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

       sigaction - examine and change a signal action

       #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

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       The sa_restorer element is obsolete and should not be used.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

		  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.

		  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,
		  non-standard synonym for this flag.

		  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.

		  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, non-standard synonym for this flag.

		  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

	   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‐

       The siginfo_t argument 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 */
	       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 */
	       int	si_band;     /* Band event */
	       int	si_fd;	     /* File descriptor */

       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:

       * POSIX.1b signals and SIGCHLD fill in si_pid and si_uid.

       * 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_cre‐

       * SIGCHLD  fills in si_status, si_utime and si_stime.  The si_utime and
	 si_stime 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)).

       * si_int and si_ptr are specified by the sender of the POSIX.1b signal.
	 See sigqueue(2) for more details.

       * 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	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‐

	   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

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

	   TRAP_BRKPT	  process breakpoint

	   TRAP_TRACE	  process trace trap

       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

       sigaction() returns 0 on success and -1 on error.

       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.

       POSIX.1-2001, SVr4.

       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

       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

       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.

       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.

       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.

       See mprotect(2).

       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),

       This  page  is  part of release 3.22 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux				  2009-07-25			  SIGACTION(2)

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