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       sigaction, rt_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 field is not intended for application use.  (POSIX does
       not  specify  a sa_restorer field.)  Some further details of purpose of
       this field can be found in sigreturn(2).

       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

                  Not  intended  for  application use.  This flag is used by C
                  libraries to indicate that the  sa_restorer  field  contains
                  the  address of a "signal trampoline".  See sigreturn(2) for
                  more details.

           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) */
               void    *si_call_addr; /* Address of system call instruction
                                         (since Linux 3.5) */
               int      si_syscall;   /* Number of attempted system call
                                         (since Linux 3.5) */
               unsigned int si_arch;  /* Architecture of attempted system call
                                         (since Linux 3.5) */

       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 times(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)).
         When  SIGTRAP  is  delivered  in  response  to  a   ptrace(2)   event
         (PTRACE_EVENT_foo),  si_addr  is not populated, but si_pid and si_uid
         are populated with the respective process ID and user ID  responsible
         for  delivering the trap.  In the case of seccomp(2), the tracee will
         be shown as delivering the event.  BUS_MCEERR_* 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;
         for further details, see the description of F_SETSIG in fcntl(2).

       * SIGSYS,  generated  (since  Linux  3.5) when a seccomp filter returns
         SECCOMP_RET_TRAP,  fills  in   si_call_addr,   si_syscall,   si_arch,
         si_errno, and other fields as described in seccomp(2).

       si_code  is  a  value  (not  a bit mask) indicating why this signal was
       sent.  For a ptrace(2) event, si_code will contain SIGTRAP and have the
       ptrace event in the high byte:

           (SIGTRAP | PTRACE_EVENT_foo << 8).

       For  a regular signal, the following list shows the values which can be
       placed in si_code for any signal, along with reason that the signal was


                  Sent by the kernel.


                  POSIX timer expired.

           SI_MESGQ (since Linux 2.6.6)
                  POSIX message queue state changed; see mq_notify(3).

                  AIO completed.

                  Queued  SIGIO  (only  in kernels up to Linux 2.2; from Linux
                  2.4 onward  SIGIO/SIGPOLL  fills  in  si_code  as  described

           SI_TKILL (since Linux 2.4.19)
                  tkill(2) or tgkill(2).

       The following values can be placed in si_code for a SIGILL signal:

                  Illegal opcode.

                  Illegal operand.

                  Illegal addressing mode.

                  Illegal trap.

                  Privileged opcode.

                  Privileged register.

                  Coprocessor error.

                  Internal stack error.

       The following values can be placed in si_code for a SIGFPE signal:

                  Integer divide by zero.

                  Integer overflow.

                  Floating-point divide by zero.

                  Floating-point overflow.

                  Floating-point underflow.

                  Floating-point inexact result.

                  Floating-point invalid operation.

                  Subscript out of range.

       The following values can be placed in si_code for a SIGSEGV signal:

                  Address not mapped to object.

                  Invalid permissions for mapped object.

       The following values can be placed in si_code for a SIGBUS signal:

                  Invalid address alignment.

                  Nonexistent physical address.

                  Object-specific hardware error.

           BUS_MCEERR_AR (since Linux 2.6.32)
                  Hardware  memory  error  consumed on a machine check; action

           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:

                  Process breakpoint.

                  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:

                  Child has exited.

                  Child was killed.

                  Child terminated abnormally.

                  Traced child has trapped.

                  Child has stopped.

           CLD_CONTINUED (since Linux 2.6.9)
                  Stopped child has continued.

       The  following  values  can  be  placed  in si_code for a SIGIO/SIGPOLL

                  Data input available.

                  Output buffers available.

                  Input message available.

                  I/O error.

                  High priority input available.

                  Device disconnected.

       The following value can be placed in si_code for a SIGSYS signal:

           SYS_SECCOMP (since Linux 3.5)
                  Triggered by a seccomp(2) filter rule.


       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, POSIX.1-2008, 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 and later allow 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
       SA_RESTART, and SA_SIGINFO.  Use of these latter values in sa_flags may
       be   less   portable   in   applications   intended   for   older  UNIX

       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.

   C library/kernel differences
       The  glibc  wrapper function for sigaction() gives an error (EINVAL) on
       attempts to change the disposition of the two  real-time  signals  used
       internally  by  the  NPTL  threading  implementation.   See nptl(7) for

       The original Linux system call was named  sigaction().   However,  with
       the  addition of real-time signals in Linux 2.2, the fixed-size, 32-bit
       sigset_t type supported by that system  call  was  no  longer  fit  for
       purpose.  Consequently, a new system call, rt_sigaction(), was added to
       support an enlarged sigset_t type.  The new system call takes a  fourth
       argument,  size_t  sigsetsize, which specifies the size in bytes of the
       signal sets  in  act.sa_mask  and  oldact.sa_mask.   This  argument  is
       currently  required  to  have  the value sizeof(sigset_t) (or the error
       EINVAL results).  The glibc sigaction() wrapper  function  hides  these
       details  from  us, transparently calling rt_sigaction() when the kernel
       provides it.

       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),  seccomp(2)
       sigaltstack(2),  signal(2),  signalfd(2),  sigpending(2), sigreturn(2),
       sigprocmask(2),  sigsuspend(2),  wait(2),  raise(3),   siginterrupt(3),
       sigqueue(3), sigsetops(3), sigvec(3), core(5), signal(7)


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