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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);

DESCRIPTION
     The signal() facility is a simplified interface to the more general
     sigaction(2) facility.

     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 ter-
     mination 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 receiv-
     ing 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
     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

     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 sig-
     nal 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 4.2 (``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
     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).

     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
     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(), 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:

     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 as long as $n positional arguments are not used.
           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
     occur:

     [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), tty(4)

HISTORY
     This signal() facility appeared in 4.0BSD.

OpenBSD 3.4                     April 19, 1994                               3

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY

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