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NAME    [Toc]    [Back]

     sigaction - software signal facilities

SYNOPSIS    [Toc]    [Back]

     #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

     sigaction(int  sig,  const  struct  sigaction  *act,  struct
sigaction *oact);

DESCRIPTION    [Toc]    [Back]

     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 invocation
 blocked, but other signals may yet occur.   A  global
signal mask
     defines  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 signals
 are ready to be delivered at the same time, any signals
that could
     be caused by traps are delivered first.  Additional  signals
may be processed
  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 handling
  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 previous
     context itself.

     When a signal is delivered to a process a new signal mask is
     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 default
 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

     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 function
  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

           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 execution
                           of the handler.

           SA_RESETHAND    If this bit is set, the handler is reset back to
                           SIG_DFL at the moment  the  signal  is

           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 structure
 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

           SA_RESTART       If a signal is caught during the system calls listed
 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
                           affected system calls include read(2),
                           sendto(2), recvfrom(2), sendmsg(2) and
                           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 already
 committed are not restarted, but
instead return
 a partial success (for example, a
short read

     After a fork(2) or vfork(2), all signals, the  signal  mask,
the signal
     stack,  and the restart/interrupt flags are inherited by the

     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

     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 (cannot
be caught or
     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

NOTE    [Toc]    [Back]

     The sa_mask field specified in act is not allowed  to  block
     SIGSTOP.  Any attempt to do so will be silently 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:

           _exit(2),  access(2),  alarm(3), cfgetispeed(3), cfgetospeed(3),
           cfsetispeed(3),  cfsetospeed(3),  chdir(2),  chmod(2),
           close(2),  creat(3),  dup(2),  dup2(2), execle(3), execve(2),
           fcntl(2), fork(2), fpathconf(2),  fstat(2),  fsync(2),
           geteuid(2),  getgid(2), getgroups(2), getpgrp(2), getpid(2),
           getppid(2),  getuid(2),  kill(2),  link(2),  lseek(2),
           mkfifo(2),  open(2),  pathconf(2),  pause(3), pipe(2),
           read(2), rename(2), rmdir(2),  setgid(2),  setpgid(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),
           tcgetattr(3), tcgetpgrp(3),  tcsendbreak(3),  tcsetattr(3),
           tcsetpgrp(3),  time(3),  times(3), umask(2), uname(3),
           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 modification
  of  the  external symbol errno by the above functions,
saving it at
     entry and restoring it on return, thus:

                   int save_errno = errno;

                   errno = save_errno;

RETURN VALUES    [Toc]    [Back]

     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    [Toc]    [Back]

     The handler routine can be declared:

                   int sig;

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

           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
     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

ERRORS    [Toc]    [Back]

     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    [Toc]    [Back]

     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    [Toc]    [Back]

     The  sigaction()  function  conforms to IEEE Std 1003.1-1990
     The SA_ONSTACK and SA_RESTART flags are Berkeley extensions,
as are the
     SIGVTALRM, SIGPROF, SIGWINCH, and  SIGINFO.   These  signals
are available
     on  most  BSD-derived systems.  The SA_NODEFER and SA_RESETHAND flags are
     intended for backwards compatibility  with  other  operating
systems.  The
     SA_NOCLDSTOP, SA_NOCLDWAIT, and SA_SIGINFO flags are options
     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 signalcatching

     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().

     ANSI C Interfaces:

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

     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.

OpenBSD      3.6                           April      3,     1994
[ Back ]
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