Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
SIGACTION(2)              FreeBSD System Calls Manual             SIGACTION(2)

     sigaction - software signal facilities

     #include <signal.h>

     struct sigaction {
             void     (*sa_handler)();  /* signal handler */
             sigset_t sa_mask;          /* signal mask to apply */
             int      sa_flags;         /* see signal options below */
     sigaction(int sig, const struct sigaction *act, struct sigaction *oact);

     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 process
     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, in
     which case its delivery is postponed 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 process.  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
     process.  The signal mask for a process is initialized from that of its
     parent (normally empty).  It may be changed with a sigprocmask(2) call,
     or when a signal is delivered to the process.

     When a signal condition arises for a process, the signal is added to a
     set of signals pending for the process.  If the signal is not currently
     blocked by the process then it is delivered to the process.  Signals may
     be delivered any time a process 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) function.  When a caught
     signal is delivered, the current state of the process 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 process will resume execution in
     the context from before the signal's delivery.  If the process wishes to
     resume in a different context, then it must arrange to restore the
     previous context itself.

     When a signal is delivered to a process a new signal mask is installed
     for the duration of the process' signal handler (or until a sigprocmask
     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.

     Sigaction() 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.

     Once a signal handler is installed, it normally remains installed until
     another sigaction() 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.  If the SA_NOCLDSTOP 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.  Further, if the SA_ONSTACK bit is set in
     sa_flags, the system will deliver the signal to the process on a signal
     stack, specified with sigaltstack(2).  If the SA_NODEFER bit is set,
     further occurrences of the delivered signal are not masked during the
     execution of the handler.  If the SA_RESETHAND bit is set, the handler is
     reset back to SIG_DFL at the moment the signal is delivered.

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

     Execve(2) 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

       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 Xr    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
     SIGSTOP         stop process            stop (cannot be caught or
     SIGTSTP         stop process            stop signal generated from
     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))
     Dv SIGXCPU      terminate process       cpu time limit exceeded (see
     Dv SIGXFSZ      terminate process       file size limit exceeded (see
     Dv SIGVTALRM    terminate process       virtual time alarm (see
     Dv SIGPROF      terminate process       profiling timer alarm (see
     Dv 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

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

     A 0 value indicated that the call succeeded.  A -1 return value indicates
     an error occurred and errno is set to indicated the reason.

     The handler routine can be declared:

           void handler(sig, code, scp)
           int sig, code;
           struct sigcontext *scp;

     Here sig is the signal number, into which the hardware faults and traps
     are mapped.  Code is a parameter that is either a constant or the code
     provided by the hardware.  Scp is a pointer to the sigcontext structure
     (defined in <signal.h>), used to restore the context from before the

     Sigaction() will fail and no new signal handler will be installed if one
     of the following occurs:

     [EFAULT]           Either act or oact points to memory that is not a
                        valid part of the process address space.

     [EINVAL]           Sig is not a valid signal number.

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

     The sigaction() function call is expected to conform to IEEE Std
     1003.1-1990 (``POSIX.1'').  The SA_ONSTACK and SA_RESTART flags are
     Berkeley extensions, as are the signals, SIGTRAP, SIGEMT, SIGBUS, SIGSYS,
     SIGINFO.  Those signals are available on most BSD-derived systems.  The
     SA_NODEFER and SA_RESETHAND are intended for backwards compatability with
     other operating systems.

     kill(1), kill(2), ptrace(2), sigaltstack(2), sigblock(2), sigpause(2),
     sigprocmask(2), sigsetmask(2), sigsuspend(2), sigvec(2), setjmp(3),
     siginterrupt(3), sigsetops(3), tty(4)

FreeBSD 11.0-PRERELEASE          April 3, 1994         FreeBSD 11.0-PRERELEASE


Want to link to this manual page? Use this URL:

home | help