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       sigaction - examine and change a signal action


       #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       The sa_restorer element is obsolete and should not be used.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
       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.   (Commonly,  the  handler  function
       doesn't  make  any  use  of  the third argument.  See getcontext(3) for
       further 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
       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

                  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 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
                  provided  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
                  establishing  a  signal handler.  SA_ONESHOT is an obsolete,
                  nonstandard 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
                  handler.   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

       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 */
               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 */
               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
         overrun count; this is the same information as is obtained by a  call
         to   timer_getoverrun(2).    These   fields   are  nonstandard  Linux

       * 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
         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  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.

       * SIGIO/SIGPOLL (the two names are synonyms on Linux) 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 I/O event occurred.

       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

           SI_USER        kill(2)

           SI_KERNEL      Sent by the kernel.

           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

       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

           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

           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.


       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.


       POSIX.1-2001, SVr4.


       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

       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   specified   only   SA_NOCLDSTOP.    POSIX.1-2001   added
       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
       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.

       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  Linux  kernel
       sources for details.  This use is obsolete now.


       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.


       See mprotect(2).


       kill(1),    kill(2),    killpg(2),    pause(2),     restart_syscall(2),
       sigaltstack(2),  signal(2), signalfd(2), sigpending(2), sigprocmask(2),
       sigsuspend(2),   wait(2),   raise(3),   siginterrupt(3),   sigqueue(3),
       sigsetops(3), sigvec(3), core(5), signal(7)


       This  page  is  part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

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