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

NAME
       sigaction - examine and change a	signal action

SYNOPSIS
       #include	<signal.h>

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

   Feature Test	Macro Requirements for glibc (see feature_test_macros(7)):

       sigaction(): _POSIX_C_SOURCE >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE

       siginfo_t: _POSIX_C_SOURCE >= 199309L

DESCRIPTION
       The  sigaction()	 system	 call  is used to change the action taken by a
       process on receipt of a specific	signal.	 (See signal(7)	for  an	 over-
       view of signals.)

       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       The sa_restorer element is obsolete and should not be used.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.  This function receives the sig-
       nal number as its only argument.

       If SA_SIGINFO is	specified in sa_flags, then sa_sigaction  (instead  of
       sa_handler)  specifies  the  signal-handling function for signum.  This
       function	receives the signal number as its first	argument, a pointer to
       a  siginfo_t as its second argument and a pointer to a ucontext_t (cast
       to void *) as its third	argument.   (Commonly,	the  handler  function
       doesn't make any	use of the third argument.  See	getcontext(3) for fur-
       ther information	about ucontext_t.)

       sa_mask specifies a mask	of signals  which  should  be  blocked	(i.e.,
       added  to  the signal mask of the thread	in which the signal handler is
       invoked)	during execution of the	signal handler.	 In addition, the sig-
       nal  which triggered the	handler	will be	blocked, unless	the SA_NODEFER
       flag is used.

       sa_flags	specifies a set	of flags which modify the behavior of the sig-
       nal.  It	is formed by the bitwise OR of zero or more of the following:

	   SA_NOCLDSTOP
		  If signum is SIGCHLD,	do not receive notification when child
		  processes stop (i.e.,	when  they  receive  one  of  SIGSTOP,
		  SIGTSTP,  SIGTTIN, or	SIGTTOU) or resume (i.e., they receive
		  SIGCONT) (see	wait(2)).  This	flag is	meaningful  only  when
		  establishing a handler for SIGCHLD.

	   SA_NOCLDWAIT	(since Linux 2.6)
		  If signum is SIGCHLD,	do not transform children into zombies
		  when they terminate.	See also  waitpid(2).	This  flag  is
		  meaningful  only when	establishing a handler for SIGCHLD, or
		  when setting that signal's disposition to SIG_DFL.

		  If the SA_NOCLDWAIT flag is set when establishing a  handler
		  for SIGCHLD, POSIX.1 leaves it unspecified whether a SIGCHLD
		  signal is generated when a  child  process  terminates.   On
		  Linux,  a  SIGCHLD signal is generated in this case; on some
		  other	implementations, it is not.

	   SA_NODEFER
		  Do not prevent the signal from being	received  from	within
		  its  own  signal handler.  This flag is meaningful only when
		  establishing a signal	handler.  SA_NOMASK  is	 an  obsolete,
		  nonstandard synonym for this flag.

	   SA_ONSTACK
		  Call	the  signal  handler on	an alternate signal stack pro-
		  vided	by sigaltstack(2).   If	 an  alternate	stack  is  not
		  available,  the  default  stack  will	be used.  This flag is
		  meaningful only when establishing a signal handler.

	   SA_RESETHAND
		  Restore the signal action to the default upon	entry  to  the
		  signal  handler.   This  flag	is meaningful only when	estab-
		  lishing a signal handler.  SA_ONESHOT	is an  obsolete,  non-
		  standard synonym for this flag.

	   SA_RESTART
		  Provide  behavior  compatible	 with  BSD signal semantics by
		  making certain  system  calls	 restartable  across  signals.
		  This flag is meaningful only when establishing a signal han-
		  dler.	  See  signal(7)  for  a  discussion  of  system  call
		  restarting.

	   SA_SIGINFO (since Linux 2.2)
		  The  signal handler takes three arguments, not one.  In this
		  case,	sa_sigaction should  be	 set  instead  of  sa_handler.
		  This flag is meaningful only when establishing a signal han-
		  dler.

       The siginfo_t argument 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 */
	       int	si_trapno;   /*	Trap number that caused
					hardware-generated signal
					(unused	on most	architectures) */
	       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	*/
	       int	si_overrun;  /*	Timer overrun count; POSIX.1b timers */
	       int	si_timerid;  /*	Timer ID; POSIX.1b timers */
	       void    *si_addr;     /*	Memory location	which caused fault */
	       long	si_band;     /*	Band event (was	int in
					glibc 2.3.2 and	earlier) */
	       int	si_fd;	     /*	File descriptor	*/
	       short	si_addr_lsb; /*	Least significant bit of address
					(since Linux 2.6.32) */
	   }

       si_signo,  si_errno and si_code are defined for all signals.  (si_errno
       is generally unused on Linux.)  The rest	of the struct may be a	union,
       so  that	 one  should  read only	the fields that	are meaningful for the
       given signal:

       * Signals sent with kill(2) and sigqueue(3) fill	in si_pid and  si_uid.
	 In  addition, signals sent with sigqueue(3) fill in si_int and	si_ptr
	 with  the  values  specified  by  the	sender	of  the	 signal;   see
	 sigqueue(3) for more details.

       * Signals  sent by POSIX.1b timers (since Linux 2.6) fill in si_overrun
	 and si_timerid.  The si_timerid field is an internal ID used  by  the
	 kernel	 to identify the timer;	it is not the same as the timer	ID re-
	 turned	by timer_create(2).  The si_overrun field is the timer overrun
	 count;	 this  is  the	same  information  as is obtained by a call to
	 timer_getoverrun(2).  These fields are	nonstandard Linux extensions.

       * Signals sent for message queue	notification (see the  description  of
	 SIGEV_SIGNAL	in  mq_notify(3))  fill	 in  si_int/si_ptr,  with  the
	 sigev_value supplied to mq_notify(3); si_pid, with the	process	ID  of
	 the  message sender; and si_uid, with the real	user ID	of the message
	 sender.

       * SIGCHLD fills in si_pid, si_uid, si_status, si_utime,	and  si_stime,
	 providing  information	 about	the  child.   The  si_pid field	is the
	 process ID of the child; si_uid is the	child's	 real  user  ID.   The
	 si_status  field contains the exit status of the child	(if si_code is
	 CLD_EXITED), or the signal number that	caused the process  to	change
	 state.	  The  si_utime	 and  si_stime contain the user	and system CPU
	 time used by the child	process; these fields do not include the times
	 used  by  waited-for children (unlike getrusage(2) and	times(2)).  In
	 kernels up to 2.6, and	since 2.6.27, these fields report CPU time  in
	 units	of  sysconf(_SC_CLK_TCK).  In 2.6 kernels before 2.6.27, a bug
	 meant that these fields reported time in units	of the	(configurable)
	 system	jiffy (see time(7)).

       * SIGILL, SIGFPE, SIGSEGV, SIGBUS, and SIGTRAP fill in si_addr with the
	 address of the	fault.	On some	architectures, these signals also fill
	 in  the  si_trapno  field.   Some  suberrors of SIGBUS, in particular
	 BUS_MCEERR_AO and BUS_MCEERR_AR,  also	 fill  in  si_addr_lsb.	  This
	 field indicates the least significant bit of the reported address and
	 therefore the extent of the corruption.  For example, if a full  page
	 was   corrupted,  si_addr_lsb	contains  log2(sysconf(_SC_PAGESIZE)).
	 BUS_MCERR_* and si_addr_lsb are Linux-specific	extensions.

       * SIGIO/SIGPOLL (the two	names are synonyms on Linux) fills in  si_band
	 and  si_fd.  The si_band event	is a bit mask containing the same val-
	 ues as	are filled in the revents field	by poll(2).  The  si_fd	 field
	 indicates the file descriptor for which the I/O event occurred.

       si_code	is  a  value  (not  a bit mask)	indicating why this signal was
       sent.  The following list shows the  values  which  can	be  placed  in
       si_code	for  any  signal, along	with reason that the signal was	gener-
       ated.

	   SI_USER	  kill(2)

	   SI_KERNEL	  Sent by the kernel.

	   SI_QUEUE	  sigqueue(3)

	   SI_TIMER	  POSIX	timer expired

	   SI_MESGQ	  POSIX	 message  queue	 state	changed	 (since	 Linux
			  2.6.6); see mq_notify(3)

	   SI_ASYNCIO	  AIO completed

	   SI_SIGIO	  Queued  SIGIO	(only in kernels up to Linux 2.2; from
			  Linux	2.4 onward SIGIO/SIGPOLL fills in  si_code  as
			  described below).

	   SI_TKILL	  tkill(2) or tgkill(2)	(since Linux 2.4.19)

       The following values can	be placed in si_code for a SIGILL signal:

	   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

       The following values can	be placed in si_code for a SIGFPE signal:

	   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

       The following values can	be placed in si_code for a SIGSEGV signal:

	   SEGV_MAPERR	  address not mapped to	object

	   SEGV_ACCERR	  invalid permissions for mapped object

       The following values can	be placed in si_code for a SIGBUS signal:

	   BUS_ADRALN	  invalid address alignment

	   BUS_ADRERR	  nonexistent physical address

	   BUS_OBJERR	  object-specific hardware error

	   BUS_MCEERR_AR (since	Linux 2.6.32)
			  Hardware  memory  error consumed on a	machine	check;
			  action required.

	   BUS_MCEERR_AO (since	Linux 2.6.32)
			  Hardware memory error	detected in  process  but  not
			  consumed; action optional.

       The following values can	be placed in si_code for a SIGTRAP signal:

	   TRAP_BRKPT	  process breakpoint

	   TRAP_TRACE	  process trace	trap

	   TRAP_BRANCH (since Linux 2.4)
			  process taken	branch trap

	   TRAP_HWBKPT (since Linux 2.4)
			  hardware breakpoint/watchpoint

       The following values can	be placed in si_code for a SIGCHLD signal:

	   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 (since Linux 2.6.9)

       The  following values can be placed in si_code for a SIGIO/SIGPOLL sig-
       nal:

	   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

RETURN VALUE
       sigaction() returns 0 on	success; on error, -1 is returned,  and	 errno
       is set to indicate the error.

ERRORS
       EFAULT act  or oldact points to memory which is not a valid part	of the
	      process address space.

       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 or	ignored.

CONFORMING TO
       POSIX.1-2001, SVr4.

NOTES
       A child created via fork(2) inherits a copy of its parent's signal dis-
       positions.   During  an	execve(2), the dispositions of handled signals
       are reset to the	default; the dispositions of ignored signals are  left
       unchanged.

       According to POSIX, the behavior	of a process is	undefined after	it ig-
       nores a SIGFPE, SIGILL, or SIGSEGV signal that  was  not	 generated  by
       kill(2)	or  raise(3).	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.1-1990 disallowed setting the  action  for	 SIGCHLD  to  SIG_IGN.
       POSIX.1-2001  allows  this possibility, so that ignoring	SIGCHLD	can be
       used to prevent the creation of zombies (see  wait(2)).	 Nevertheless,
       the  historical BSD and System V	behaviors for ignoring SIGCHLD differ,
       so that the only	completely portable method of ensuring that terminated
       children	 do not	become zombies is to catch the SIGCHLD signal and per-
       form a wait(2) or similar.

       POSIX.1-1990 specified only SA_NOCLDSTOP.  POSIX.1-2001 added SA_NOCLD-
       WAIT,  SA_RESETHAND,  SA_NODEFER,  and SA_SIGINFO.  Use of these	latter
       values in sa_flags may be less portable in  applications	 intended  for
       older UNIX implementations.

       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 implementa-
       tion allowed the	receipt	of any signal, not just	the  one  we  are  in-
       stalling	(effectively overriding	any sa_mask settings).

       sigaction() can be called with a	NULL second argument to	query the cur-
       rent signal handler.  It	can also be used to check whether a given sig-
       nal is valid for	the current machine by calling it with NULL second and
       third arguments.

       It is not possible to block SIGKILL or SIGSTOP (by specifying  them  in
       sa_mask).  Attempts to do so are	silently ignored.

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

       See signal(7) for a list	of the async-signal-safe functions that	can be
       safely called inside from inside	a signal handler.

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

BUGS
       In  kernels  up	to  and	 including  2.6.13,  specifying	 SA_NODEFER in
       sa_flags	prevents not only the delivered	signal from being masked  dur-
       ing  execution  of  the	handler,  but  also  the  signals specified in
       sa_mask.	 This bug was fixed in kernel 2.6.14.

EXAMPLE
       See mprotect(2).

SEE ALSO
       kill(1),	 kill(2),  killpg(2),  pause(2),  restart_syscall(2),  sigalt-
       stack(2),  signal(2),  signalfd(2), sigpending(2), sigprocmask(2), sig-
       suspend(2), wait(2),  raise(3),	siginterrupt(3),  sigqueue(3),	sigse-
       tops(3),	sigvec(3), core(5), signal(7)

COLOPHON
       This  page  is  part of release 3.74 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at
       http://www.kernel.org/doc/man-pages/.

Linux				  2014-04-27			  SIGACTION(2)

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

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