sigaction


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

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       The sa_restorer element is obsolete and should not be used.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(2) 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-

           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
                  only  meaningful 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 only meaningful 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
                  only meaningful when establishing a signal handler.

           SA_RESETHAND
                  Restore the signal action to the default state once the sig-
                  nal  handler  has been called.  This flag is only meaningful
                  when establishing a signal handler.  SA_ONESHOT is an  obso-
                  lete, nonstandard synonym for this flag.

           SA_RESTART
                  Provide  behavior  compatible  with  BSD signal semantics by
                  making certain  system  calls  restartable  across  signals.
                  This flag is only meaningful when establishing a signal han-
                  dler.   See  signal(7)  for  a  discussion  of  system  call
                  restarting.

           SA_SIGINFO (since Linux 2.2)
                  The  signal  handler  takes  3  arguments, not one.  In this
                  case, sa_sigaction should  be  set  instead  of  sa_handler.
                  This flag is only meaningful when establishing a signal han-
                  dler.

       The siginfo_t argument 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 */
               int      si_trapno;   /* Trap number that caused
                                        hardware-generated signal
                                        (unused on most architectures) */
                                        glibc 2.3.2 and earlier) */
               int      si_fd;       /* File descriptor */
               short    si_addr_lsb; /* Least significant bit of address
                                        (since kernel 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  only read 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 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  filed.   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.

       * SIGPOLL/SIGIO fills in si_band and si_fd.  The si_band event is a bit
         mask containing the same values as are filled in the revents field by
         poll(2).  The si_fd field indicates the file descriptor for which the

           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

           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 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

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.

       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 only specified 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
       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.

   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  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).


Linux                             2011-09-08                      SIGACTION(2)
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