interface cross-referred to
sigaction(). This issue provides a complete
description of the function as defined in ISO C
standard.
Another change is incorporated as follows:
- The APPLICATION USAGE section is added.
Issue 4, Version 2
The following changes are incorporated for X/OPEN UNIX conformance:
- The sighold(), sigignore(), sigpause(), sigrelse(), and
sigset() functions are added to the SYNOPSIS.
- The DESCRIPTION is updated to describe semantics of the
above interfaces.
- Additional text is added to the RETURN VALUE section to
describe possible returns from the sigset() function
specifically, and all of the above functions in
general.
- The ERRORS section is restructured to describe possible
error returns from each of the above functions
individually.
- The APPLICATION USAGE section is updated to describe
certain programming considerations associated with the
X/OPEN UNIX functions.
- 4 - Formatted: July 18, 2000
signal(2) signal(2)
HP-UX EXTENSIONS
SYNOPSIS
void (*signal(int sig, void (*action)(int)))(int);
void (*sigset(int sig, void (*func)(int)))(int);
DESCRIPTION
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(2), and sigspace(2).
Acceptable values for sig are defined in <signal.h>.
SIG_DFL 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 SIG_DFL but the default
action of the pending signal is to ignore the signal
(as in the case of SIGCLD).
SIG_IGN Ignore the signal.
When signal() is called with action set to SIG_IGN
and an instance of the signal sig is pending, the
pending signal is discarded, whether or not it is
blocked.
SIGKILL and SIGSTOP signals cannot be ignored.
address Catch the signal.
Upon receipt of signal sig, reset the value of action
for the caught signal to SIG_DFL (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.
The signal-catching function is called with the
following three parameters:
sig The signal number.
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code A word of information usually provided
by the hardware.
scp A pointer to the machine-dependent
structure sigcontext defined in
<signal.h>.
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
SIGILL and SIGFPE. For SIGILL, code has the
following values:
8 illegal instruction trap;
9 break instruction trap;
10 privileged operation trap;
11 privileged register trap.
For SIGFPE, code has the following values:
12 overflow trap;
13 conditional trap;
14 assist exception trap;
22 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 isinf(3M).
The signals SIGKILL and SIGSTOP cannot be caught.
sigset() allows the calling process to choose one of four ways to
handle the receipt of a specific signal. sig specifies the
signal and func specifies the choice.
sig can be any one of the signals described under signal(5)
except SIGKILL or SIGSTOP.
func is assigned one of four values: SIG_DFL, SIG_IGN, SIG_HOLD,
or a function address. The actions prescribed by SIG_DFL and
SIG_IGN are described under signal(5). The action prescribed by
SIG_HOLD and function address are described below:
SIG_HOLD Hold signal. The signal sig is held
upon receipt. Any pending signal of
this signal type remains held. Only
one signal of each type is held.
Note: the signals SIGKILL, SIGCONT,
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signal(2) signal(2)
and SIGSTOP cannot be held.
function address
Catch signal. func must be a pointer to a function,
the signal- catching handler, that is called when
signal sig occurs. sigset() specifies that the process
calls this function upon receipt of signal sig. 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 function pointed to by
funcasdescribedunder signal(5) with the following
differences:
Before calling the signal-catching handler, the system
signal action of sig is set to SIG_HOLD. During a normal
return from the signal-catching handler, the system signal
action is restored to func and any held signal of this type
is released. If a non-local goto (longjmp(3C)) is taken,
sigrelse() must be called to restore the system signal
action to func and release any held signal of this type.
sighold() holds the signal sig. sigrelse() restores the system
signal action of sig to that specified previously by sigset().
sighold() and sigrelse() are used to establish critical regions
of code. sighold() is analogous to raising the priority level and
deferring or holding a signal until the priority is lowered by
sigrelse().
sigignore() sets the action for signal sig to SIG_IGN (see
signal(5)).
sigpause() suspends the calling process until it receives an
unblocked signal. If the signal sig is held, it is released
before the process pauses. sigpause() is useful for testing
variables that are changed when a signal occurs. For example,
sighold() should be used to block the signal first, then test the
variables. If they have not changed, call sigpause() to wait for
the signal.
These functions can be linked into a program by giving the -lV3
option to the ld command (see ld(1)).
ERRORS
sigset() fails and the system signal action for sig is not changed if
any of the following occur:
[EFAULT] The func argument points to memory that
is not a valid part of the process
address space. Reliable detection of
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this error is implementation-dependent.
sigset(), sighold(), sigrelse(), sigignore(), and sigpause() fail and
the system signal action for sig is not changed if any of the
following occur:
[EINVAL] An attempt is made to ignore, hold, or
supply a handler for a signal that
cannot be ignored, held, or caught; see
signal(5).
sigpause returns when the following occurs:
[EINTR] A signal was caught.
EXAMPLES
The following call to signal() sets up a signal-catching function for
the SIGINT signal:
void myhandler();
(void) signal(SIGINT, myhandler);
WARNINGS
signal() should not be used in conjunction with the facilities
described under bsdproc(2), sigaction(2), sigset(2V), or sigvector(2).
signal() does not detect an invalid value for action, and if it does
not equal SIG_DFL or SIG_IGN, or point to a valid function address,
subsequent receipt of the signal sig causes undefined results.
AUTHOR
signal() was developed by HP, AT&T, and the University of California,
Berkeley.
SEE ALSO
kill(1), init(1M), exit(2), kill(2), lseek(2), pause(2), sigaction(2),
sigvector(2), wait(2), abort(3C), setjmp(3C), signal(5).
STANDARDS CONFORMANCE
signal(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, ANSI C
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