The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here

NAME

signal.h - signals

SYNOPSIS

#include <signal.h>

DESCRIPTION

[CX] [Option Start] Some of the functionality described on this reference page extends the ISO C standard. Applications shall define the appropriate feature test macro (see the System Interfaces volume of IEEE Std 1003.1-2001, Section 2.2, The Compilation Environment) to enable the visibility of these symbols in this header. [Option End]

The <signal.h> header shall define 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
[CX] [Option Start] Request that signal be held. [Option End]
SIG_IGN
Request that signal be ignored.

The following data types shall be defined through typedef:

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] [Option Start] Integer or structure type of an object used to represent sets of signals. [Option End]
pid_t
[CX] [Option Start] As described in <sys/types.h>. [Option End]

[RTS] [Option Start] The <signal.h> header shall define the sigevent structure, which has at least the following members:

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

The following values of sigev_notify shall be defined:

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. 

This header shall also declare the macros SIGRTMIN and SIGRTMAX, which evaluate to integer expressions, and specify a range of signal numbers that are reserved for application use and for which the realtime signal behavior specified in this volume of IEEE Std 1003.1-2001 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.

It is implementation-defined whether realtime signal behavior is supported for other signals. [Option End]

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 integer values. The value 0 is reserved for use as the null signal (see kill()). Additional implementation-defined signals may occur in the system.

[CX] [Option Start] The ISO C standard only requires the signal names SIGABRT, SIGFPE, SIGILL, SIGINT, SIGSEGV, and SIGTERM to be defined. [Option End]

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] [Option Start]  

 

or continued. [Option End]

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.

[XSI] [Option Start] SIGPOLL

T

Pollable event.

SIGPROF

T

Profiling timer expired.

SIGSYS

A

Bad system call.

SIGTRAP

A

Trace/breakpoint trap. [Option End]

SIGURG

I

High bandwidth data is available at a socket.

[XSI] [Option Start] SIGVTALRM

T

Virtual timer expired.

SIGXCPU

A

CPU time limit exceeded.

SIGXFSZ

A

File size limit exceeded. [Option End]

The default actions are as follows:

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.
A
Abnormal termination of the process.
[XSI] [Option Start] Additionally, implementation-defined abnormal termination actions, such as creation of a core file, may occur. [Option End]
I
Ignore the signal.
S
Stop the process.
C
Continue the process, if it is stopped; otherwise, ignore the signal.

[CX] [Option Start] The header shall provide a declaration of struct sigaction, including at least the following members:

void (*sa_handler)(int)  Pointer to a signal-catching function or one of the macros 
                         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. 

[Option End]

[XSI] [Option Start] The storage occupied by sa_handler and sa_sigaction may overlap, and a conforming application shall not use both simultaneously. [Option End]

The following shall be declared as constants:

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

[XSI] [Option Start] The ucontext_t structure shall be defined through typedef as described in <ucontext.h>.

The mcontext_t type shall be defined through typedef as described in <ucontext.h>.

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

int       ss_onstack  Non-zero when signal stack is in use. 
void     *ss_sp       Signal stack pointer. 

[Option End]

[CX] [Option Start] The <signal.h> header shall define the siginfo_t type as a structure that includes at least the following members: [Option End]

[CX][Option Start]
int           si_signo  Signal number. 
int           si_code   Signal code. 
[Option End]
[XSI][Option Start]
int           si_errno  If non-zero, an errno value associated with 
                        this signal, as defined in <errno.h>. 
pid_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. 
[Option End]
[RTS][Option Start]
union sigval  si_value  Signal value. 
[Option End]

The macros specified in the Code column of the following table are defined for use as values of si_code that are [XSI] [Option Start]  signal-specific or [Option End] non-signal-specific reasons why the signal was generated.


Signal

Code

Reason

[XSI] [Option Start] 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 abnormally and created a core file.

 

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. [Option End]

[CX] [Option Start] Any

SI_USER

Signal sent by kill().

 

SI_QUEUE

Signal sent by the 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. [Option End]

[XSI] [Option Start] 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.

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

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. [Option End]

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

[XSI][Option Start]
void (*bsd_signal(int, void (*)(int)))(int);
[Option End]
[CX][Option Start]
int    kill(pid_t, int);
[Option End]
[XSI][Option Start]
int    killpg(pid_t, int);
[Option End]
[THR][Option Start]
int    pthread_kill(pthread_t, int);
int    pthread_sigmask(int, const sigset_t *, sigset_t *);
[Option End]
int    raise(int);
[CX][Option Start]
int    sigaction(int, const struct sigaction *restrict,
           struct sigaction *restrict);
int    sigaddset(sigset_t *, int);
[Option End]
[XSI][Option Start]
int    sigaltstack(const stack_t *restrict, stack_t *restrict);
[Option End]
[CX][Option Start]
int    sigdelset(sigset_t *, int);
int    sigemptyset(sigset_t *);
int    sigfillset(sigset_t *);
[Option End]
[XSI][Option Start]
int    sighold(int);
int    sigignore(int);
int    siginterrupt(int, int);
[Option End]
[CX][Option Start]
int    sigismember(const sigset_t *, int);
[Option End]
void (*signal(int, void (*)(int)))(int);
[XSI][Option Start]
int    sigpause(int);
[Option End]
[CX][Option Start]
int    sigpending(sigset_t *);
int    sigprocmask(int, const sigset_t *restrict, sigset_t *restrict);
[Option End]
[RTS][Option Start]
int    sigqueue(pid_t, int, const union sigval);
[Option End]
[XSI][Option Start]
int    sigrelse(int);
void (*sigset(int, void (*)(int)))(int);
[Option End]
[CX][Option Start]
int    sigsuspend(const sigset_t *);
[Option End]
[RTS][Option Start]
int    sigtimedwait(const sigset_t *restrict, siginfo_t *restrict,
           const struct timespec *restrict);
[Option End]
[CX][Option Start]
int    sigwait(const sigset_t *restrict, int *restrict);
[Option End]
[RTS][Option Start]
int    sigwaitinfo(const sigset_t *restrict, siginfo_t *restrict);
[Option End]

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


The following sections are informative.

APPLICATION USAGE

None.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

<errno.h>, <stropts.h>, <sys/types.h>, <time.h>, <ucontext.h>, the System Interfaces volume of IEEE Std 1003.1-2001, alarm(), bsd_signal(), ioctl(), kill(), killpg(), raise(), sigaction(), sigaddset(), sigaltstack(), sigdelset(), sigemptyset(), sigfillset(), siginterrupt(), sigismember(), signal(), sigpending(), sigprocmask(), sigqueue(), sigsuspend(), sigwaitinfo(), wait(), waitid()

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 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 struct sigaction.

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

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.

End of informative text.

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POSIX ® is a registered Trademark of The IEEE.
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