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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);

DESCRIPTION
       The sigaction() system call is used to change the  action  taken	 by  a
       process on receipt of a specific signal.

       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       The sa_restorer element is obsolete and should not be used.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 gives a mask of signals which should be blocked	during	execu-
       tion  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  behaviour  of  the
       signal handling process. 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)).

	      SA_NOCLDWAIT
		     (Linux 2.6 and later) If signum is SIGCHLD, do not trans-
		     form children into zombies when they terminate.  See also
		     waitpid(2).

	      SA_RESETHAND
		     Restore  the  signal action to the default state once the
		     signal handler has been called.  SA_ONESHOT is  an	 obso-
		     lete, non-standard 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.

	      SA_RESTART
		     Provide behaviour compatible with BSD signal semantics by
		     making certain system calls restartable across signals.

	      SA_NODEFER
		     Do not prevent the signal from being received from within
		     its  own  signal handler.	SA_NOMASK is an obsolete, non-
		     standard synonym for this flag.

	      SA_SIGINFO
		     The signal handler takes 3 arguments, not one.   In  this
		     case,  sa_sigaction  should be set instead of sa_handler.
		     (The sa_sigaction field was added in Linux 2.1.86.)

       The siginfo_t parameter 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 */
		  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 */
		  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_signo
       is 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 sig-
       nal.  POSIX.1b signals and SIGCHLD fill in si_pid and si_uid.   SIGCHLD
       also  fills in si_status, si_utime and si_stime.	 si_int and si_ptr are
       specified by the	 sender	 of  the  POSIX.1b  signal.   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 indicates why this signal was sent.  It is a value, not a  bit-
       mask.   The values which are possible for any signal are listed in this
       table:

       +-------------------------------------------------------------------+
       |			     si_code				   |
       +-----------+-------------------------------------------------------+
       |Value	   | Signal origin					   |
       +-----------+-------------------------------------------------------+
       |SI_USER	   | kill(), sigsend(), or raise()			   |
       +-----------+-------------------------------------------------------+
       |SI_KERNEL  | The kernel						   |
       +-----------+-------------------------------------------------------+
       |SI_QUEUE   | sigqueue()						   |
       +-----------+-------------------------------------------------------+
       |SI_TIMER   | POSIX timer expired				   |
       +-----------+-------------------------------------------------------+
       |SI_MESGQ   | POSIX message queue state changed (since Linux 2.6.6) |
       +-----------+-------------------------------------------------------+
       |SI_ASYNCIO | AIO completed					   |
       +-----------+-------------------------------------------------------+
       |SI_SIGIO   | queued SIGIO					   |
       +-----------+-------------------------------------------------------+
       |SI_TKILL   | tkill() or tgkill() (since Linux 2.4.19)		   |
       +-----------+-------------------------------------------------------+

       +-------------------------------------+
       |	       SIGILL		     |
       +-----------+-------------------------+
       |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    |
       +-----------+-------------------------+

       +----------------------------------------------+
       |		   SIGFPE		      |
       +-----------+----------------------------------+
       |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	      |
       +-----------+----------------------------------+

       +----------------------------------------------------+
       |		      SIGSEGV			    |
       +------------+---------------------------------------+
       |SEGV_MAPERR | address not mapped to object	    |
       +------------+---------------------------------------+
       |SEGV_ACCERR | invalid permissions for mapped object |
       +------------+---------------------------------------+

       +--------------------------------------------+
       |		  SIGBUS		    |
       +-----------+--------------------------------+
       |BUS_ADRALN | invalid address alignment	    |
       +-----------+--------------------------------+
       |BUS_ADRERR | non-existent physical address  |
       +-----------+--------------------------------+
       |BUS_OBJERR | object specific hardware error |
       +-----------+--------------------------------+

       +--------------------------------+
       |	    SIGTRAP		|
       +-----------+--------------------+
       |TRAP_BRKPT | process breakpoint |
       +-----------+--------------------+
       |TRAP_TRACE | process trace trap |
       +-----------+--------------------+

       +----------------------------------------------------------------+
       |			    SIGCHLD				|
       +--------------+-------------------------------------------------+
       |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) |
       +--------------+-------------------------------------------------+

       +-----------------------------------------+
       |		SIGPOLL			 |
       +---------+-------------------------------+
       |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.

NOTES
       According  to  POSIX,  the behaviour of a process is undefined after it
       ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not	 generated  by
       kill()  or raise().  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 behaviours 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.

       Support for SA_SIGINFO was added in Linux 2.2.

       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.

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

CONFORMING TO
       POSIX.1-2001, SVr4.

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.

SEE ALSO
       kill(1),	 kill(2),  pause(2), sigaltstack(2), signal(2), sigpending(2),
       sigprocmask(2),	 sigqueue(2),	sigsuspend(2),	 wait(2),   killpg(3),
       raise(3), siginterrupt(3), sigsetops(3), sigvec(3), core(5), signal(7)



Linux 2.6.14			  2005-09-15			  SIGACTION(2)
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