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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

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       The sa_restorer element is obsolete and should not be used.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.

       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	only  meaningful  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
		  only	meaningful 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 only meaningful 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
		  only meaningful when establishing a signal handler.

	   SA_RESETHAND
		  Restore the signal action to the default state once the sig-
		  nal  handler	has been called.  This flag is only meaningful
		  when establishing a signal handler.  SA_ONESHOT is an	 obso-
		  lete,	nonstandard synonym for	this flag.

	   SA_RESTART
		  Provide  behavior  compatible	 with  BSD signal semantics by
		  making certain  system  calls	 restartable  across  signals.
		  This flag is only meaningful 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  3  arguments, not one.  In this
		  case,	sa_sigaction should  be	 set  instead  of  sa_handler.
		  This flag is only meaningful 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 kernel 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  only read	the fields that	are meaningful for the
       given signal:

       * Signals sent with kill(2) and sigqueue(2) fill	in si_pid and  si_uid.
	 In  addition, signals sent with sigqueue(2) fill in si_int and	si_ptr
	 with the values specified by the sender the signal;  see  sigqueue(2)
	 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
	 returned by timer_create(2).  The si_overrun field is the timer over-
	 run  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	time(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 filed.  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.

       * SIGPOLL/SIGIO fills in	si_band	and si_fd.  The	si_band	event is a bit
	 mask containing the same values 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) or raise(3)

	   SI_KERNEL	  Sent by the kernel.

	   SI_QUEUE	  sigqueue(2)

	   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

	   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 SIGPOLL signal:

	   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	and -1 on 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
       ignores	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 only specified 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
       installing (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 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), sigaltstack(2), signal(2), sig-
       nalfd(2), sigpending(2),	 sigprocmask(2),  sigqueue(2),	sigsuspend(2),
       wait(2),	 raise(3),  siginterrupt(3), sigsetops(3), sigvec(3), core(5),
       signal(7)

COLOPHON
       This page is part of release 3.25 of the	Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found	at http://www.kernel.org/doc/man-pages/.

Linux				  2010-06-16			  SIGACTION(2)

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

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