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

       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 the 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 three arguments, as described below.

       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 han-
              dler 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 mean-
              ingful 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  imple-
              mentations, it is not.

              Do  not  prevent  the signal from being received from within its
              own signal handler.  This flag is meaningful  only  when  estab-
              lishing 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 de-
              fault stack will be used.  This flag is meaningful only when es-
              tablishing a signal handler.

              Restore  the signal action to the default upon entry to the sig-
              nal 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  sig-
              nal(7) for a discussion of system call restarting.

              Not  intended  for  application use.  This flag is used by C li-
              braries to indicate that the sa_restorer field contains the  ad-
              dress  of  a "signal trampoline".  See sigreturn(2) for more de-

       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 handler.

   The siginfo_t argument to a SA_SIGINFO handler
       When the SA_SIGINFO flag is specified in act.sa_flags, the signal  han-
       dler  address  is  passed via the act.sa_sigaction field.  This handler
       takes three arguments, as follows:

           handler(int sig, siginfo_t *info, void *ucontext)

       These three arguments are as follows

       sig    The number of the signal that caused invocation of the handler.

       info   A pointer to a siginfo_t, which is a structure  containing  fur-
              ther information about the signal, as described below.

              This  is  a  pointer  to a ucontext_t structure, cast to void *.
              The structure pointed to by this field contains  signal  context
              information  that  was saved on the user-space stack by the ker-
              nel; for details, see sigreturn(2).  Further  information  about
              the  ucontext_t  structure  can be found in getcontext(3).  Com-
              monly, the handler function doesn't make any use  of  the  third

       The siginfo_t data type is a structure with the following fields:

           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_lower;     /* Lower bound when address violation
                                         occurred (since Linux 3.19) */
               void    *si_upper;     /* Upper bound when address violation
                                         occurred (since Linux 3.19) */
               int      si_pkey;      /* Protection key on PTE that caused
                                         fault (since Linux 4.6) */
               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 re-
         turned 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 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 ex-
         tent 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 deliv-
         ering the event.  BUS_MCEERR_* and si_addr_lsb are Linux-specific ex-

         The SEGV_BNDERR suberror of SIGSEGV populates si_lower and si_upper.

         The SEGV_PKUERR suberror of SIGSEGV populates si_pkey.

       * 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_er-
         rno, and other fields as described in seccomp(2).

   The si_code field
       The  si_code  field  inside  the siginfo_t argument that is passed to a
       SA_SIGINFO signal handler is a value (not a bit  mask)  indicating  why
       this signal was sent.  For a ptrace(2) event, si_code will contain SIG-
       TRAP and have the ptrace event in the high byte:

           (SIGTRAP | PTRACE_EVENT_foo << 8).

       For a non-ptrace(2) event, the values that can appear  in  si_code  are
       described in the remainder of this section.  Since glibc 2.20, the def-
       initions of most of these  symbols  are  obtained  from  <signal.h>  by
       defining feature test macros (before including any header file) as fol-

       *  _XOPEN_SOURCE with the value 500 or greater;


       *  _POSIX_C_SOURCE with the value 200809L or greater.

       For the TRAP_* constants, the symbol definitions are provided  only  in
       the  first  two  cases.  Before glibc 2.20, no feature test macros were
       required to obtain these symbols.

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


                  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 be-

           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.

           SEGV_BNDERR (since Linux 3.19)
                  Failed address bound checks.

           SEGV_PKUERR (since Linux 4.6)
                  Access was denied by memory protection keys.  See  pkeys(7).
                  The protection key which applied to this access is available
                  via si_pkey.

       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, IA64 only))
                  Process taken branch trap.

           TRAP_HWBKPT (since Linux 2.4, IA64 only))
                  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 ig-
       nores  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 in-
       stalling (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-safety(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 de-

       On architectures where the signal trampoline resides in the C  library,
       the  glibc  wrapper  function for sigaction() places the address of the
       trampoline code in the act.sa_restorer field and sets  the  SA_RESTORER
       flag in the act.sa_flags field.  See sigreturn(2).

       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 about the signal.  This was done by providing an
       sa_handler signal handler with a second argument of type struct sigcon-
       text, which is the same structure as the one  that  is  passed  in  the
       uc_mcontext  field  of  the  ucontext  structure  that is passed (via a
       pointer) in the third argument of the sa_sigaction  handler.   See  the
       relevant Linux kernel sources for details.  This use is obsolete now.

       When delivering a signal with a SA_SIGINFO handler, the kernel does not
       always provide meaningful values for all of the fields of the siginfo_t
       that are relevant for that signal.

       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), pause(2),  pidfd_send_signal(2),  restart_syscall(2),
       seccomp(2), sigaltstack(2), signal(2), signalfd(2), sigpending(2), sig-
       procmask(2), sigreturn(2), sigsuspend(2), wait(2), killpg(3), raise(3),
       siginterrupt(3),  sigqueue(3),  sigsetops(3),  sigvec(3), core(5), sig-

       This page is part of release 5.07 of the Linux  man-pages  project.   A
       description  of  the project, information about reporting bugs, and the
       latest    version    of    this    page,    can     be     found     at

Linux                             2020-04-11                      SIGACTION(2)

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