NAME

signal − specify what to do upon receipt of a signal

SYNOPSIS

#include <signal.h>

int (∗signal (sig, func))()
int sig;
int (∗func)(); func(sig [, code, scp ] )
int sig, code;
struct sigcontext ∗scp;

DESCRIPTION

Signal allows the calling process to choose one of three ways to handle the receipt of a specific signal.  Sig specifies the signal and func specifies the choice. 

Sig can be assigned any one of the following except SIGKILL or SIGSTOP:

SIGHUP01hangup
SIGINT02interrupt
SIGQUIT03∗quit
SIGILL04∗•illegal instruction
SIGTRAP05∗•trace trap
SIGIOT06∗software generated (sent by abort(3C))
SIGEMT07∗software generated
SIGFPE08∗floating point exception
SIGKILL09‡+%kill
SIGBUS10∗bus error
SIGSEGV11∗segmentation violation
SIGSYS12∗bad argument to system call
SIGPIPE13write on a pipe with no one to read it
SIGALRM14alarm clock
SIGTERM15software termination signal
SIGUSR116user defined signal 1
SIGUSR217user defined signal 2
SIGCLD18†death of a child (see WARNING below)
SIGPWR19•†power fail (see WARNING below)
SIGVTALRM20virtual timer alarm; see getitimer(2)
SIGPROF21profiling timer alarm; see getitimer(2)
SIGIO22†asynchronous I/O signal; see select(2)
SIGWINDOW23†window change or mouse signal; see windowing package
SIGSTOP24‡+#stop
SIGTSTP25#stop signal generated from keyboard
SIGCONT26‡†%continue after stop
SIGTTIN27#background read attempted from control terminal
SIGTTOU28#background write attempted to control terminal
SIGURG29†urgent data arrived on an I/O channel

∗ Indicates that a core dump can be generated. 

† Indicates that the action on SIG_DFL is to ignore the signal, rather than terminate the process. 

# Indicates that the action on SIG_DFL is to stop rather than terminate the process. 

% Indicates that the signal will not be held off by a stopped process. 

• Indicates that the signal is not reset when it is caught by signal.

‡ Indicates that the signal cannot be ignored. 

+ Indicates that the signal cannot be caught. 

See below for details. 

Func is assigned one of three values: SIG_DFL, SIG_IGN, or a function address. The actions prescribed by these values are as follows:

SIG_DFL − (usually) terminate process upon receipt of a signal. 
For those signals not flagged with a dagger († ) or a pound (#) above, upon receipt of the signal sig, the receiving process is to be terminated with all of the consequences outlined in exit(2). In addition a “core image” will be made in the current working directory of the receiving process if sig is one for which an asterisk appears in the above list and the following conditions are met:

The effective user ID and the real user ID of the receiving process are equal. 

The effective group ID and the real group ID of the receiving process are equal. 

An ordinary file named core exists and is writable or can be created.  If the file must be created, it will have the following properties:

a mode of 0666 modified by the file creation mask, see umask(2)

a file owner ID that is the same as the effective user ID of the receiving process

a file group ID that is the same as the effective group ID of the receiving process

SIG_DFL - (#) stop process upon receipt of a signal. 
For those signals flagged with a pound (#) above, upon receipt of the signal sig, the receiving process is to be stopped. While a process is stopped, any additional signals (except those marked with a percent (%) above that are processed immediately) that are sent to the process will be held off until the process is restarted.  When the process is restarted, pending signals will be processed.  When a process whose parent is the initialization process (proc1) stops as the result of receiving the SIGTSTP, SIGTTIN, or SIGTTOU signals, it is sent the SIGKILL signal, which causes the process to terminate. 

SIG_DFL - (†) no action upon receipt of a signal. 
For those signals flagged with a dagger († ) above, neither terminate nor stop action is taken. 

SIG_IGN - ignore signal. 
The signal sig is to be ignored. 

Note:  the signals SIGKILL, SIGCONT and SIGSTOP cannot be ignored. 

function address - catch signal. 
Upon receipt of the signal sig, the receiving process is to execute the signal-catching function pointed to by func. The signal number sig will be passed as the first parameter to the signal-catching function.  The HP-UX kernel will also pass two additional (optional) parameters to signal handler routines.  The complete parameter list for func is:

sig signal number. 

code a word of information usually provided by the hardware. 

scp a pointer to the machine dependent structure sigcontext defined in the include file signal.h. 

Depending on the value of sig, code can be zero and/or scp can be NULL.  The meanings of code and scp and the conditions upon which they are other than zero or NULL are implementation dependent.  It is permissible for code to always be zero, and scp to always be NULL. 

The pointer scp will only be valid during the context of the signal handler. 

The optional parameters can be omitted from the handler parameter list, in which case the handler is exactly compatible with System V UNIX. 

Truly portable software should not use the optional parameters in signal-catching routines. 

Before entering the signal-catching function, the value of func for the caught signal will be set to SIG_DFL unless the signal is one of those flagged with a bullet (• ) above. 

Upon return from the signal-catching function, the receiving process will resume execution at the point it was interrupted. 

When a signal that is to be caught occurs during the execution of calls such as read, a write, an open, or an ioctl system call on a slow device (like a terminal; but not a file), during a pause system call, or during a wait system call that does not return immediately due to the existence of a previously stopped or zombie process, the signal catching function will be executed and then the interrupted system call can return a −1 to the calling process with errno set to EINTR. 

Note:  The signals SIGKILL and SIGSTOP cannot be caught. 

SIGKILL may be sent by the system in the event of an unsuccessful exec, if the original program has already been deleted.  When signal is called with func equal to SIG_IGN and a signal sig is pending, the pending signal is cleared. 

EXAMPLES

The following call to signal sets up a signal handler for the SIGINT signal:

int myhandler();

signal (SIGINT, myhandler);

RETURN VALUE

Upon successful completion, signal returns the previous value of func for the specified signal sig. Otherwise, a value of −1 is returned and errno is set to indicate the error. 

ERRORS

Signal will fail if one or more of the following are true:

­[EINVAL] Sig is an illegal signal number, or is equal to SIGKILL or SIGSTOP. 

­[EINVAL] An attempt is made to ignore SIGCONT (by default SIGCONT is ignored). 

WARNINGS

Two other signals that behave differently than the signals described above exist in this release of the system; they are:

SIGCLD18†death of a child
SIGPWR19•†power fail

There is no guarantee that, in future releases of the HP-UX system, these signals will continue to behave as described below; they are included only for compatibility with other versions of the UNIX system.  Their use in new programs is strongly discouraged. 

For these signals, func is assigned one of three values:  SIG_DFL, SIG_IGN, or a function address. The actions prescribed by these values are as follows:

SIG_DFL - ignore signal. 
The signal is to be ignored.

SIG_IGN - ignore signal. 
The signal is to be ignored. Also, if sig is SIGCLD, the calling process’s child processes will not create zombie processes when they terminate, see exit(2).

function address - catch signal. 
If the signal is SIGPWR, the action to be taken is the same as that described above for func equal to function address. The same is true if the signal is  SIGCLD.  In addition, if signal is called to catch SIGCLD in a process that currently has terminated (zombie) children, a SIGCLD signal is delivered to the process immediately.  Thus if the signal-catching function re-installs itself, the apparent effect is that any SIGCLD signals received due to the death of children while the function is executing are queued and the signal-catching function is continually reentered until the queue is empty.  Note that the function must re-install itself after it has called wait(2) or wait3(2). Otherwise the presence of the child that caused the original signal will cause another signal immediately, resulting in infinite recursion.

The SIGCLD affects two other system calls (wait(2), and exit(2)) in the following ways:

wait If the func value of SIGCLD is set to SIG_IGN and a wait is executed, the wait will block until all of the calling process’s child processes terminate; it will then return a value of −1 with errno set to ECHILD. 

exit If in the exiting process’s parent process the func value of SIGCLD is set to SIG_IGN, the exiting process will not create a zombie process. 

When processing a pipeline, the shell makes the last process in the pipeline the parent of the preceding processes.  A process that can be piped into in this manner (and thus become the parent of other processes) should take care not to set SIGCLD to be caught. 

Some implementations do not generate SIGPWR.  For systems without non-volatile memory, it is not useful.  If SIGPWR is generated, it occurs when power is restored and the system has done all necessary re-initialization.  Processes will re-start by responding to SIGPWR. 

DEPENDENCIES

Series 300, 500
The SIGURG, SIGSTOP, SIGTSTP, SIGCONT, SIGTTIN, and SIGTTOU signals are not supported.

Series 300
The signal SIGPWR is not currently generated.

The code word is always zero for all signals except signal 4 (SIGILL) and signal 8 (SIGFPE).  For SIGILL, code has the following values:

Refer to the MC6800xx processor documentation for more detailed information about the meaning of the SIGILL errors. 

For SIGFPE, code has the following values:

0 software floating point exception;

5 integer divide-by-zero. 

0x8xxxxxxx
any value with the high-order bit set indicates an exception while using the HP98248A floating point accelerator.  The value of (code &~ 0x8000000) is the value of the HP98248A status register.  Refer to the HP98248A documentation for more detailed information. 

other
any other value indicates an exception while using the MC68881 floating point coprocessor.  The value of code is the value of the MC68881 status register.  Refer to the MC68881 documentation for more detailed information. 

Series 500
The SIGEMT signal means “out of memory,” and is generated by the HP-UX Operating System.  When sent by the system, this signal is always fatal to the process, and cannot be caught or ignored.

SIGIOT can be sent if an invalid string operation is attempted, or if a bounds range check trap is encountered. 

The signal SIGBUS is not currently generated by the operating system. 

The signal handler parameter code contains the trap number provided by the hardware in the event a trap occurs in the user’s program; see trapno(2) for a list of these trap numbers.  Otherwise, code will be zero. 

The structure pointer scp is defined when a trap occurs in the user’s program, and points to the structure sigcontext defined in signal.h.  If no trap occurs, scp will be NULL. 

A zero value is returned on floating point underflow.  Floating point overflow, divide-by-zero, integer divide-by-zero, and illegal floating point operation exceptions result in the signal SIGFPE being sent to the process.  An undefined value is returned as the result of the operation if the signal SIGFPE is ignored or caught. 

SIGFPE is not sent on integer overflow.  Instead, a wrapped integer result is returned. 

Series 800
The structure pointer scp is always defined. 

The code word is always zero for all signals except signal 4 (SIGILL) and signal 8 (SIGFPE). For SIGILL, code has the following values:

For SIGFPE, code has the following values:

Refer to the Series 800 processor documentation provided with your system for more detailed information about the meaning of these errors. 

AUTHOR

Signal was developed by HP, AT&T, and the University of California, Berkeley. 

SEE ALSO

kill(1), kill(2), lseek(2), pause(2), wait(2), abort(3C), setjmp(3C). 

Hewlett-Packard Company  —  Version B.1,  May 11, 2021