MirOS Manual: sigaction(2)

SIGACTION(2)               BSD Programmer's Manual                SIGACTION(2)

NAME

     sigaction - software signal facilities

SYNOPSIS

     #include <signal.h>

     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 de-
     fines 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.

     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 de-
     fault 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. 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.

           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>. The siginfo_t struc-
                           ture is a part of IEEE Std 1003.1b ("POSIX"). 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 list-
                           ed 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 af-
                           fected system calls include 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 al-
                           ready committed are not restarted, but instead re-
                           turn 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

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 sig-
     nals and are async-signal safe. Therefore applications may invoke them,
     without restriction, from signal-catching functions:

           _exit(2), access(2), alarm(3), cfgetispeed(3), cfgetospeed(3),
           cfsetispeed(3), cfsetospeed(3), chdir(2), chmod(2), chown(2),
           close(2), creat(3), dup(2), dup2(2), execle(3), execve(2),
           fcntl(2), fork(2), fpathconf(2), fstat(2), fsync(2), getegid(2),
           geteuid(2), getgid(2), getgroups(2), getpgrp(2), getpid(2),
           getppid(2), getuid(2), kill(2), link(2), lseek(2), mkdir(2),
           mkfifo(2), open(2), pathconf(2), pause(3), pipe(2), raise(3),
           read(2), rename(2), rmdir(2), setgid(2), setpgid(2), setsid(2),
           setuid(2), sigaction(2), sigaddset(3), sigdelset(3),
           sigemptyset(3), sigfillset(3), sigismember(3), signal(3),
           sigpause(3), sigpending(2), sigprocmask(2), sigsuspend(2),
           sleep(3), stat(2), sysconf(3), tcdrain(3), tcflow(3), tcflush(3),
           tcgetattr(3), tcgetpgrp(3), tcsendbreak(3), tcsetattr(3),
           tcsetpgrp(3), time(3), times(3), umask(2), uname(3), unlink(2),
           utime(3), wait(2), waitpid(2), write(2).

     Please see signal(3) for a more detailed list.

     All functions not in the above list 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.

     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(sig)
           {
                   int save_errno = errno;

                   ...
                   errno = save_errno;
           }

RETURN VALUES

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

EXAMPLES

     The handler routine can be declared:

           void
           handler(sig)
                   int sig;

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

           void
           handler(sig, sip, scp)
                   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 1003.1-1990 ("POSIX"). The
     SA_ONSTACK and SA_RESTART flags are Berkeley extensions, as are the sig-
     nals SIGTRAP, SIGEMT, SIGBUS, SIGSYS, SIGURG, SIGIO, SIGXCPU, SIGXFSZ,
     SIGVTALRM, SIGPROF, SIGWINCH, and SIGINFO. These signals are available on
     most BSD-derived systems. The SA_NODEFER and SA_RESETHAND flags are in-
     tended for backwards compatibility with other operating systems. The
     SA_NOCLDSTOP, SA_NOCLDWAIT, and SA_SIGINFO flags are options commonly
     found in other operating systems. 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:

     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(), get-
     pid(), getppid(), getuid(), kill(), link(), lseek(), mkdir(), mkfifo(),
     open(), pathconf(), pause(), pipe(), raise(), read(), rename(), rmdir(),
     setgid(), setpgid(), setsid(), setuid(), sigaction(), sigaddset(), sig-
     delset(), sigemptyset(), sigfillset(), sigismember(), signal(), sigpend-
     ing(), sigprocmask(), sigsuspend(), sleep(), stat(), sysconf(),
     tcdrain(), tcflow(), tcflush(), tcgetattr(), tcgetpgrp(), tcsendbreak(),
     tcsetattr(), tcsetpgrp(), time(), times(), umask(), uname(), unlink(),
     utime(), wait(), waitpid(), write().

     ANSI C Interfaces:

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

     Extension Interfaces:

     strlcat(), strlcpy().

     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.

     Additionally, inside the signal handler it is also considered safer to
     make a copy of the global variable errno and restore it before returning
     from the signal handler.

     A few other functions are signal race safe in OpenBSD but probably not on
     other systems:

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

MirOS BSD #10-current           April 3, 1994                                5

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