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           #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


           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
           overview 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           The sa_restorer element is obsolete and should not be used.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.   (Commonly,  the  handler  function
           doesn't make any use of the third argument.  See getcontext(3) for fur-
           ther information about ucontext_t.)
                      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.
                      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.
                      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.
                      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.
                      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
               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-
           The  siginfo_t  argument to sa_sigaction is a struct with the following
               siginfo_t {
                   int      si_signo;    /* Signal number */
                   int      si_errno;    /* An errno value */
                   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) */
           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
           * 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 pro-
             cess 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 time(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)).
             BUS_MCERR_* and si_addr_lsb are Linux-specific extensions.
               SI_QUEUE       sigqueue(3)
               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 (only in kernels up to Linux 2.2; from
                              Linux 2.4 onward SIGIO/SIGPOLL fills in  si_code  as
                              described below).
               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
               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  stopped child has continued (since Linux 2.6.9)
           The  following values can be placed in si_code for a SIGIO/SIGPOLL sig-
               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; on error, -1 is returned,  and  errno
           is set to indicate the error.
           positions.   During  an  execve(2), the dispositions of handled signals
           are reset to the default; the dispositions of ignored signals are  left
           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 per-
           form a wait(2) or similar.
           POSIX.1-1990 specified only 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.
           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 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.
           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.



    Linux 2013-07-30 SIGACTION(2)


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