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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 overview 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_sigac-
       tion.

       The sa_restorer element is obsolete and should not be used.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 signal number as its only argument.

       If  SA_SIGINFO  is specified in sa_flags, then sa_sigaction (instead of sa_handler) speci-
       fies 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 further 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 signal 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 signal.  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 termi-
		  nate.  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  han-
		  dler.   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  provided  by  sigalt-
		  stack(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 establishing a signal handler.  SA_ONESHOT is
		  an obsolete, 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 meaningful only when establish-
		  ing a signal handler.  See signal(7) for a discussion of system  call  restart-
		  ing.

	   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  estab-
		  lishing a signal handler.

       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, sig-
	 nals 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 returned 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
	 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 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 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 generated.

	   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 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; 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  dispositions.   During
       an  execve(2),  the dispositions of handled signals are reset to the default; the disposi-
       tions of ignored signals are left unchanged.

       According to POSIX, the behavior of a process is undefined after it ignores a SIGFPE, SIG-
       ILL, 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  perform  a  wait(2)  or
       similar.

       POSIX.1-1990  specified only SA_NOCLDSTOP.  POSIX.1-2001 added SA_NOCLDWAIT, 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 implementation 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 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.

       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 informa-
       tion, namely by using a sa_handler with second argument 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 during execution of the handler, but also the  sig-
       nals 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), sigaltstack(2), signal(2), sig-
       nalfd(2),  sigpending(2),  sigprocmask(2),  sigsuspend(2),  wait(2),  raise(3),	siginter-
       rupt(3), sigqueue(3), sigsetops(3), sigvec(3), core(5), signal(7)

COLOPHON
       This  page  is  part of release 3.55 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					    2013-07-30				     SIGACTION(2)
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