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NAME    [Toc]    [Back]

     signal - simplified software signal facilities

SYNOPSIS    [Toc]    [Back]

     #include <signal.h>

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

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

DESCRIPTION    [Toc]    [Back]

     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
     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
     SIGPIPE         terminate process       write on a pipe with
no reader
     SIGALRM          terminate process       real-time timer expired
     SIGTERM         terminate process       software termination
     SIGURG           discard  signal           urgent  condition
present on
     SIGSTOP          stop  process             stop  (cannot  be
caught or
     SIGTSTP         stop process            stop signal generated from
     SIGCONT         discard signal          continue after stop
     SIGCHLD         discard  signal           child  status  has
     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
                                             (see fcntl(2))
     SIGXCPU         terminate process       CPU time  limit  exceeded (see
     SIGXFSZ          terminate process       file size limit exceeded (see
     SIGVTALRM       terminate process       virtual  time  alarm
     SIGPROF           terminate  process        profiling  timer
alarm (see
     SIGWINCH        discard signal          window size change
     SIGINFO         discard signal          status request  from
     SIGUSR1          terminate process       user-defined signal
     SIGUSR2         terminate process       user-defined  signal

     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
     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    [Toc]    [Back]

     as the X/Open Portability Guide Issue 4.2 (``XPG4.2'').

     The handled signal is unblocked when func  returns  and  the
process continues
 from where it left off when the signal occurred.  Unlike
     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),
     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

     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(),
     cfsetospeed(), chdir(), chmod(), chown(), close(),  creat(),
     dup2(),  execle(),  execve(),  fcntl(), fork(), fpathconf(),
     fsync(),  getegid(),   geteuid(),   getgid(),   getgroups(),
     getpid(),  getppid(),  getuid(),  kill(),  link(),  lseek(),
     mkfifo(),  open(),  pathconf(),  pause(),  pipe(),  raise(),
read(), rename(),
     rmdir(),    setgid(),    setpgid(),    setsid(),   setuid(),
     sigaddset(),   sigdelset(),   sigemptyset(),   sigfillset(),
     signal(),    sigpending(),    sigprocmask(),   sigsuspend(),
sleep(), stat(),
     sysconf(),  tcdrain(),  tcflow(),  tcflush(),   tcgetattr(),
     tcsendbreak(),  tcsetattr(),  tcsetpgrp(),  time(), times(),
     uname(), unlink(), utime(), wait(), waitpid(), write().

     Realtime Interfaces:

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

     ANSI C Interfaces:

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

     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 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.

RETURN VALUES    [Toc]    [Back]

     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    [Toc]    [Back]

     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.

SEE ALSO    [Toc]    [Back]

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

HISTORY    [Toc]    [Back]

     This signal() facility appeared in 4.0BSD.

OpenBSD      3.6                          April      19,     1994
[ Back ]
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