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

       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  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 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 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(3) 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-
       nal 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:

                  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  pro-
                  vided  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  estab-
                  lishing  a  signal handler.  SA_ONESHOT is an obsolete, non-
                  standard 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 han-
                  dler.   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 han-

       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 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

       * 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  val-
         ues  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 sig-

                  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 dis-
       positions.   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)).   Never-
       theless, 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 sig-
       nal and perform a wait(2) or similar.

       POSIX.1-1990 specified only SA_NOCLDSTOP.  POSIX.1-2001 added SA_NOCLD-
       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

       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.

   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 pur-
       pose.  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 cur-
       rently 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

       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 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 dur-
       ing 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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Linux                             2015-08-08                      SIGACTION(2)
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