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NAME    [Toc]    [Back]

       sigaction,  sigprocmask, sigpending, sigsuspend - POSIX signal handling

SYNOPSIS    [Toc]    [Back]

       #include <signal.h>

       int sigaction(int signum, const struct sigaction *act, struct sigaction

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

       int sigpending(sigset_t *set);

       int sigsuspend(const sigset_t *mask);

DESCRIPTION    [Toc]    [Back]

       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_sigaction.

       The sa_restorer element is obsolete and should not be used.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

		     Restore  the  signal action to the default state once the
		     signal handler has been called.   (This  is  the  default
		     behavior of the signal(2) system call.)

		     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 handler.

		     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

	      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_sta-
       tus, 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_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

       |	      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-existant 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 behaviour of the call is dependent on the value of how, as

		     The  set  of  blocked signals is the union of the current
		     set and the set argument.

		     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.

RETURN VALUE    [Toc]    [Back]

       The  functions sigaction, sigprocmask, sigpending and sigsuspend return
       0 on success and -1 on error.  (In the case of sigsuspend there will be
       no success, and only the error return with EINTR is possible.)

ERRORS    [Toc]    [Back]

       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 or oldset point to memory which is not a valid
	      part of the process address space.

       EINTR  System call was interrupted.

NOTES    [Toc]    [Back]

       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

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

       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.

CONFORMING TO    [Toc]    [Back]

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

UNDOCUMENTED    [Toc]    [Back]

       Before the introduction of SA_SIGINFO it was also possible to get  some
       additional  information, 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.

SEE ALSO    [Toc]    [Back]

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

Linux 2.4			  2000-08-02			  SIGACTION(2)
[ Back ]
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