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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):

           ┌──────────┬────────────────┬────────────────────────────────────────────────────┐
           │ SignalDefault ActionDescription                     │
           ├──────────┼────────────────┼────────────────────────────────────────────────────┤
           │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.

       ┌───────┬─────────────┬──────────────────────────────────────────────────────────────────┐
       │SignalCodeReason                               │
       ├───────┼─────────────┼──────────────────────────────────────────────────────────────────┤
       │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:

             ┌────────┬────────────────┬───────────────────────────────────────────────────┐
             │SignalMemberValue                       │
             ├────────┼────────────────┼───────────────────────────────────────────────────┤
             │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 .