signal



SIGNAL(3)                 OpenBSD Programmer's Manual                SIGNAL(3)


NAME

     signal - simplified software signal facilities


SYNOPSIS

     #include <signal.h>

     void
     (*signal(int sigcatch, void (*func)(int sigraised)))(int);

     void
     (*bsd_signal(int sigcatch, void (*func)(int sigraised)))(int);


DESCRIPTION

     The signal() and bsd_signal() facilities are simplified interfaces to the
     more general sigaction(2) facility.  The bsd_signal() interface is
     provided for source compatibility only.  It is mainly used on systems
     where the standard signal() does not have BSD semantics.  On OpenBSD the
     two interfaces are identical.

     Signals allow the manipulation of a process from outside its domain as
     well as allowing the process to manipulate itself or copies of itself
     (children).  There are two general types of signals: those that cause
     termination of a process and those that do not.  Signals which cause
     termination of a program might result from an irrecoverable error or
     might be the result of a user at a terminal typing the ``interrupt''
     character.

     Signals are used when a process is stopped because it wishes to access
     its control terminal while in the background (see tty(4)).  Signals are
     optionally generated when a process resumes after being stopped, when the
     status of child processes changes, or when input is ready at the control
     terminal.  Most signals result in the termination of the process
     receiving them if no action is taken; some signals instead cause the
     process receiving them to be stopped, or are simply discarded if the
     process has not requested otherwise.

     Except for the SIGKILL and SIGSTOP signals, the signal() function allows
     for any signal to be caught, to be ignored, or to generate an interrupt.
     These signals are defined in the 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

     The func argument is a function to be called as the action upon receipt
     of the signal sigcatch.  The function will be called with one argument,
     sigraised, which is the signal raised (thus the same function, func, can
     be used by more than one signal).  To set the default action of the
     signal to occur as listed above, func should be SIG_DFL.  A SIG_DFL
     resets the default action.  To ignore the signal, func should be SIG_IGN.
     This will cause subsequent instances of the signal to be ignored and
     pending instances to be discarded.  If SIG_IGN is not used, further
     occurrences of the signal are automatically blocked and func is called.

     If the func is set to SIG_IGN 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.  This differs from
     historical BSD behavior but is consistent with AT&T System V UNIX as well
     as the X/Open Portability Guide Issue 4, Version 2 (``XPG4.2'').

     The handled signal is unblocked when func returns and the process
     continues from where it left off when the signal occurred.  Unlike
     previous signal facilities, the handler func() remains installed after a
     signal has been delivered.

     For some system calls, if a signal is caught while the call is executing
     and the call is prematurely terminated, the call is automatically
     restarted.  (The handler is installed using the SA_RESTART flag with
     sigaction(2).) The affected system calls include read(2), write(2),
     sendto(2), recvfrom(2), sendmsg(2), and recvmsg(2) on a communications
     channel or a low-speed device and during a ioctl(2) or wait(2).  However,
     calls that have already committed are not restarted, but instead return a
     partial success (for example, a short read count).  The siginterrupt(3)
     function can be used to change the system call restart behavior for a
     specific signal.

     When a process which has installed signal handlers forks, the child
     process inherits the signals.  All caught signals may be reset to their
     default action by a call to the execve(2) function; ignored signals
     remain 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:

     Base Interfaces:

     _exit(), 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(), fork(), fpathconf(), fstat(), fstatat(),
     fsync(), ftruncate(), futimens(), futimes(), getegid(), geteuid(),
     getgid(), getgroups(), getpeername(), getpgrp(), getpid(), getppid(),
     getsockname(), getsockopt(), getuid(), kill(), link(), linkat(),
     listen(), lseek(), lstate(), mkdir(), mkdirat(), mkfifo(), mkfifoat(),
     mknod(), mknodat(), open(), openat(), pathconf(), pause(), pipe(),
     poll(), 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(), socket(), socketpair(), stat(), symlink(),
     symlinkat(), sysconf(), tcdrain(), tcflow(), tcflush(), tcgetattr(),
     tcgetpgrp(), tcsendbreak(), tcsetattr(), tcsetpgrp(), time(), times(),
     umask(), uname(), unlink(), unlinkat(), utime(), utimensat(), utimes(),
     wait(), waitpid(), write().

     ANSI C Interfaces:

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

     Extension Interfaces:

     chflags(), fchflags(), getresgid(), getresuid(), 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
     modification 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:

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


RETURN VALUES

     The previous action is returned on a successful call.  Otherwise, SIG_ERR
     is returned and the global variable errno is set to indicate the error.


ERRORS

     signal() will fail and no action will take place if one of the following
     occurs:

     [EINVAL]           A specified signal 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), sigaction(2), sigaltstack(2),
     sigprocmask(2), sigsuspend(2), setjmp(3), siginterrupt(3), tty(4)


HISTORY

     A signal() system call first appeared in Version 4 AT&T UNIX.  In 4.2BSD,
     it was reimplemented as a wrapper around the former sigvec() system call,
     and for 4.3BSD-Reno, it was rewritten to use sigaction(2) instead.

OpenBSD 5.4                      July 17, 2013                     OpenBSD 5.4

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