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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);

   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
       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       The sa_restorer element is obsolete and should not be used.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
       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 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
       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
       signal.   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
                  provided  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
                  signal  handler  has  been  called.   This  flag   is   only
                  meaningful  when  establishing a signal handler.  SA_ONESHOT
                  is an obsolete, 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
                  handler.   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
                  handler.

       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_create(2).

       * 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
       generated.

           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

       A  child  created  via  fork(2)  inherits a copy of its parent’s signal
       dispositions.  During an execve(2), the dispositions of handled signals
       are  reset to the default; the dispositions of ignored signals are left
       unchanged.

       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
       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.

       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.

       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
       argument  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
       during 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),
       signalfd(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 3.21 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/.