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       sigaction - examine and change a signal action


       #include <signal.h>

       int sigaction(int signum, const struct sigaction *act,
                     struct sigaction *oldact);


       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       The sa_restorer element is obsolete and should not be used.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
       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  gives  a  mask  of  signals  which  should  be  blocked during
       execution of  the  signal  handler.   In  addition,  the  signal  which
       triggered  the  handler  will be blocked, unless the SA_NODEFER flag is

       sa_flags specifies a set of flags which  modify  the  behavior  of  the
       signal  handling  process.   It  is formed by the bitwise OR of zero or
       more of the following:

                  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)).

                  (Linux 2.6 and later) If signum is SIGCHLD, do not transform
                  children  into  zombies  when  they  terminate.   See   also

                  Restore  the  signal  action  to  the default state once the
                  signal handler has been called.  SA_ONESHOT is an  obsolete,
                  non-standard synonym for this flag.

                  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.

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

                  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.

                  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

           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_errno
       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

       * 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.
         See sigqueue(2) for more details.

       * 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 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

           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

       The following values can be placed in si_code for a SIGTRAP signal:

           TRAP_BRKPT     process breakpoint

           TRAP_TRACE     process trace trap

       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


       sigaction() returns 0 on success and -1 on error.


       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.


       POSIX.1-2001, SVr4.


       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  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   only   specified   SA_NOCLDSTOP.    POSIX.1-2001   added
       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

       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.

       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.


       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.


       See mprotect(2).


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


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