MirBSD manpage: bsd_signal(3), signal(3)

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


     signal - simplified software signal facilities


     #include <signal.h>

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

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


     The signal() and bsd_signal() facilities are simplified interfaces to the
     more general sigaction(2) facility. The bsd_signal() interface is provid-
     ed for source compatibility only. It is mainly used on systems where the
     standard signal() does not have BSD semantics. On OpenBSD the two inter-
     faces 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 ter-
     mination of a process and those that do not. Signals which cause termina-
     tion 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 re-
     ceiving 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
     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
     SIGTTOU     stop process        background write attempted to control
     SIGIO       discard signal      I/O is possible on a descriptor (see
     SIGXCPU     terminate process   CPU time limit exceeded (see
     SIGXFSZ     terminate process   file size limit exceeded (see
     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

     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 in-
     stances 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 histor-
     ical 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 contin-
     ues from where it left off when the signal occurred. Unlike previous sig-
     nal 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 res-
     tarted. (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 pro-
     cess inherits the signals. All caught signals may be reset to their de-
     fault action by a call to the execve(2) function; ignored signals remain

     The following functions are either reentrant or not interruptible by sig-
     nals and are asyncronous-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(), readv(), 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(), writev().

     Realtime Interfaces:

     aio_error(), clock_gettime(), sigpause(), timer_getoverrun(),
     aio_return(), fdatasync(), sigqueue(), timer_gettime(), aio_suspend(),
     sem_post(), sigset(), timer_settime().

     ANSI C Interfaces:

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

     Extension Interfaces:

     getentropy(), strlcpy(), strlcat(), syslog_r().

     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; most other actions are
     not safe.

     Additionally, inside the signal handler it is also considered more safe
     to make a copy of the global variable errno and restore it before return-
     ing 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.


     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.


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

     [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.


     kill(1), kill(2), ptrace(2), sigaction(2), sigaltstack(2),
     sigprocmask(2), sigsuspend(2), setjmp(3), siginterrupt(3), tty(4)


     This signal() facility appeared in 4.0BSD.

MirBSD #10-current             December 5, 2021                              2

Generated on 2021-12-07 11:07:08 by $MirOS: src/scripts/roff2htm,v 1.103 2021/01/23 20:24:35 tg Exp $ — This product includes material provided by mirabilos.

These manual pages and other documentation are copyrighted by their respective writers; their sources are available at the project’s CVSweb, AnonCVS and other mirrors. The rest is Copyright © 2002–2021 MirBSD.

This manual page’s HTML representation is supposed to be valid XHTML/1.1; if not, please send a bug report — diffs preferred.