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PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of
this interface may differ (consult the corresponding Linux manual page for details of
Linux behavior), or the interface may not be implemented on Linux.
NAME
signal.h — signals
SYNOPSIS
#include <signal.h>
DESCRIPTION
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().
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.
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
The SIGPOLL and SIGPROF signals may be removed in a future version.
SEE ALSO
<errno.h>, <stropts.h>, <sys_types.h>, <time.h>
The System Interfaces volume of POSIX.1‐2017, Section 2.2, The Compilation Environment,
alarm(), ioctl(), kill(), killpg(), psiginfo(), pthread_kill(), pthread_sigmask(),
raise(), sigaction(), sigaddset(), sigaltstack(), sigdelset(), sigemptyset(),
sigfillset(), sighold(), siginterrupt(), sigismember(), signal(), sigpending(),
sigqueue(), sigsuspend(), sigtimedwait(), sigwait(), timer_create(), wait(), waitid()
The Shell and Utilities volume of POSIX.1‐2017, kill
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std
1003.1-2017, Standard for Information Technology -- Portable Operating System Interface
(POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by
the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and The Open Group
Standard, the original IEEE and The Open Group Standard is the referee document. The
original Standard can be obtained online at http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page are most likely to have
been introduced during the conversion of the source files to man page format. To report
such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .