signal(5)

NAME

signal: signal.h — description of signals

SYNOPSIS

#include <signal.h>

DESCRIPTION

The <signal.h> header defines the following symbolic constants, each of which expands to a distinct constant expression of the type:

void (*)(int)

whose value matches no declarable function.

SIG_DFL: Request for default signal handling.
SIG_ERR: Return value from signal() in case of error.
SIG_HOLD: Request that signal be held.
SIG_IGN: Request that signal be ignored.

The following data types are defined through typedef:

sig_atomic_t: Integral type of an object that can be accessed as an atomic entity, even in the presence of asynchronous interrupts
sigset_t: Integral or structure type of an object used to represent sets of signals.
pid_t: As described in <sys/types.h>.

This header also declares the constants that are used to refer to the signals that occur in the system. Signals defined here begin with the letters SIG. Each of the signals have distinct positive integral values. The value 0 is reserved for use as the null signal (see kill(2)). Additional implementation-dependent signals may occur in the system.

HP-UX supports multiple signal interfaces (see sigaction(2), signal(2), and sigset(3C)) that allow a process to specify the action taken upon receipt of a signal.

The following signals are supported on all implementations. The default actions A (Abort), C (Continue), I (Ignore), S (Stop) and T (Abnormal Termination) are explained after the table.

	`Default`
`Signal`	`Action`	`Description`
`SIGABRT`	A	Process abort signal.
		Same as `SIGIOT`.
`SIGALRM`	T	Alarm clock.
`SIGBUS`	A	Access to undefined portion of a memory object.
`SIGCHLD`	I	Child process terminated or stopped.
		Same as `SIGCLD`.
`SIGCLD`	I	Same as `SIGCHLD` (also see `WARNINGS` below).
`SIGCONT`	C	Continue executing, if stopped.
`SIGEMT`	A	Software generated signal.
`SIGFPE`	A	Erroneous arithmetic operation.
`SIGHUP`	T	Hangup.
`SIGILL`	A	Illegal instruction.
`SIGIO`	I	Asynchronous I/O signal; see select(2).
		Same as `SIGPOLL`.
`SIGIOT`	A	Process abort signal. Same as `SIGABRT`.
`SIGINT`	T	Terminal interrupt signal.
`SIGKILL`	T	Kill (cannot be caught or ignored).
`SIGLOST`	A	File lock lost (NFS file locking).
`SIGPIPE`	T	Write on a pipe with no one to read it.
`SIGPOLL`	T	Pollable event.
`SIGQUIT`	A	Terminal quit signal.
`SIGSEGV`	A	Invalid memory reference.
`SIGSTOP`	S	Stop executing (cannot be caught or ignored).
`SIGTERM`	T	Termination signal.
`SIGTSTP`	S	Terminal stop signal.
`SIGTTIN`	S	Background process attempting read.
`SIGTTOU`	S	Background process attempting write.
`SIGUSR1`	T	User-defined signal 1.
`SIGUSR2`	T	User-defined signal 2.
`SIGPOLL`	T	Pollable event.
`SIGPROF`	T	Profiling timer expired.
`SIGPWR`	I	Power fail (see `WARNINGS` below).
`SIGSYS`	A	Bad system call.
`SIGTRAP`	A	Trace/breakpoint trap.
`SIGURG`	I	High bandwidth data is available at a socket.
`SIGVTALRM`	T	Virtual timer expired.
`SIGWINCH`	I	Window size change; see termio(7).
`SIGXCPU`	A	CPU time limit exceeded.
`SIGXFSZ`	A	File size limit exceeded.
`SIGRTMIN`	T	First realtime signal.
`SIGRTMAX`	T	Last realtime signal.

The macros SIGRTMIN and SIGRTMAX evaluate to integral expressions, and specify a range that includes at least {RTSIG_MAX} signal numbers that are reserved for application use and for which the realtime signal extensions are supported (see sigaction(2)).

The default actions are as follows:

A

Abnormal termination of the process. Additionally, for some signals having this default action, If the following conditions are met, generate a core image file (see core(4)) in the current working directory of the receiving process:

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.
A regular file named core does not exist and can be created, or exists and is writable.

If the file is created, it has the following properties:

The file mode is 0600, modified by the file creation mode mask (see umask(2)).
The file user ID is equal to the effective user ID of the receiving process.
The file group ID is equal to the effective group ID of the receiving process.

C

Continue the process, if it is stopped; otherwise ignore the signal.

I

Ignore the signal.

S

Stop the process. While a process is stopped, any additional signals sent to the process are suspended until the process is restarted (except SIGKILL, which is processed immediately). However, when the process is restarted, pending signals are processed. When a process that is in an orphaned process group (see glossary(9)) receives a SIGTSTP, SIGTTIN, or SIGTTOU signal, the process is not stopped because a process in an orphaned process group is not allowed to stop. Instead, a SIGHUP signal is sent to the process, and the SIGTSTP, SIGTTIN, or SIGTTOU is discarded.

T

Abnormal termination of the process. The process is terminated with all the consequences of exit() except that the status made available to wait() and waitpid() indicates abnormal termination by the specified signal. See exit(2), wait(2), waitpid(2).

The header provides a declaration of struct sigaction, including at least the following members:

`void`	`(*sa_handler)(int)`	what to do on receipt of signal
`sigset_t`	`sa_mask`	set of signals to be blocked during execution of the signal handling function
`int`	`sa_flags`	special flags
`void (*)`	`sa_sigaction`	pointer to signal
`(int, siginfo_t , void )`		handler function
		or one of the macros
		`SIG_IGN` or `SIG_DFL`

The storage occupied by sa_handler and sa_sigaction may overlap, and a portable program must not use both simultaneously.

The following are declared as constants:

SA_NOCLDSTOP: Do not generate SIGCHLD when children stop.
SIG_BLOCK: The resulting set is the union of the current set and the signal set pointed to by the argument set.
SIG_UNBLOCK: The resulting set is the intersection of the current set and the complement of the signal set pointed to by the argument set.
SIG_SETMASK: The resulting set is the signal set pointed to by the argument set. SA_ONSTACK Causes signal delivery to occur on an alternate stack.
SA_RESETHAND: Causes signal dispositions to be set to SIG_DFL on entry to signal handlers.
SA_RESTART: Causes certain functions to become restartable.
SA_SIGINFO: Causes extra information to be passed to signal handlers at the time of receipt of a signal.
SA_NOCLDWAIT: Causes implementations not to create zombie processes on child death.
SA_NODEFER: Causes signal not to be automatically blocked on entry to signal handler.
SS_ONSTACK: Process is executing on an alternate signal stack.
SS_DISABLE: Alternate signal stack is disabled.
MINSIGSTKSZ: Minimum stack size for a signal handler.
SIGSTKSZ: Default size in bytes for the alternate signal stack.

The ucontext_t structure is defined through typedef as described in <ucontext.h>.

The mcontext_t structure is defined through typedef as described in <ucontext.h>.

The <signal.h> header defines the stack_t type as a structure that includes at least the following members:

`void`	`*ss_sp`	stack base or pointer
`size_t`	`ss_size`	stack size
`int`	`ss_flags`	flags

The <signal.h> header defines the sigstack structure that includes at least the following members:

`int`	`ss_onstack`	nonzero when signal stack is in use
`void`	`*ss_sp`	signal stack pointer

The <signal.h> header defines the sigevent structure that includes at least the following members:

`int`	`sigev_notify`	Notification type
`int`	`sigev_signo`	Signal number
`union sigval`	`sigev_value`	Signal value.

The sigev_notify member specifies the notification mechanism to use when an asynchronous event occurs. The following values are defined for the sigev_notify member:

SIGEV_NONE: No asynchronous notification will be delivered when the event of interest occurs.
SIGEV_SIGNAL: The signal specified in sigev_signo will be generated for the process when the event of interest occurs. If SA_SIGINFO is set for that signal number, then the signal will be queued to the process, and the value specified in sigev_value will be the sigev_value component of the generated signal. If SA_SIGINFO is not set for that signal number, it is unspecified whether the signal is queued, and what value, if any, is sent.

The sigev_signo member specifies the signal to be generated. The sigev_value member is the application-defined value to be passed to the signal-catching function at the time of the signal delivery or to be returned at signal acceptance as the si_value member of the siginfo_t structure.

The <signal.h> header defines sigval as a union that includes at least the following members:

`int`	`sival_int`	Integer signal value
`void *`	`sival_ptr`	Pointer signal value.

The <signal.h> header defines the siginfo_t type as a structure that includes at least the following members:

`int`	`si_signo`	signal number
`int`	`si_errno`	if nonzero, an `errno` value associated with this signal, as defined in `<errno.h>`
`int`	`si_code`	signal code
`union sigval`	`si_value`	signal value
`id_t`	`si_pid`	sending process ID
`uid_t`	`si_uid`	real user ID of sending process
`void`	`*si_addr`	address of faulting instruction
`int`	`si_status`	exit value or signal
`long`	`si_band`	band event for SIGPOLL

The si_code member contains a code identifying the cause of the signal. The following values are defined for si_code:

SI_USER: The signal was sent by kill(). The si_code may be set to SI_USER also if the signal was sent by raise() (see kill(2)) or similar functions that are provided as implementation extensions of kill().
SI_QUEUE: The signal was sent by sigqueue().
SI_TIMER: The signal was generated by the expiration of a timer set by timer_settime().
SI_ASYNCIO: The signal was generated by the completion of an asynchronous I/O request.
SI_MESGQ: The signal was generated by the arrival of a message on an empty message queue.

If the signal was not generated by one of the functions or events listed above (such as kill(), raise(), sigqueue(), and other functions), the si_code will be set to an implementation-defined value (see the Code column below) that is not equal to any of the values defined above for si_code.

If si_code is one of SI_QUEUE, SI_TIMER, SI_ASYNCIO, or SI_MESGQ, then si_value will contain the application-specified signal value. Otherwise, the contents of si_value are undefined.

The macros specified in the Code column of the following table are defined for use as values of si_code that are signal-specific reasons why the signal was generated.

Signal	Code	Reason
`SIGILL`	`ILL_ILLOPC`	illegal opcode
	`ILL_ILLOPN`	illegal operand
	`ILL_ILLADR`	illegal addressing mode
	`ILL_ILLTRP`	illegal trap
	`ILL_PRVOPC`	privileged opcode
	`ILL_PRVREG`	privileged register
	`ILL_COPROC`	coprocessor error
	`ILL_BADSTK`	internal stack error
`SIGFPE`	`FPE_INTDIV`	integer divide by zero
	`FPE_INTOVF`	integer overflow
	`FPE_FLTDIV`	floating point divide by zero
	`FPE_FLTOVF`	floating point overflow
	`FPE_FLTUND`	floating point underflow
	`FPE_FLTRES`	floating point inexact result
	`FPE_FLTINV`	invalid floating point operation
	`FPE_FLTSUB`	subscript out of range
`SIGSEGV`	`SEGV_MAPERR`	address not mapped to object
	`SEGV_ACCERR`	invalid permissions for mapped object
`SIGBUS`	`BUS_ADRALN`	invalid address alignment
	`BUS_ADRERR`	nonexistent physical address
	`BUS_OBJERR`	object specific hardware error
`SIGTRAP`	`TRAP_BRKPT`	process breakpoint
	`TRAP_TRACE`	process trace trap
`SIGCHLD`	`CLD_EXITED`	child has exited
	`CLD_KILLED`	child has terminated abnormally and
		did not create a core file
	`CLD_DUMPED`	child has terminated and created a core file
	`CLD_KILLED`	child was killed
	`CLD_DUMPED`	child has terminated abnormally
	`CLD_TRAPPED`	traced child has trapped
	`CLD_STOPPED`	child has stopped
	`CLD_CONTINUED`	stopped child has continued
`SIGPOLL`	`POLL_IN`	data input available
	`POLL_OUT`	output buffers available
	`POLL_MSG`	input message available
	`POLL_ERR`	I/O error
	`POLL_PRI`	high priority input available
	`POLL_HUP`	device disconnected

Implementations may support additional si_code values not included in this list, may generate values included in this list under circumstances other than those described in this list, and may contain extensions or limitations that prevent some values from being generated. Implementations will not generate a different value from the ones described in this list for circumstances described in this list.

In addition, the following signal-specific information will be available:

Signal	Member	Value
`SIGILL`	`void * si_addr`	address of faulting instruction
`SIGFP`
`SIGSEGV`	`void * si_addr`	address of faulting memory reference
`SIGBUS`
`SIGCHLD`	`pid_t si_pid`	child process ID
	`int si_status`	exit value or signal
	`uid_t si_uid`	real user ID of the process that sent the signal
`SIGPOLL`	`long si_band`	band event for `POLL_IN`, `POLL_OUT`, or `POLL_MSG`

For some implementations, the value of si_addr may be inaccurate.

The following are declared as functions and may also be defined as macros:

void  (*bsd_signal(int sig, void (*func)(int)))(int);
int   kill(pid_t pid, int sig);
int   killpg(pid_t pgrp, int sig);
int   raise(int sig);
int   sigaction(int sig, const struct sigaction
              *act, struct sigaction *oact);
int   sigaddset(sigset_t *set, int signo);
int   sigaltstack(const stack_t *ss, stack_t *oss);
int   sigdelset(sigset_t *set, int signo);
int   sigemptyset(sigset_t *set);
int   sigfillset(sigset_t *set);
int   sighold(int sig);
int   sigignore(int sig);
int   siginterrupt(int sig, int flag);
int   sigismember(const sigset_t *set, int signo);
int   sigmask(int signum);
void  (*signal(int sig, void (*func)(int)))(int);
int   sigpause(int sig);
int   sigpending(sigset_t *set);
int   sigprocmask(int how, const sigset_t *set, sigset_t *oset);
int   sigqueue(pid_t pid, int sig, const union sigval value);
int   sigrelse(int sig);
void  *sigset(int sig, void (*disp)(int)))(int);
int   sigstack(struct sigstack *ss,
                  struct sigstack *oss);
int   sigsuspend(const sigset_t *sigmask);
int   sigtimedwait(const sigset_t *set, siginfo_t * info,
                        const struct timespec *timeout);
int   sigwait(const sigset_t *set, int *sig);
int   sigwaitinfo(const sigset_t *set, siginfo_t * info);

APPLICATION USAGE

Upon receipt of the signal, if signal() is used to set the action to the address of a signal handler, the action for the signal caught is reset to SIG_DFL (except SKGILL, SIGTRAP, and SIGPWR). Then, the signal-catching function is executed. Signal interface routines other than signal() normally do not reset the action for the signal caught. However, sigaction() provides a way of specifying this behavior.

The parameters passed to the signal-catching function where signal() is used are:

`int`	`sig`	The signal number.
`int`	`code`	A word of information usually provided
		by the hardware.
`struct sigcontext`	`*scp`	A pointer to the machine-dependent
		structure `sigcontext` defined in
		`<signal.h>`.

The parameters passed to the signal-catching function where sigaction() is used and SA_SIGINFO is not specified are:

int sig The signal number.

The parameters passed to the signal-catching function where sigaction() is used and SA_SIGINFO is specified are:

`int`	`sig`	The signal number.
`siginfo_t`	`*siginfop`	Information on why the signal was
		generated.
`void`	`*contextp`	Can be cast to a pointer to a `ucontext_t`
		to refer to the thread's context at the
		time of the interruption.

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 determining when they are other than zero or NULL are implementation-dependent. It is possible for code to always be zero, and scp to always be NULL.

The pointer scp, siginfop, and contextp are valid only during the context of the signal-catching function.

Optional parameters can be omitted from the signal-catching function parameter list, in which case the signal-catching function is exactly compatible with UNIX System V. Truly portable software should not use the optional parameters in signal-catching routines.

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

When a signal is caught during the execution of system calls such as read(), write(), open(), or ioctl() on a slow device (such as a terminal, but not a file on a local filesystem), during a pause() system call or a wait() system call that does not return immediately because a previously stopped or zombie process already exists, the signal-catching function is executed and the interrupted system call returns a -1 to the calling process with errno set to EINTR.

When any stop signal (SIGSTOP, SIGTSTP, SIGTTIN, SIGTTOU) is generated for a process, pending SIGCONT signals for that process are discarded. Conversely, when SIGCONT is generated for a process, all pending stop signals for that process are discarded. When SIGCONT is generated for a stopped process, the process is continued, even if the SIGCONT signal is blocked or ignored. If SIGCONT is blocked and not ignored, the signal remains pending until it is either unblocked or a stop signal is generated.

Note: When any stop signal (SIGSTOP, SIGTSTP, SIGTIN, SIGTTOU) is posted to threads created with process contention scope, using pthread_kill(), pending SIGCONT signals may not be discarded. Similarly, when SIGCONT is posted to threads created with process contention scope, using pthread_kill(), pending stop signals may not be discarded. However, stop and continue signals posted to threads created with system contention scope, using pthread_kill(), will continue to adhere to the semantics described in the preceding paragraph.

SIGKILL is sent by the system if an exec() system call is unsuccessful and the original program has already been deleted.

Threads Considerations

The following summarizes the signal model for threads:

A signal mask which specifies the signals blocked from delivery is associated with each thread.
The signal disposition, catch/ignore/default, is a process attribute and is shared by all threads in the process.
If the signal action for a signal specifies termination, stop or continue, all threads within the process are terminated, stopped or continued, respectively. This is the case regardless of whether the signal was directed at the process or a specific thread within the process.
Signals which are generated by some action associated with a particular thread, such as an invalid pointer dereference, will be delivered to the thread which caused the generation of the signal. These signals are referred to as synchronously generated signals.
Signals that are posted to the process by kill() or some asynchronous event such as terminal activity will be delivered to exactly one thread in the process which does not block delivery of the signal; if there is more than one eligible thread, which thread the signal is delivered to may not be able to be determined by an application. If all threads in the process block the signal, then the signal remains pending on the process until a thread unblocks the signal, issues a sigwait() call for the signal or sets the signal disposition to ignore the signal. These signals are referred to as asynchronously generated signals.
A thread can post a signal to a particular thread in the same process using pthread_kill(). If the thread which the signal is posted to blocks delivery of the signal, the signal remains pending on the thread.
The sigpending() function returns a union of the set of signals pending on the process and on the calling thread.
Each PTHREAD_SCOPE_SYSTEM thread may define an alternate signal handling stack. Use of alternate signal stacks in PTHREAD_SCOPE_PROCESS threads will result in indeterminate behavior.

Threadsafe Considerations

Refer to thread_safety(5) for a list of libc and libpthread interfaces which are not thread-safe, cancellation points, cancel safe, async signal safe, and async cancel safe.

WARNINGS

The signals SIGCLD and SIGPWR behave differently than those described above in the list of signals in DESCRIPTION.

The actions for these signals is modified as follows:

SIGCLD

Setting the action for SIGCLD to SIG_IGN in a parent process prevents exiting children of the calling process from creating a zombie process. If the parent process executes the wait() function, the calling process blocks until all of the child processes of the calling processes terminate. The wait() function then returns a value of -1 with errno set to ECHILD (see wait(2)).

If one of the signal interface routines is used to set the action for SIGCLD to be caught (that is, a function address is supplied) in a process that currently has terminated (zombie) children, a SIGCLD signal is delivered to the parent process immediately.

Signal-catching functions set by signal() are reset when the signal is caught. These functions generally reinstall themselves. If the function associated with SIGCLD reinstalls itself before it has called one of the child wait functions (such as wait(), wait3(), or waitpid()), another SIGCLD signal will be delivered and the handler immediately invoked again (unless the signal is masked). Thus, a signal-catching function for SIGCLD should reinstall itself only after it has called one of the child wait functions. (This similarly applies to signal-catching functions set with sigaction() where SA_RESETHAND or SV_RESETHAND has been specified.)

SIGPWR

The SIGPWR signal is sent to all processes after a power interruption when power is restored and the system has done all necessary reinitialization. Processes restart by catching (or ignoring) SIGPWR.

Applications that wish to recover from power failures should catch SIGPWR and take whatever necessary steps to reinitialize itself.

Some implementations do not generate SIGPWR. Only systems with nonvolatile memory can recover from power failures.

The hardware program counter is not advanced when a trap occurs. If the signal generated by a hardware trap is received by a signal-catching function in the user program, the instruction that caused the trap is re-executed upon return from the signal-catching function, causing the trap again, unless program flow is altered by the signal-catching function. For example, the longjmp() routine (see setjmp(3C)) can be called. Using longjmp() ensures software portability across different hardware architectures.

If the signal generated by a hardware trap is not received by a signal-catching function, that is, if the signal is masked or ignored, there may be one of two results:

1.: The instruction causing the trap is executed again, resulting in an infinite program loop
2.: The system detects the situation and causes the program to terminate.

AUTHOR

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

STANDARDS CONFORMANCE

<signal.h>: AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1, ANSI C