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 POSIX.1‐2017, Section 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_HOLD Request that signal be held.
SIG_IGN Request that signal be ignored.
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 Integer or structure type of an object used to
represent sets of signals.
pid_t As described in <sys/types.h>.
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.1‐2017 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.
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. 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(), kill(), the General Terminal Interface (see
Section 11.1.9, Special Characters), and the kill utility, unless
otherwise stated (see, for example, the System Interfaces volume
of POSIX.1‐2017, Section 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, │
│ │ │ 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. │
│SIGPOLL │ T │ Pollable event. │
│SIGPROF │ T │ Profiling timer expired. │
│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. │
│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 the
System Interfaces volume of POSIX.1‐2017, Section 2.4.3, Signal
Actions.
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.
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 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.
The <signal.h> header shall also define the following symbolic
constants:
SA_NOCLDSTOP Do not generate SIGCHLD when children stop
or stopped children continue.
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 or status information on child
termination. See sigaction(3p).
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 <signal.h> header shall define the mcontext_t type through
typedef.
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.
The <signal.h> header shall define the siginfo_t type as a
structure, which shall include at least the following members:
int si_signo Signal number.
int si_code Signal code.
int si_errno If non-zero, an errno value associated with
this signal, as described 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.
union sigval si_value Signal value.
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 │
├───────┼─────────────┼──────────────────────────────────────────────────────────────────┤
│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. │
├───────┼─────────────┼──────────────────────────────────────────────────────────────────┤
│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. │
└───────┴─────────────┴──────────────────────────────────────────────────────────────────┘
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 │ 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. │
├────────┼────────────────┼───────────────────────────────────────────────────┤
│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 shall be declared as functions and may also be
defined as macros. Function prototypes shall be provided.
int kill(pid_t, int);
int killpg(pid_t, int);
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);
int sigaction(int, const struct sigaction *restrict,
struct sigaction *restrict);
int sigaddset(sigset_t *, int);
int sigaltstack(const stack_t *restrict, stack_t *restrict);
int sigdelset(sigset_t *, int);
int sigemptyset(sigset_t *);
int sigfillset(sigset_t *);
int sighold(int);
int sigignore(int);
int siginterrupt(int, int);
int sigismember(const sigset_t *, int);
void (*signal(int, void (*)(int)))(int);
int sigpause(int);
int sigpending(sigset_t *);
int sigprocmask(int, const sigset_t *restrict, sigset_t *restrict);
int sigqueue(pid_t, int, union sigval);
int sigrelse(int);
void (*sigset(int, void (*)(int)))(int);
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);
Inclusion of the <signal.h> header may make visible all symbols
from the <time.h> header.
The following sections are informative.
