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SIGACTION(2)                BSD System Calls Manual               SIGACTION(2)

NAME
     sigaction -- software signal facilities

SYNOPSIS
     #include <&lt;signal.h>&gt;

     struct sigaction {
             union {         /* signal handler */
                     void    (*__sa_handler)(int);
                     void    (*__sa_sigaction)(int, siginfo_t *, void *);
             } __sigaction_u;
             sigset_t sa_mask;          /* signal mask to apply */
             int      sa_flags;         /* see signal options below */
     };

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

     int
     sigaction(int sig, const struct sigaction *act, struct sigaction *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 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) 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 han-
     dling 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 previ-
     ous 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) 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 sa_mask associated with the handler to be invoked, but always
     excluding SIGKILL and SIGSTOP.

     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.  The
     value of sa_handler (or, if the SA_SIGINFO flag is set, the value of
     sa_sigaction instead) indicates what action should be performed when a
     signal arrives.  A signal-specific default action may be reset by setting
     sa_handler to SIG_DFL.  Alternately, if the SA_RESETHAND flag is set the
     default action will be reinstated when the signal is first posted.  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.  If sig is SIGCHLD and sa_handler is
     set to SIG_IGN, the SA_NOCLDWAIT flag (described below) is implied.

     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, though existing zombies will remain.  If the
                           calling process subsequently issues a waitpid(2)
                           (or equivalent) and there are no previously exist-
                           ing zombie child processes that match the
                           waitpid(2) criteria, it blocks until all of the
                           calling process's child processes that would match
                           terminate, and then returns a value of -1 with
                           errno set to ECHILD.

           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_SIGINFO      If this bit is set, the 2nd argument of the handler
                           is set to be a pointer to a siginfo_t structure as
                           described in <sys/siginfo.h>.  It provides much
                           more information about the causes and attributes of
                           the signal that is being delivered.

           SA_RESTART      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.  Restarting of pending calls is
                           requested by setting the SA_RESTART bit in
                           sa_flags.  The affected system calls include
                           read(2), write(2), sendto(2), recvfrom(2),
                           sendmsg(2) and recvmsg(2) on a communications chan-
                           nel 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
     <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 (cannot be caught or
                                                       ignored)
     SIGBUS        create core image    bus error
     SIGSEGV       create core image    segmentation violation
     SIGSYS        create core image    system call given invalid argument
     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
     SIGTHR        discard signal       thread AST

RETURN VALUES
     Upon successful completion, the value 0 is returned; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.

EXAMPLES
     The handler routine can be declared:

           void
           handler(int sig)

     If the SA_SIGINFO option is enabled, the canonical way to declare it is:

           void
           handler(int sig, siginfo_t *sip, struct sigcontext *scp)

     Here sig is the signal number, into which the hardware faults and traps
     are mapped.  If the SA_SIGINFO option is set, sip is a pointer to a
     siginfo_t as described in <sys/siginfo.h>.  If SA_SIGINFO is not set,
     this pointer will be NULL instead.  The function specified in
     sa_sigaction will be called instead of the function specified by
     sa_handler (Note that in some implementations these are in fact the
     same).  scp is a pointer to the sigcontext structure (defined in
     <signal.h>), used to restore the context from before the signal.

ERRORS
     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.

SEE ALSO
     kill(1), kill(2), ptrace(2), sigaltstack(2), sigprocmask(2),
     sigsuspend(2), wait(2), setjmp(3), sigblock(3), sigpause(3),
     sigsetops(3), sigvec(3), tty(4)

STANDARDS
     The sigaction() function conforms to IEEE Std 1003.1-2008 (``POSIX.1'').

     The SA_ONSTACK flag and the SIGPROF, SIGSYS, SIGTRAP, SIGVTALRM, SIGXCPU,
     and SIGXFSZ signals conform to the X/Open System Interfaces option of
     that standard.  The standard marks SIGPROF as obsolescent.  The signals
     SIGEMT, SIGINFO, SIGIO, and SIGWINCH are Berkeley extensions.  These sig-
     nals are available on most BSD-derived systems.  The SIGTHR signal is
     specific to OpenBSD and is part of the implementation of thread cancella-
     tion; sigaction and other signal interfaces may reject attempts to use or
     alter the handling of SIGTHR.

     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:

     Standard Interfaces:

     _exit(), _Exit(), abort(), accept(), access(), alarm(), bind(),
     cfgetispeed(), cfgetospeed(), cfsetispeed(), cfsetospeed(), chdir(),
     chmod(), chown(), clock_gettime(), close(), connect(), creat(), dup(),
     dup2(), execl(), execle(), execv(), execve(), faccessat(), fchdir(),
     fchmod(), fchmodat(), fchown(), fchownat(), fcntl(), fdatasync(), fork(),
     fpathconf(), fstat(), fstatat(), fsync(), ftruncate(), futimens(),
     futimes(), 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(), pathconf(),
     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(), sigpause(), sigpending(), sigprocmask(), sigsuspend(), sleep(),
     sockatmark(), socket(), socketpair(), stat(), strcat(), strcpy(),
     strncat(), strncpy(), symlink(), symlinkat(), sysconf(), tcdrain(),
     tcflow(), tcflush(), tcgetattr(), tcgetpgrp(), tcsendbreak(),
     tcsetattr(), tcsetpgrp(), time(), times(), umask(), uname(), unlink(),
     unlinkat(), utime(), utimensat(), utimes(), wait(), waitpid(), write(),
     and perhaps some others.

     Extension Interfaces:

     accept4(), chflags(), dup3(), fchflags(), getentropy(), getresgid(),
     getresuid(), pipe2(), ppoll(), sendsyslog(), setresgid(), setresuid(),
     strlcat(), strlcpy(), wait3(), wait4().

     In addition, access and updates to errno are guaranteed to be safe.  Most
     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, ideally of type volatile
     sig_atomic_t; most other actions are not safe.

     Additionally, it is advised that signal handlers guard against modifica-
     tion of the external symbol errno by the above functions, saving it at
     entry and restoring it on return, thus:

           void
           handler(int sig)
           {
                   int save_errno = errno;

                   ...
                   errno = save_errno;
           }

     The functions below are async-signal-safe in OpenBSD except when used
     with floating-point arguments or directives, but are probably unsafe on
     other systems:

           dprintf()     Safe.
           vdprintf()    Safe.
           snprintf()    Safe.
           vsnprintf()   Safe.
           syslog_r()    Safe if the syslog_data struct is initialized as a
                         local variable.

BSD                             March 27, 2017                             BSD