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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       The sa_restorer element is obsolete and should not be used.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
       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
       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
                     transform 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
                     obsolete, non-standard 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.

              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
       signal.   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
       bitmask.  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
       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.

       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
       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.

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, SVr4.  SVr4 does not document the EINTR condition.

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.

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), signal(7)