manpagez: man pages & more
man sigaction(2)
Home | html | info | man
sigaction(2)                BSD System Calls Manual               sigaction(2)


     sigaction -- software signal facilities


     Standard C Library (libc, -lc)


     #include <signal.h>

     struct  sigaction {
             union __sigaction_u __sigaction_u;  /* signal handler */
             sigset_t sa_mask;               /* signal mask to apply */
             int     sa_flags;               /* see signal options below */

     union __sigaction_u {
             void    (*__sa_handler)(int);
             void    (*__sa_sigaction)(int, struct __siginfo *,
                            void *);

     #define sa_handler      __sigaction_u.__sa_handler
     #define sa_sigaction    __sigaction_u.__sa_sigaction

     sigaction(int sig, const struct sigaction *restrict act,
         struct sigaction *restrict 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 invo-
     cation 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 sig-
     nals are ready to be delivered at the same time, any signals that could
     be caused by traps are delivered first.  Additional signals may be pro-
     cessed 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 process is saved, a
     new signal mask is calculated (as described below), and the signal han-
     dler is invoked.  The call to the handler is arranged so that if the sig-
     nal 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(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 sig-
     nal.  If oact is non-zero, the previous handling information for the sig-
     nal is returned to the user.

     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 sig-
     nal 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 sig-
     nal 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 func-
                           tion 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

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

           SA_NODEFER      If this bit is set, further occurrences of the
                           delivered signal are not masked during the execu-
                           tion 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

     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.

     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

     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
     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))
     SIGXCPU         terminate process       cpu time limit exceeded (see
     SIGXFSZ         terminate process       file size limit exceeded (see
     SIGVTALRM       terminate process       virtual time alarm (see
     SIGPROF         terminate process       profiling timer alarm (see
     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.

     The following functions are either reentrant or not interruptible by sig-
     nals and are async-signal safe.  Therefore applications may invoke them,
     without restriction, from signal-catching functions:

     Base Interfaces:

     _exit(), access(), alarm(), cfgetispeed(), cfgetospeed(), cfsetispeed(),
     cfsetospeed(), chdir(), chmod(), chown(), close(), creat(), dup(),
     dup2(), execle(), execve(), fcntl(), fork(), fpathconf(), fstat(),
     fsync(), getegid(), geteuid(), getgid(), getgroups(), getpgrp(),
     getpid(), getppid(), getuid(), kill(), link(), lseek(), mkdir(),
     mkfifo(), open(), pathconf(), pause(), pipe(), raise(), read(), rename(),
     rmdir(), setgid(), setpgid(), setsid(), setuid(), sigaction(),
     sigaddset(), sigdelset(), sigemptyset(), sigfillset(), sigismember(),
     signal(), sigpending(), sigprocmask(), sigsuspend(), sleep(), stat(),
     sysconf(), tcdrain(), tcflow(), tcflush(), tcgetattr(), tcgetpgrp(),
     tcsendbreak(), tcsetattr(), tcsetpgrp(), time(), times(), umask(),
     uname(), unlink(), utime(), wait(), waitpid(), write().

     Realtime Interfaces:

     aio_error(), sigpause(), aio_return(), aio_suspend(), sem_post(),

     ANSI C Interfaces:

     strcpy(), strcat(), strncpy(), strncat(), and perhaps some others.

     Extension Interfaces:

     strlcpy(), strlcat().

     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 when
     called from a signal handler is undefined.  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.


     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


     There are three possible prototypes the handler may match:

           ANSI C:
                  void handler(int);

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

     The handler function should match the SA_SIGINFO prototype if the SA_SIG-
     INFO bit is set in 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 capa-
     bilities are a full subset of a SA_SIGINFO handler, its use is depre-

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

     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/sig-
     nal.h> or codes specific to a signal, i.e. one of the FPE_... values for

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


     The sigaction() system call 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]           The sig argument is not a valid signal number.

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

     [EINVAL]           An attempt was made to set the action to SIG_DFL for a
                        signal that cannot be caught or ignored (or both).


     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 exten-
     sions, as are the signals, SIGTRAP, SIGEMT, SIGBUS, SIGSYS, SIGURG,
     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.


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

BSD                           September 18, 2008                           BSD

Mac OS X 10.9.1 - Generated Mon Jan 6 18:15:12 CST 2014
© 2000-2024
Individual documents may contain additional copyright information.