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NAME

       sigaction, rt_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

       siginfo_t: _POSIX_C_SOURCE >= 199309L

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 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 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  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 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  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.

           SA_NODEFER
                  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.

           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 meaningful only when establishing a signal handler.

           SA_RESETHAND
                  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.

           SA_RESTART
                  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
                  restarting.

           SA_RESTORER
                  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 handler.

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

           void
           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 further information about
              the signal, as described below.

       ucontext
              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 kernel; for  details,  see  sigreturn(2).   Further  information
              about  the  ucontext_t  structure  can  be  found  in getcontext(3).  Commonly, the
              handler function doesn't make any use of the third argument.

       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 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 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 sender.

       * 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.

         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 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).

   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 SIGTRAP 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 definitions of most of these symbols are
       obtained  from  <signal.h>  by  defining  feature test macros (before including any header
       file) as follows:

       *  _XOPEN_SOURCE with the value 500 or greater;

       *  _XOPEN_SOURCE and _XOPEN_SOURCE_EXTENDED; or

       *  _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.

           SI_USER
                  kill(2).

           SI_KERNEL
                  Sent by the kernel.

           SI_QUEUE
                  sigqueue(3).

           SI_TIMER
                  POSIX timer expired.

           SI_MESGQ (since Linux 2.6.6)
                  POSIX message queue state changed; 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 (since Linux 2.4.19)
                  tkill(2) or tgkill(2).

       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.

           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:

           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, 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:

           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 (since Linux 2.6.9)
                  Stopped child has continued.

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

       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.

RETURN VALUE

       sigaction() returns 0 on success; on error, -1 is returned, and errno is set  to  indicate
       the 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, POSIX.1-2008, 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 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_NOCLDSTOP,  SA_NOCLDWAIT,
       SA_NODEFER,  SA_ONSTACK,  SA_RESETHAND,  SA_RESTART,  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-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 details.

       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 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.

   Undocumented
       Before the introduction of SA_SIGINFO,  it  was  also  possible  to  get  some  additional
       information,  namely  by  using  a  sa_handler  with  a  second  argument  of  type struct
       sigcontext.  See the relevant Linux 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),  pause(2),  restart_syscall(2),  seccomp(2)  sigaltstack(2),  signal(2),
       signalfd(2),   sigpending(2),   sigprocmask(2),   sigreturn(2),   sigsuspend(2),  wait(2),
       killpg(3),  raise(3),  siginterrupt(3),  sigqueue(3),  sigsetops(3),  sigvec(3),  core(5),
       signal(7)

COLOPHON

       This  page  is  part of release 4.16 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 https://www.kernel.org/doc/man-pages/.