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SIGACTION(2)			    Linux Programmer's Manual			     SIGACTION(2)

       sigaction, sigprocmask, sigpending, sigsuspend - POSIX signal handling functions.

       #include <signal.h>

       int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact);

       int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);

       int sigpending(sigset_t *set);

       int sigsuspend(const sigset_t *mask);

       The  sigaction system call is used to change the action taken by a process on receipt of a
       specific signal.

       signum specifies the signal and can be any valid signal except SIGKILL and SIGSTOP.

       If act is non-null, the new action for signal signum is installed from act.  If oldact  is
       non-null, the previous action is saved in oldact.

       The sigaction structure is defined as something like

	      struct sigaction {
		  void (*sa_handler)(int);
		  void (*sa_sigaction)(int, siginfo_t *, void *);
		  sigset_t sa_mask;
		  int sa_flags;
		  void (*sa_restorer)(void);

       On some architectures a union is involved - do not assign to both sa_handler and sa_sigac-

       The sa_restorer element is obsolete and should not be used.   POSIX  does  not  specify	a
       sa_restorer element.

       sa_handler  specifies  the  action to be associated with signum and may be SIG_DFL for the
       default action, SIG_IGN to ignore this signal, or a pointer to a signal handling function.

       sa_mask gives a mask of signals which should be blocked during  execution  of  the  signal
       handler.   In addition, the signal which triggered the handler will be blocked, unless the
       SA_NODEFER or SA_NOMASK flags are used.

       sa_flags specifies a set of flags which	modify	the  behaviour	of  the  signal  handling
       process. It is formed by the bitwise OR of zero or more of the following:

		     If  signum is SIGCHLD, do not receive notification when child processes stop
		     (i.e., when child processes receive one  of  SIGSTOP,  SIGTSTP,  SIGTTIN  or

		     Restore  the  signal action to the default state once the signal handler has
		     been called.

		     Call the signal handler on an alternate signal  stack  provided  by  sigalt-
		     stack(2).	If an alternate stack is not available, the default stack will be

		     Provide behaviour compatible with BSD signal  semantics  by  making  certain
		     system calls restartable across signals.

		     Do  not  prevent  the  signal from being received from within its own signal

		     The signal handler takes 3 arguments, not one.  In this  case,  sa_sigaction
		     should  be  set instead of sa_handler.  (The sa_sigaction field was added in
		     Linux 2.1.86.)

       The siginfo_t parameter to sa_sigaction is a struct with the following elements

	      siginfo_t {
		  int	   si_signo;  /* Signal number */
		  int	   si_errno;  /* An errno value */
		  int	   si_code;   /* Signal code */
		  pid_t    si_pid;    /* Sending process ID */
		  uid_t    si_uid;    /* Real user ID of sending process */
		  int	   si_status; /* Exit value or signal */
		  clock_t  si_utime;  /* User time consumed */
		  clock_t  si_stime;  /* System time consumed */
		  sigval_t si_value;  /* Signal value */
		  int	   si_int;    /* POSIX.1b signal */
		  void *   si_ptr;    /* POSIX.1b signal */
		  void *   si_addr;   /* Memory location which caused fault */
		  int	   si_band;   /* Band event */
		  int	   si_fd;     /* File descriptor */

       si_signo, si_errno and si_code are defined for all signals.  The rest of the struct may be
       a union, so that one should only read the fields that are meaningful for the given signal.
       kill(2), POSIX.1b signals and SIGCHLD fill in si_pid and si_uid.   SIGCHLD also	fills  in
       si_status,  si_utime  and  si_stime.  si_int and si_ptr are specified by the sender of the
       POSIX.1b signal.  SIGILL, SIGFPE, SIGSEGV and SIGBUS fill in si_addr with the  address  of
       the fault.  SIGPOLL fills in si_band and si_fd.

       si_code	indicates  why	this  signal was sent.	It is a value, not a bitmask.  The values
       which are possible for any signal are listed in this table:

       |	      si_code		    |
       |Value	   | Signal origin	    |
       |SI_USER    | kill, sigsend or raise |
       |SI_KERNEL  | The kernel 	    |
       |SI_QUEUE   | sigqueue		    |
       |SI_TIMER   | timer expired	    |
       |SI_MESGQ   | mesq state changed     |
       |SI_ASYNCIO | AIO completed	    |
       |SI_SIGIO   | queued SIGIO	    |

       |	       SIGILL		     |
       |ILL_ILLOPC | illegal opcode	     |
       |ILL_ILLOPN | illegal operand	     |
       |ILL_ILLADR | illegal addressing mode |
       |ILL_ILLTRP | illegal trap	     |
       |ILL_PRVOPC | privileged opcode	     |
       |ILL_PRVREG | privileged register     |
       |ILL_COPROC | coprocessor error	     |
       |ILL_BADSTK | internal stack error    |

       |		   SIGFPE		      |
       |FPE_INTDIV | integer divide by zero	      |
       |FPE_INTOVF | integer overflow		      |
       |FPE_FLTDIV | floating point divide by zero    |
       |FPE_FLTOVF | floating point overflow	      |
       |FPE_FLTUND | floating point underflow	      |
       |FPE_FLTRES | floating point inexact result    |
       |FPE_FLTINV | floating point invalid operation |
       |FPE_FLTSUB | subscript out of range	      |

       |		      SIGSEGV			    |
       |SEGV_MAPERR | address not mapped to object	    |
       |SEGV_ACCERR | invalid permissions for mapped object |

       |		  SIGBUS		    |
       |BUS_ADRALN | invalid address alignment	    |
       |BUS_ADRERR | non-existent physical address  |
       |BUS_OBJERR | object specific hardware error |

       |	    SIGTRAP		|
       |TRAP_BRKPT | process breakpoint |
       |TRAP_TRACE | process trace trap |

       |		  SIGCHLD		    |
       |CLD_EXITED    | child has exited	    |
       |CLD_KILLED    | child was killed	    |
       |CLD_DUMPED    | child terminated abnormally |
       |CLD_TRAPPED   | traced child has trapped    |
       |CLD_STOPPED   | child has stopped	    |
       |CLD_CONTINUED | stopped child has continued |

       |		SIGPOLL 		 |
       |POLL_IN  | data input available 	 |
       |POLL_OUT | output buffers available	 |
       |POLL_MSG | input message available	 |
       |POLL_ERR | i/o error			 |
       |POLL_PRI | high priority input available |
       |POLL_HUP | device disconnected		 |

       The sigprocmask call is used to change the list of currently blocked signals.  The  behav-
       iour of the call is dependent on the value of how, as follows.

		     The set of blocked signals is the union of the current set and the set argu-

		     The signals in set are removed from the current set of blocked signals.   It
		     is legal to attempt to unblock a signal which is not blocked.

		     The set of blocked signals is set to the argument set.

       If oldset is non-null, the previous value of the signal mask is stored in oldset.

       The sigpending call allows the examination of pending signals (ones which have been raised
       while blocked).	The signal mask of pending signals is stored in set.

       The sigsuspend call temporarily replaces the signal mask for the process with  that  given
       by mask and then suspends the process until a signal is received.

       The  functions sigaction, sigprocmask, and sigpending return 0 on success and -1 on error.
       The function sigsuspend always returns -1, normally with the error EINTR.

       EINVAL An invalid signal was specified.	This will also be generated if an attempt is made
	      to change the action for SIGKILL or SIGSTOP, which cannot be caught.

       EFAULT act,  oldact,  set, oldset or mask point to memory which is not a valid part of the
	      process address space.

       EINTR  System call was interrupted.

       It is not possible to block SIGKILL or SIGSTOP with the sigprocmask call.  Attempts to  do
       so will be silently ignored.

       According  to  POSIX,  the  behaviour of a process is undefined after it ignores a SIGFPE,
       SIGILL, or SIGSEGV signal that was not generated by the kill() or the  raise()  functions.
       Integer	division  by zero has undefined result.  On some architectures it will generate a
       SIGFPE signal.  (Also dividing the most negative  integer  by  -1  may  generate  SIGFPE.)
       Ignoring this signal might lead to an endless loop.

       POSIX  (B.  disallows setting the action for SIGCHLD to SIG_IGN.  The BSD and SYSV
       behaviours differ, causing BSD software that sets the action for  SIGCHLD  to  SIG_IGN  to
       fail on Linux.

       The POSIX spec only defines SA_NOCLDSTOP.  Use of other sa_flags is non-portable.

       The SA_RESETHAND flag is compatible with the SVr4 flag of the same name.

       The  SA_NODEFER flag is compatible with the SVr4 flag of the same name under kernels 1.3.9
       and newer.  On older kernels the Linux implementation allowed the receipt of  any  signal,
       not just the one we are installing (effectively overriding any sa_mask settings).

       The  SA_RESETHAND  and SA_NODEFER names for SVr4 compatibility are present only in library
       versions 3.0.9 and greater.

       The SA_SIGINFO flag is specified by POSIX.1b.  Support for it was added in Linux 2.2.

       sigaction can be called with a null second argument to query the current  signal  handler.
       It  can	also  be used to check whether a given signal is valid for the current machine by
       calling it with null second and third arguments.

       See sigsetops(3) for details on manipulating signal sets.

       POSIX, SVr4.  SVr4 does not document the EINTR condition.

       Before the introduction of SA_SIGINFO it was also possible to get some additional informa-
       tion,  namely  by  using a sa_handler with second argument of type struct sigcontext.  See
       the relevant kernel sources for details.  This use is obsolete now.

       kill(1), kill(2), killpg(2), pause(2),  sigaltstack(2),	raise(3),  siginterrupt(3),  sig-
       nal(2), signal(7), sigsetops(3), sigvec(2)

Linux 2.4				    2001-12-29				     SIGACTION(2)
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