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NAME

     sigaction — software signal facilities

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <signal.h>

     struct  sigaction {
             void    (*sa_handler)(int);
             void    (*sa_sigaction)(int, siginfo_t *, void *);
             int     sa_flags;               /* see signal options below */
             sigset_t sa_mask;               /* signal mask to apply */
     };

     int
     sigaction(int sig, const struct sigaction * restrict act, struct sigaction * restrict oact);

DESCRIPTION

     The system defines a set of signals that may be delivered to a process.  Signal delivery
     resembles the occurrence of a hardware interrupt: the signal is normally blocked from
     further occurrence, the current thread context is saved, and a new one is built.  A process
     may specify a handler to which a signal is delivered, or specify that a signal is to be
     ignored.  A process may also specify that a default action is to be taken by the system when
     a signal occurs.  A signal may also be blocked for a thread, in which case it will not be
     delivered to that thread until it is unblocked.  The action to be taken on delivery is
     determined at the time of delivery.  Normally, signal handlers execute on the current stack
     of the thread.  This may be changed, on a per-handler basis, so that signals are taken on a
     special signal stack.

     Signal routines normally execute with the signal that caused their invocation blocked, but
     other signals may yet occur.  A global signal mask defines the set of signals currently
     blocked from delivery to a thread.  The signal mask for a thread is initialized from that of
     its parent (normally empty).  It may be changed with a sigprocmask(2) or pthread_sigmask(3)
     call, or when a signal is delivered to the thread.

     When a signal condition arises for a process or thread, the signal is added to a set of
     signals pending for the process or thread.  Whether the signal is directed at the process in
     general or at a specific thread depends on how it is generated.  For signals directed at a
     specific thread, if the signal is not currently blocked by the thread then it is delivered
     to the thread.  For signals directed at the process, if the signal is not currently blocked
     by all threads then it is delivered to one thread that does not have it blocked (the
     selection of which is unspecified).  Signals may be delivered any time a thread enters the
     operating system (e.g., during a system call, page fault or trap, or clock interrupt).  If
     multiple signals are ready to be delivered at the same time, any signals that could be
     caused by traps are delivered first.  Additional signals may be processed at the same time,
     with each appearing to interrupt the handlers for the previous signals before their first
     instructions.  The set of pending signals is returned by the sigpending(2) system call.
     When a caught signal is delivered, the current state of the thread is saved, a new signal
     mask is calculated (as described below), and the signal handler is invoked.  The call to the
     handler is arranged so that if the signal handling routine returns normally the thread will
     resume execution in the context from before the signal's delivery.  If the thread wishes to
     resume in a different context, then it must arrange to restore the previous context itself.

     When a signal is delivered to a thread a new signal mask is installed for the duration of
     the process' signal handler (or until a sigprocmask(2) system call is made).  This mask is
     formed by taking the union of the current signal mask set, the signal to be delivered, and
     the signal mask associated with the handler to be invoked.

     The sigaction() system call assigns an action for a signal specified by sig.  If act is non-
     zero, it specifies an action (SIG_DFL, SIG_IGN, or a handler routine) and mask to be used
     when delivering the specified signal.  If oact is non-zero, the previous handling
     information for the signal is returned to the user.

     The above declaration of struct sigaction is not literal.  It is provided only to list the
     accessible members.  See <sys/signal.h> for the actual definition.  In particular, the
     storage occupied by sa_handler and sa_sigaction overlaps, and an application can not use
     both simultaneously.

     Once a signal handler is installed, it normally remains installed until another sigaction()
     system call is made, or an execve(2) is performed.  A signal-specific default action may be
     reset by setting sa_handler to SIG_DFL.  The defaults are process termination, possibly with
     core dump; no action; stopping the process; or continuing the process.  See the signal list
     below for each signal's default action.  If sa_handler is SIG_DFL, the default action for
     the signal is to discard the signal, and if a signal is pending, the pending signal is
     discarded even if the signal is masked.  If sa_handler is set to SIG_IGN current and pending
     instances of the signal are ignored and discarded.

     Options may be specified by setting sa_flags.  The meaning of the various bits is as
     follows:

           SA_NOCLDSTOP    If this bit is set when installing a catching function for the SIGCHLD
                           signal, the SIGCHLD signal will be generated only when a child process
                           exits, not when a child process stops.

           SA_NOCLDWAIT    If this bit is set when calling sigaction() for the SIGCHLD signal,
                           the system will not create zombie processes when children of the
                           calling process exit.  If the calling process subsequently issues a
                           wait(2) (or equivalent), it blocks until all of the calling process's
                           child processes terminate, and then returns a value of -1 with errno
                           set to ECHILD.  The same effect of avoiding zombie creation can also
                           be achieved by setting sa_handler for SIGCHLD to SIG_IGN.

           SA_ONSTACK      If this bit is set, the system will deliver the signal to the process
                           on a signal stack, specified by each thread with sigaltstack(2).

           SA_NODEFER      If this bit is set, further occurrences of the delivered signal are
                           not masked during the execution of the handler.

           SA_RESETHAND    If this bit is set, the handler is reset back to SIG_DFL at the moment
                           the signal is delivered.

           SA_RESTART      See paragraph below.

           SA_SIGINFO      If this bit is set, the handler function is assumed to be pointed to
                           by the sa_sigaction member of struct sigaction and should match the
                           prototype shown above or as below in EXAMPLES.  This bit should not be
                           set when assigning SIG_DFL or SIG_IGN.

     If a signal is caught during the system calls listed below, the call may be forced to
     terminate with the error EINTR, the call may return with a data transfer shorter than
     requested, or the call may be restarted.  Restart of pending calls is requested by setting
     the SA_RESTART bit in sa_flags.  The affected system calls include open(2), read(2),
     write(2), sendto(2), recvfrom(2), sendmsg(2) and recvmsg(2) on a communications channel or a
     slow device (such as a terminal, but not a regular file) and during a wait(2) or ioctl(2).
     However, calls that have already committed are not restarted, but instead return a partial
     success (for example, a short read count).

     After a pthread_create(3) the signal mask is inherited by the new thread and the set of
     pending signals and the signal stack for the new thread are empty.

     After a fork(2) or vfork(2) all signals, the signal mask, the signal stack, and the
     restart/interrupt flags are inherited by the child.

     The execve(2) system call reinstates the default action for all signals which were caught
     and resets all signals to be caught on the user stack.  Ignored signals remain ignored; the
     signal mask remains the same; signals that restart pending system calls continue to do so.

     The following is a list of all signals with names as in the include file <signal.h>:

     NAME            Default Action          Description
     SIGHUP          terminate process       terminal line hangup
     SIGINT          terminate process       interrupt program
     SIGQUIT         create core image       quit program
     SIGILL          create core image       illegal instruction
     SIGTRAP         create core image       trace trap
     SIGABRT         create core image       abort(3) call (formerly SIGIOT)
     SIGEMT          create core image       emulate instruction executed
     SIGFPE          create core image       floating-point exception
     SIGKILL         terminate process       kill program
     SIGBUS          create core image       bus error
     SIGSEGV         create core image       segmentation violation
     SIGSYS          create core image       non-existent system call invoked
     SIGPIPE         terminate process       write on a pipe with no reader
     SIGALRM         terminate process       real-time timer expired
     SIGTERM         terminate process       software termination signal
     SIGURG          discard signal          urgent condition present on socket
     SIGSTOP         stop process            stop (cannot be caught or ignored)
     SIGTSTP         stop process            stop signal generated from keyboard
     SIGCONT         discard signal          continue after stop
     SIGCHLD         discard signal          child status has changed
     SIGTTIN         stop process            background read attempted from control terminal
     SIGTTOU         stop process            background write attempted to control terminal
     SIGIO           discard signal          I/O is possible on a descriptor (see fcntl(2))
     SIGXCPU         terminate process       cpu time limit exceeded (see setrlimit(2))
     SIGXFSZ         terminate process       file size limit exceeded (see setrlimit(2))
     SIGVTALRM       terminate process       virtual time alarm (see setitimer(2))
     SIGPROF         terminate process       profiling timer alarm (see setitimer(2))
     SIGWINCH        discard signal          window size change
     SIGINFO         discard signal          status request from keyboard
     SIGUSR1         terminate process       user defined signal 1
     SIGUSR2         terminate process       user defined signal 2

NOTE

     The sa_mask field specified in act is not allowed to block SIGKILL or SIGSTOP.  Any attempt
     to do so will be silently ignored.

     The following functions are either reentrant or not interruptible by signals and are async-
     signal safe.  Therefore applications may invoke them, without restriction, from signal-
     catching functions or from a child process after calling fork(2) in a multi-threaded
     process:

     Base Interfaces:

     _Exit(), _exit(), accept(), access(), alarm(), bind(), cfgetispeed(), cfgetospeed(),
     cfsetispeed(), cfsetospeed(), chdir(), chmod(), chown(), close(), connect(), creat(), dup(),
     dup2(), execl(), execle(), execv(), execve(), faccessat(), fchdir(), fchmod(), fchmodat(),
     fchown(), fchownat(), fcntl(), fork(), fstat(), fstatat(), fsync(), ftruncate(), getegid(),
     geteuid(), getgid(), getgroups(), getpeername(), getpgrp(), getpid(), getppid(),
     getsockname(), getsockopt(), getuid(), kill(), link(), linkat(), listen(), lseek(), lstat(),
     mkdir(), mkdirat(), mkfifo(), mkfifoat(), mknod(), mknodat(), open(), openat(), pause(),
     pipe(), poll(), pselect(), pthread_sigmask(), raise(), read(), readlink(), readlinkat(),
     recv(), recvfrom(), recvmsg(), rename(), renameat(), rmdir(), select(), send(), sendmsg(),
     sendto(), setgid(), setpgid(), setsid(), setsockopt(), setuid(), shutdown(), sigaction(),
     sigaddset(), sigdelset(), sigemptyset(), sigfillset(), sigismember(), signal(),
     sigpending(), sigprocmask(), sigsuspend(), sleep(), sockatmark(), socket(), socketpair(),
     stat(), symlink(), symlinkat(), tcdrain(), tcflow(), tcflush(), tcgetattr(), tcgetpgrp(),
     tcsendbreak(), tcsetattr(), tcsetpgrp(), time(), times(), umask(), uname(), unlink(),
     unlinkat(), utime(), wait(), waitpid(), write().

     X/Open Systems Interfaces:

     sigpause(), sigset(), utimes().

     Realtime Interfaces:

     aio_error(), clock_gettime(), timer_getoverrun(), aio_return(), fdatasync(), sigqueue(),
     timer_gettime(), aio_suspend(), sem_post(), timer_settime().

     Base Interfaces not specified as async-signal safe by POSIX:

     fpathconf(), pathconf(), sysconf().

     Base Interfaces not specified as async-signal safe by POSIX, but planned to be:

     ffs(), htonl(), htons(), memccpy(), memchr(), memcmp(), memcpy(), memmove(), memset(),
     ntohl(), ntohs(), stpcpy(), stpncpy(), strcat(), strchr(), strcmp(), strcpy(), strcspn(),
     strlen(), strncat(), strncmp(), strncpy(), strnlen(), strpbrk(), strrchr(), strspn(),
     strstr(), strtok_r(), wcpcpy(), wcpncpy(), wcscat(), wcschr(), wcscmp(), wcscpy(),
     wcscspn(), wcslen(), wcsncat(), wcsncmp(), wcsncpy(), wcsnlen(), wcspbrk(), wcsrchr(),
     wcsspn(), wcsstr(), wcstok(), wmemchr(), wmemcmp(), wmemcpy(), wmemmove(), wmemset().

     Extension Interfaces:

     accept4(), bindat(), closefrom(), connectat(), eaccess(), ffsl(), ffsll(), flock(), fls(),
     flsl(), flsll(), futimesat(), pipe2(), strlcat().  strlcpy(), strsep().

     In addition, reading or writing errno is async-signal safe.

     All functions not in the above lists are considered to be unsafe with respect to signals.
     That is to say, the behaviour of such functions is undefined when they are called from a
     signal handler that interrupted an unsafe function.  In general though, signal handlers
     should do little more than set a flag; most other actions are not safe.

     Also, it is good practice to make a copy of the global variable errno and restore it before
     returning from the signal handler.  This protects against the side effect of errno being set
     by functions called from inside the signal handler.

RETURN VALUES

     The sigaction() function returns the value 0 if successful; otherwise the value -1 is
     returned and the global variable errno is set to indicate the error.

EXAMPLES

     There are three possible prototypes the handler may match:

           ANSI C:
                  void handler(int);

           Traditional BSD style:
                  void handler(int, int code, struct sigcontext *scp);

           POSIX SA_SIGINFO:
                  void handler(int, siginfo_t *info, ucontext_t *uap);

     The handler function should match the SA_SIGINFO prototype if the SA_SIGINFO bit is set in
     sa_flags.  It then should be pointed to by the sa_sigaction member of struct sigaction.
     Note that you should not assign SIG_DFL or SIG_IGN this way.

     If the SA_SIGINFO flag is not set, the handler function should match either the ANSI C or
     traditional BSD prototype and be pointed to by the sa_handler member of struct sigaction.
     In practice, FreeBSD always sends the three arguments of the latter and since the ANSI C
     prototype is a subset, both will work.  The sa_handler member declaration in FreeBSD include
     files is that of ANSI C (as required by POSIX), so a function pointer of a BSD-style
     function needs to be casted to compile without warning.  The traditional BSD style is not
     portable and since its capabilities are a full subset of a SA_SIGINFO handler, its use is
     deprecated.

     The sig argument is the signal number, one of the SIG... values from <signal.h>.

     The code argument of the BSD-style handler and the si_code member of the info argument to a
     SA_SIGINFO handler contain a numeric code explaining the cause of the signal, usually one of
     the SI_... values from <sys/signal.h> or codes specific to a signal, i.e., one of the
     FPE_... values for SIGFPE.

     The scp argument to a BSD-style handler points to an instance of struct sigcontext.

     The uap argument to a POSIX SA_SIGINFO handler points to an instance of ucontext_t.

ERRORS

     The sigaction() system call will fail and no new signal handler will be installed if one of
     the following occurs:

     [EINVAL]           The sig argument is not a valid signal number.

     [EINVAL]           An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.

SEE ALSO

     kill(1), kill(2), ptrace(2), setitimer(2), setrlimit(2), sigaltstack(2), sigpending(2),
     sigprocmask(2), sigsuspend(2), wait(2), fpsetmask(3), setjmp(3), siginfo(3),
     siginterrupt(3), sigsetops(3), ucontext(3), tty(4)

STANDARDS

     The sigaction() system call is expected to conform to ISO/IEC 9945-1:1990 (“POSIX.1”).  The
     SA_ONSTACK and SA_RESTART flags are Berkeley extensions, as are the signals, SIGTRAP,
     SIGEMT, SIGBUS, SIGSYS, SIGURG, SIGIO, SIGXCPU, SIGXFSZ, SIGVTALRM, SIGPROF, SIGWINCH, and
     SIGINFO.  Those signals are available on most BSD-derived systems.  The SA_NODEFER and
     SA_RESETHAND flags are intended for backwards compatibility with other operating systems.
     The SA_NOCLDSTOP, and SA_NOCLDWAIT flags are featuring options commonly found in other
     operating systems.  The flags are approved by Version 2 of the Single UNIX Specification
     (“SUSv2”), along with the option to avoid zombie creation by ignoring SIGCHLD.