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SIGACTION(2)               Linux Programmer's Manual              SIGACTION(2)



NAME
       sigaction - examine and change a signal action

SYNOPSIS
       #include <&lt;signal.h>&gt;

       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       The sa_restorer element is obsolete and should not be used.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 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.

       sa_mask  gives  a mask of signals which should be blocked during execu-
       tion 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 behavior of the sig-
       nal.  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)).  This flag is only meaningful 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
                  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,
                  non-standard 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, 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 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) */
               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 */
               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 */
               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  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:

       * POSIX.1b signals and SIGCHLD fill in si_pid and si_uid.

       * 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_cre-
         ate(3).

       * SIGCHLD 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  gener-
       ated.

           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

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.

CONFORMING TO
       POSIX.1-2001, SVr4.

NOTES
       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 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).

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

COLOPHON
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-07-08                      SIGACTION(2)