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 functions described in this reference page provide simplified signal management:

The signal() function chooses one of three ways in which receipt of the signal number sig is to be subsequently handled.
The sigset() function is used to modify signal dispositions.
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.

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), sigblock(2), sigsetmask(2), sigpause(3C), and sigspace(2).

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:

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(), raise(), pthread_kill(), or sigqueue(), the behaviour is undefined if the signal handler calls any function in the standard library other than one of the functions listed on the thread_safety(5) 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, 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).

Acceptable values for sig are described in signal(5).

Acceptable values for func are:

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.

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 <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.

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 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 to the delivery of the signal.

If disp is equal to SIG_HOLD, sig will be added to the calling process' signal mask and sig's disposition will remain unchanged.

If disp is not equal to SIG_HOLD, sig will be removed from the calling process' signal mask.

If disp is not SIG_DFL, SIG_IGN, or SIG_HOLD, 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 SIG_HOLD. 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 (longjmp(3C)) is taken, sigrelse() must be called to restore the system signal action to disp and release any held signal of this type.

For either signal(2) or sigset(3C), 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 honored, 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 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() function will fail if:

[EINVAL]: The sig argument is an illegal signal number.
[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).
[EFAULT]: 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 sighold(), sigrelse(), sigignore(), and sigpause() functions will fail if:

[EINVAL]: The sig argument is an illegal signal number.
[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).

The sigpause() returns when the following occurs:

[EINTR]: A signal was caught.

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.

The sigpause() function 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.

Threads Considerations

The signal disposition (such as catch/ignore/default) established by signal() is shared by all threads in the process. Blocked signal masks are maintained by each thread.

If signal() is used to set the signal disposition for sig to SIG_IGN or to SIG_DFL for a signal whose default action is to ignore the signal, any instances of sig pending on the process or any of the threads will be discarded. The signals are discarded regardless of whether the signal is blocked by any of the threads.

For more information regarding signals and threads, refer to signal(5).

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(3C), sigaction(2), or sigset(3C).

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.

STANDARDS CONFORMANCE

signal(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, ANSI C