Yolinux.com

sigaction manpage

Search topic Section


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



NAME
       sigaction, rt_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	 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 field is not intended for application use.  (POSIX does
       not  specify  a sa_restorer field.)  Some further details of purpose of
       this field can be found in sigreturn(2).

       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.  (Commonly, the handler function
       doesn't make any use of the third argument.  See getcontext(3) for fur-
       ther 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 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 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  terminate.   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 handler.  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  pro-
		  vided	 by  sigaltstack(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	estab-
		  lishing  a  signal handler.  SA_ONESHOT is an obsolete, non-
		  standard 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 establishing a signal han-
		  dler.	  See  signal(7)  for  a  discussion  of  system  call
		  restarting.

	   SA_RESTORER
		  Not intended for application use.  This flag is  used	 by  C
		  libraries  to	 indicate  that the sa_restorer field contains
		  the address of a "signal trampoline".	 See sigreturn(2)  for
		  more details.

	   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 establishing a signal han-
		  dler.

       The siginfo_t argument to sa_sigaction is a struct with	the  following
       fields:

	   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) */
	       void    *si_call_addr; /* Address of system call instruction
					 (since Linux 3.5) */
	       int	si_syscall;   /* Number of attempted system call
					 (since Linux 3.5) */
	       unsigned int si_arch;  /* Architecture of attempted system call
					 (since Linux 3.5) */
	   }

       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, signals 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 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 times(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)).
	 When  SIGTRAP	is  delivered  in  response  to	 a   ptrace(2)	 event
	 (PTRACE_EVENT_foo),  si_addr  is not populated, but si_pid and si_uid
	 are populated with the respective process ID and user ID  responsible
	 for  delivering the trap.  In the case of seccomp(2), the tracee will
	 be shown as delivering the event.  BUS_MCEERR_* 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  val-
	 ues  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;  for
	 further details, see the description of F_SETSIG in fcntl(2).

       * SIGSYS,  generated  (since  Linux  3.5) when a seccomp filter returns
	 SECCOMP_RET_TRAP,  fills  in	si_call_addr,	si_syscall,   si_arch,
	 si_errno, and other fields as described in seccomp(2).

       si_code	is  a  value  (not  a bit mask) indicating why this signal was
       sent.  For a ptrace(2) event, si_code will contain SIGTRAP and have the
       ptrace event in the high byte:

	   (SIGTRAP | PTRACE_EVENT_foo << 8).

       For  a regular signal, 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 (since Linux 2.6.6)
		  POSIX message queue state changed; 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 (since Linux 2.4.19)
		  tkill(2) or tgkill(2).

       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 (since Linux 2.6.9)
		  Stopped child has continued.

       The  following values can be placed in si_code for a SIGIO/SIGPOLL sig-
       nal:

	   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.

       The following value can be placed in si_code for a SIGSYS signal:

	   SYS_SECCOMP (since Linux 3.5)
		  Triggered by a seccomp(2) filter rule.

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, POSIX.1-2008, 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 and later allow this possibility, so that ignoring SIGCHLD
       can be used to prevent the creation of zombies (see  wait(2)).	Never-
       theless, 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 sig-
       nal and perform a wait(2) or similar.

       POSIX.1-1990 specified only SA_NOCLDSTOP.  POSIX.1-2001 added SA_NOCLD-
       STOP,  SA_NOCLDWAIT,  SA_NODEFER, SA_ONSTACK, SA_RESETHAND, SA_RESTART,
       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.

   C library/kernel differences
       The glibc wrapper function for sigaction() gives an error  (EINVAL)  on
       attempts	 to  change  the disposition of the two real-time signals used
       internally by the  NPTL	threading  implementation.   See  nptl(7)  for
       details.

       The  original  Linux  system call was named sigaction().	 However, with
       the addition of real-time signals in Linux 2.2, the fixed-size,	32-bit
       sigset_t	 type supported by that system call was no longer fit for pur-
       pose.  Consequently, a new system call, rt_sigaction(),	was  added  to
       support	an enlarged sigset_t type.  The new system call takes a fourth
       argument, size_t sigsetsize, which specifies the size in bytes  of  the
       signal  sets  in act.sa_mask and oldact.sa_mask.	 This argument is cur-
       rently required to have the value sizeof(sigset_t) (or the error EINVAL
       results).   The	glibc sigaction() wrapper function hides these details
       from us, transparently calling rt_sigaction() when the kernel  provides
       it.

   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 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 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), restart_syscall(2), seccomp(2)
       sigaltstack(2), signal(2),  signalfd(2),	 sigpending(2),	 sigreturn(2),
       sigprocmask(2),	 sigsuspend(2),	 wait(2),  raise(3),  siginterrupt(3),
       sigqueue(3), sigsetops(3), sigvec(3), core(5), signal(7)

COLOPHON
       This page is part of release 4.04 of the Linux  man-pages  project.   A
       description  of	the project, information about reporting bugs, and the
       latest	 version    of	  this	  page,	   can	   be	  found	    at
       http://www.kernel.org/doc/man-pages/.



Linux				  2015-08-08			  SIGACTION(2)