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       The sa_restorer element is obsolete and should not be used.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 signal 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 signal which triggered the handler will be blocked, unless the  SA_NODE-
       FER 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 follow-
       ing:

	   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 han-
		  dler 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 mean-
		  ingful 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  implementa-
		  tions, 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 establish-
		  ing 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 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 signal handler has been called.  This flag is only meaningful 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
		  only meaningful when establishing a signal handler.  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 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 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 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 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 generated.

	   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 dispositions.  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 divid-
       ing 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 dif-
       fer, 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  only specified 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  implemen-
       tation 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 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.

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 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), signalfd(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)