rt_sigaction

       #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
                  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:

               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)
         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_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.
                  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:

              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
       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)
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