Provided by: manpages-dev_6.03-1_all bug


       sigaction, rt_sigaction - examine and change a signal action


       Standard C library (libc, -lc)


       #include <signal.h>

       int sigaction(int signum,
                     const struct sigaction *_Nullable restrict act,
                     struct sigaction *_Nullable restrict oldact);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):


           _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

       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

       sa_handler specifies the action to be associated  with  signum  and  can  be  one  of  the

       •  SIG_DFL for the default action.

       •  SIG_IGN to ignore this signal.

       •  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:

              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 add the signal to the thread's signal mask while the handler  is  executing,
              unless the signal is specified in act.sa_mask.  Consequently, a further instance of
              the signal may be delivered to the thread while it is executing the 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 restarting.

              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.

       SA_UNSUPPORTED (since Linux 5.11)
              Used to dynamically probe for flag bit support.

              If an attempt to register a handler succeeds with this flag  set  in  act->sa_flags
              alongside  other  flags  that  are  potentially  unsupported  by the kernel, and an
              immediately subsequent sigaction() call specifying the same signal number and  with
              a  non-NULL  oldact  argument yields SA_UNSUPPORTED clear in oldact->sa_flags, then
              oldact->sa_flags may be used as a  bitmask  describing  which  of  the  potentially
              unsupported  flags  are,  in fact, supported.  See the section "Dynamically probing
              for flag bit support" below for more details.

       SA_EXPOSE_TAGBITS (since Linux 5.11)
              Normally, when delivering a signal, an architecture-specific set of  tag  bits  are
              cleared from the si_addr field of siginfo_t.  If this flag is set, an architecture-
              specific subset of the tag bits will be preserved in si_addr.

              Programs that need to be compatible with Linux versions older than  5.11  must  use
              SA_UNSUPPORTED to probe for support.

   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:

           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.

              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)  and  signal(7).
              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 */
               union sigval 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)).  Up to Linux 2.6, and  since  Linux  2.6.27,  these
          fields  report  CPU time in units of sysconf(_SC_CLK_TCK).  In Linux 2.6 kernels before
          Linux  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

   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;


       •  _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 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:

                  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 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:

                  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 signal:

                  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.

   Dynamically probing for flag bit support
       The  sigaction()  call  on  Linux accepts unknown bits set in act->sa_flags without error.
       The behavior of the kernel starting with Linux 5.11 is  that  a  second  sigaction()  will
       clear  unknown  bits  from  oldact->sa_flags.  However, historically, a second sigaction()
       call would typically leave those bits set in oldact->sa_flags.

       This means that support for new flags cannot be detected simply by testing for a  flag  in
       sa_flags,  and  a program must test that SA_UNSUPPORTED has been cleared before relying on
       the contents of sa_flags.

       Since the behavior of the signal handler cannot be guaranteed unless the check passes,  it
       is  wise  to either block the affected signal while registering the handler and performing
       the check in this case, or where this is not  possible,  for  example  if  the  signal  is
       synchronous, to issue the second sigaction() in the signal handler itself.

       In  kernels  that  do not support a specific flag, the kernel's behavior is as if the flag
       was not set, even if the flag was set in act->sa_flags.

       SA_RESETHAND,  and, if defined by the architecture, SA_RESTORER may not be reliably probed
       for using this mechanism, because they were introduced before  Linux  5.11.   However,  in
       general,  programs  may  assume  that  these flags are supported, since they have all been
       supported since Linux 2.6, which was released in the year 2003.

       See EXAMPLES below for a demonstration of the use of SA_UNSUPPORTED.


       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

       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 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

       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 later.  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.

       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 sigcontext, 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

       Up  to and including Linux 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 Linux 2.6.14.


       See mprotect(2).

   Probing for flag support
       The  following  example  program  exits  with  status EXIT_SUCCESS if SA_EXPOSE_TAGBITS is
       determined to be supported, and EXIT_FAILURE otherwise.

       #include <signal.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       handler(int signo, siginfo_t *info, void *context)
           struct sigaction oldact;

           if (sigaction(SIGSEGV, NULL, &oldact) == -1
               || (oldact.sa_flags & SA_UNSUPPORTED)
               || !(oldact.sa_flags & SA_EXPOSE_TAGBITS))

           struct sigaction act = { 0 };

           act.sa_flags = SA_SIGINFO | SA_UNSUPPORTED | SA_EXPOSE_TAGBITS;
           act.sa_sigaction = &handler;
           if (sigaction(SIGSEGV, &act, NULL) == -1) {



       kill(1),  kill(2),   pause(2),   pidfd_send_signal(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)