NAME

signal.h — signals

SYNOPSIS

#include <signal.h>

DESCRIPTION

^[CX] Some of the functionality described on this reference page extends the ISO C standard. Applications shall define the appropriate feature test macro (see XSH 2.2 The Compilation Environment ) to enable the visibility of these symbols in this header.

The <signal.h> header shall define the following macros, which shall expand to constant expressions with distinct values that have a type compatible with the second argument to, and the return value of, the signal() function, and whose values shall compare unequal to the address of any declarable function.

SIG_DFL

Request for default signal handling.

SIG_ERR

Return value from signal() in case of error.

SIG_IGN

Request that signal be ignored.

^[CX] The <signal.h> header shall define the pthread_t, size_t, and uid_t types as described in <sys/types.h>.

The <signal.h> header shall define the timespec structure as described in <time.h>.

The <signal.h> header shall define the following data types:

sig_atomic_t

Possibly volatile-qualified integer type of an object that can be accessed as an atomic entity, even in the presence of asynchronous interrupts.

sigset_t

^[CX] Integer or structure type of an object used to represent sets of signals.

pid_t

^[CX] As described in <sys/types.h>.

^[CX] The <signal.h> header shall define the pthread_attr_t type as described in <sys/types.h>.

The <signal.h> header shall define the sigevent structure, which shall include at least the following members:

int              sigev_notify            Notification type.
int              sigev_signo             Signal number.
union sigval     sigev_value             Signal value.
void           (*sigev_notify_function)(union sigval)
                                         Notification function.
pthread_attr_t *sigev_notify_attributes  Notification attributes.

The <signal.h> header shall define the following symbolic constants for the values of sigev_notify:

SIGEV_NONE

No asynchronous notification is delivered when the event of interest occurs.

SIGEV_SIGNAL

A queued signal, with an application-defined value, is generated when the event of interest occurs.

SIGEV_THREAD

A notification function is called to perform notification.

The sigval union shall be defined as:

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

The <signal.h> header shall declare the SIGRTMIN and SIGRTMAX macros, which shall expand to positive integer expressions with type int, but which need not be constant expressions. These macros specify a range of signal numbers that are reserved for application use and for which the realtime signal behavior specified in this volume of POSIX.1-2024 is supported. The signal numbers in this range do not overlap any of the signals specified in the following table.

The range SIGRTMIN through SIGRTMAX inclusive shall include at least {RTSIG_MAX} signal numbers. The value of SIGRTMAX shall be less than the value returned by sysconf(_SC_NSIG).

It is implementation-defined whether realtime signal behavior is supported for other signals.

The <signal.h> header shall define the following symbolic constant. The value shall be suitable for use in #if preprocessing directives:

SIG2STR_MAX

Maximum size of a signal name returned by sig2str(), including the terminating null byte.

The <signal.h> header shall define the following macros that are used to refer to the signals that occur in the system. Signals defined here begin with the letters SIG followed by an uppercase letter. The macros shall expand to positive integer constant expressions with type int and distinct values ^[CX]  less than the value of {NSIG_MAX} defined in <limits.h>. The value 0 is reserved for use as the null signal (see kill()). Additional implementation-defined signals may occur in the system.

The ISO C standard only requires the signal names SIGABRT, SIGFPE, SIGILL, SIGINT, SIGSEGV, and SIGTERM to be defined. An implementation need not generate any of these six signals, except as a result of explicit use of interfaces that generate signals, such as raise(), ^[CX] kill(), the General Terminal Interface (see 11.1.9 Special Characters ), and the kill utility, unless otherwise stated (see, for example, XSH 2.8.3.3 Memory Protection ).

The following signals shall be supported on all implementations (default actions are explained below the table):

Signal	Default Action	Description
SIGABRT	A	Process abort signal.
SIGALRM	T	Alarm clock.
SIGBUS	A	Access to an undefined portion of a memory object.
SIGCHLD	I	Child process terminated, stopped,
[XSI]		or continued.
SIGCONT	C	Continue executing, if stopped.
SIGFPE	A	Erroneous arithmetic operation.
SIGHUP	T	Hangup.
SIGILL	A	Illegal instruction.
SIGINT	T	Terminal interrupt signal.
SIGKILL	T	Kill (cannot be caught or ignored).
SIGPIPE	T	Write on a pipe with no one to read it.
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.
SIGWINCH	I	Terminal window size changed.
[XSI] SIGSYS	A	Bad system call.
SIGTRAP	A	Trace/breakpoint trap.
SIGURG	I	High bandwidth data is available at a socket.
[XSI] SIGVTALRM	T	Virtual timer expired.
SIGXCPU	A	CPU time limit exceeded.
SIGXFSZ	A	File size limit exceeded.

The default actions are as follows:

T

Abnormal termination of the process.

A

Abnormal termination of the process with additional actions.

I

Ignore the signal.

S

Stop the process.

C

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

The effects on the process in each case are described in XSH 2.4.3 Signal Actions .

^[CX] The <signal.h> header shall declare the sigaction structure, which shall include at least the following members:

void   (*sa_handler)(int)  Pointer to a signal-catching function
                           or one of the SIG_IGN or SIG_DFL.
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)(int, siginfo_t *, void *)
                           Pointer to a signal-catching function.

^[CX] The storage occupied by sa_handler and sa_sigaction may overlap, and a conforming application shall not use both simultaneously.

The <signal.h> header shall define the following macros which shall expand to integer constant expressions that need not be usable in #if preprocessing directives:

SIG_BLOCK

^[CX] The resulting set is the union of the current set and the signal set pointed to by the argument set.

SIG_UNBLOCK

^[CX] 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

^[CX] The resulting set is the signal set pointed to by the argument set.

The <signal.h> header shall also define the following symbolic constants:

SA_NOCLDSTOP

^[CX] Do not generate SIGCHLD when children stop
^[XSI]  or stopped children continue.

SA_ONSTACK

^[XSI] Causes signal delivery to occur on an alternate stack.

SA_RESETHAND

^[CX] Causes signal dispositions to be set to SIG_DFL on entry to signal handlers.

SA_RESTART

^[CX] Causes certain functions to become restartable.

SA_SIGINFO

^[CX] Causes extra information to be passed to signal handlers at the time of receipt of a signal.

SA_NOCLDWAIT

^[XSI] Causes implementations not to create zombie processes or status information on child termination. See sigaction .

SA_NODEFER

^[CX] Causes signal not to be automatically blocked on entry to signal handler.

SS_ONSTACK

^[XSI] Process is executing on an alternate signal stack.

SS_DISABLE

^[XSI] Alternate signal stack is disabled.

MINSIGSTKSZ

^[XSI] Minimum stack size for a signal handler.

SIGSTKSZ

^[XSI] Default size in bytes for the alternate signal stack.

^[CX] The <signal.h> header shall define the mcontext_t type through typedef.

^[CX] The <signal.h> header shall define the ucontext_t type as a structure that shall include at least the following members:

ucontext_t *uc_link     Pointer to the context that is resumed
                        when this context returns.
sigset_t    uc_sigmask  The set of signals that are blocked when this
                        context is active.
stack_t     uc_stack    The stack used by this context.
mcontext_t  uc_mcontext A machine-specific representation of the saved
                        context.

The <signal.h> header shall define the stack_t type as a structure, which shall include at least the following members:

void     *ss_sp       Stack base or pointer.
size_t    ss_size     Stack size.
int       ss_flags    Flags.

^[CX] The <signal.h> header shall define the siginfo_t type as a structure, which shall include at least the following members:

[CX]
int           si_signo  Signal number.
int           si_code   Signal code.

[XSI]
int           si_errno  If non-zero, an errno value associated with
                        this signal, as described in <errno.h>.

[CX]
pid_t         si_pid    Sending process ID.
uid_t         si_uid    Real user ID of sending process.
void         *si_addr   Address that caused fault.
int           si_status Exit value or signal.
union sigval  si_value  Signal value.

^[CX] The <signal.h> header shall define the symbolic constants in the Code column of the following table for use as values of si_code that are signal-specific or non-signal-specific reasons why the signal was generated.

---

Signal	Code	Reason
[CX] 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.
[XSI] SIGTRAP	TRAP_BRKPT	Process breakpoint.
	TRAP_TRACE	Process trace trap.
[CX] SIGCHLD	CLD_EXITED	Child has exited.
	CLD_KILLED	Child has terminated abnormally with no additional actions.
	CLD_DUMPED	Child has terminated abnormally and additional actions may have been taken.
	CLD_TRAPPED	Traced child has trapped.
	CLD_STOPPED	Child has stopped.
	CLD_CONTINUED	Stopped child has continued.
Any	SI_USER	Signal sent by kill().
	SI_QUEUE	Signal sent by sigqueue().
	SI_TIMER	Signal generated by expiration of a timer set by timer_settime().
	SI_ASYNCIO	Signal generated by completion of an asynchronous I/O
		request.
	SI_MESGQ	Signal generated by arrival of a message on an empty message
		queue.

^[CX] 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 do not generate a different value from the ones described in this list for circumstances described in this list.

^[CX] In addition, the following signal-specific information shall be available:

Signal	Member	Value
SIGILL	void * si_addr	Address of faulting instruction.
SIGFPE
SIGSEGV	void * si_addr	Address of faulting memory reference.
SIGBUS
SIGCHLD	pid_t si_pid	Child process ID.
	int si_status	If si_code is equal to CLD_EXITED, then si_status holds the exit value of the process; otherwise, it is equal to the signal that caused the process to change state. The exit value in si_status shall be equal to the full exit value (that is, the value passed to _exit(), _Exit(), or exit(), or returned from main()); it shall not be limited to the least significant eight bits of the value.
	uid_t si_uid	Real user ID of the process that sent the signal.

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

The following shall be declared as functions and may also be defined as macros. Function prototypes shall be provided.

[CX]
int    kill(pid_t, int);

[XSI]
int    killpg(pid_t, int);

[CX]
void   psiginfo(const siginfo_t *, const char *);
void   psignal(int, const char *);
int    pthread_kill(pthread_t, int);
int    pthread_sigmask(int, const sigset_t *restrict,
           sigset_t *restrict);

int    raise(int);
[CX]
int    sig2str(int, char *);
int    sigaction(int, const struct sigaction *restrict,
           struct sigaction *restrict);
int    sigaddset(sigset_t *, int);

[XSI]
int    sigaltstack(const stack_t *restrict, stack_t *restrict);

[CX]
int    sigdelset(sigset_t *, int);
int    sigemptyset(sigset_t *);
int    sigfillset(sigset_t *);
int    sigismember(const sigset_t *, int);

void (*signal(int, void (*)(int)))(int);
[CX]
int    sigpending(sigset_t *);
int    sigprocmask(int, const sigset_t *restrict, sigset_t *restrict);
int    sigqueue(pid_t, int, union sigval);
int    sigsuspend(const sigset_t *);
int    sigtimedwait(const sigset_t *restrict, siginfo_t *restrict,
           const struct timespec *restrict);
int    sigwait(const sigset_t *restrict, int *restrict);
int    sigwaitinfo(const sigset_t *restrict, siginfo_t *restrict);
int    str2sig(const char *restrict, int *restrict);

^[CX] Inclusion of the <signal.h> header may make visible all symbols from the <time.h> header.

APPLICATION USAGE

On systems not supporting the XSI option, the si_pid and si_uid members of siginfo_t are only required to be valid when si_code is SI_USER or SI_QUEUE. On XSI-conforming systems, they are also valid for all si_code values less than or equal to 0; however, it is unspecified whether SI_USER and SI_QUEUE have values less than or equal to zero, and therefore XSI applications should check whether si_code has the value SI_USER or SI_QUEUE or is less than or equal to 0 to tell whether si_pid and si_uid are valid.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

<errno.h> , <limits.h> , <sys/types.h> , <time.h>

XSH 2.2 The Compilation Environment , alarm , kill , killpg , psiginfo , pthread_kill , pthread_sigmask , raise , sig2str , sigaction , sigaddset , sigaltstack , sigdelset , sigemptyset , sigfillset , sigismember , signal , sigpending , sigqueue , sigsuspend , sigtimedwait , sigwait , timer_create , wait , waitid

XCU kill

CHANGE HISTORY

First released in Issue 1.

Issue 5

The DESCRIPTION is updated for alignment with the POSIX Realtime Extension and the POSIX Threads Extension.

The default action for SIGURG is changed from i to iii. The function prototype for sigmask() is removed.

Issue 6

The Open Group Corrigendum U035/2 is applied. In the DESCRIPTION, the wording for abnormal termination is clarified.

The Open Group Corrigendum U028/8 is applied, correcting the prototype for the sigset() function.

The Open Group Corrigendum U026/3 is applied, correcting the type of the sigev_notify_function function member of the sigevent structure.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

• The SIGCHLD, SIGCONT, SIGSTOP, SIGTSTP, SIGTTIN, and SIGTTOU signals are now mandated. This is also a FIPS requirement.
• The pid_t definition is mandated.

The RT markings are changed to RTS to denote that the semantics are part of the Realtime Signals Extension option.

The restrict keyword is added to the prototypes for sigaction(), sigaltstack(), sigprocmask(), sigtimedwait(), sigwait(), and sigwaitinfo().

IEEE PASC Interpretation 1003.1 #85 is applied, adding the statement that symbols from <time.h> may be made visible when <signal.h> is included.

Extensions beyond the ISO C standard are marked.

IEEE Std 1003.1-2001/Cor 1-2002, item XBD/TC1/D6/14 is applied, changing the descriptive text for members of the sigaction structure.

IEEE Std 1003.1-2001/Cor 1-2002, item XBD/TC1/D6/15 is applied, correcting the definition of the sa_sigaction member of the sigaction structure.

IEEE Std 1003.1-2001/Cor 2-2004, item XBD/TC2/D6/24 is applied, reworking the ordering of the siginfo_t type structure in the DESCRIPTION. This is an editorial change and no normative change is intended.

Issue 7

SD5-XBD-ERN-5 is applied.

SD5-XBD-ERN-39 is applied, removing the sigstack structure which should have been removed at the same time as the LEGACY sigstack() function.

SD5-XBD-ERN-56 is applied, adding a reference to <sys/types.h> for the size_t type.

Austin Group Interpretation 1003.1-2001 #034 is applied.

The ucontext_t and mcontext_t structures are added here from the obsolescent <ucontext.h> header.

The psiginfo() and psignal() functions are added from The Open Group Technical Standard, 2006, Extended API Set Part 1.

The SIGPOLL and SIGPROF signals and text relating to the XSI STREAMS option are marked obsolescent.

The SA_RESETHAND, SA_RESTART, SA_SIGINFO, SA_NOCLDWAIT, and SA_NODEFER constants are moved from the XSI option to the Base.

Functionality relating to the Realtime Signals Extension option is moved to the Base.

This reference page is clarified with respect to macros and symbolic constants, and declarations for the pthread_attr_t, pthread_t, and uid_t types and the timespec structure are added.

SIGRTMIN and SIGRTMAX are required to be positive integer expressions.

The APPLICATION USAGE section is updated to describe the si_pid and si_uid members of siginfo_t.

POSIX.1-2008, Technical Corrigendum 1, XBD/TC1-2008/0062 [208], XBD/TC1-2008/0063 [80], and XBD/TC1-2008/0064 [157] are applied.

POSIX.1-2008, Technical Corrigendum 2, XBD/TC2-2008/0070 [536], XBD/TC2-2008/0071 [690], XBD/TC2-2008/0072 [594], XBD/TC2-2008/0073 [844], and XBD/TC2-2008/0074 [536] are applied.

Issue 8

Austin Group Defect 741 is applied, restricting the value of SIGRTMAX to less than the value returned by sysconf(_SC_NSIG) and the value of macros that are used to refer to the signals to less than {NSIG_MAX}.

Austin Group Defect 1138 is applied, adding sig2str() and str2sig().

Austin Group Defect 1141 is applied, changing the descriptions of CLD_KILLED and CLD_DUMPED.

Austin Group Defect 1151 is applied, adding SIGWINCH.

Austin Group Defect 1215 is applied, removing XSI shading from text relating to abnormal process termination with additional actions.

Austin Group Defect 1330 is applied, removing obsolescent interfaces.

Austin Group Defect 1775 is applied, changing the description of the si_addr member of siginfo_t.