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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 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 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:
         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 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
       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 deliv-
       ering the event.  BUS_MCEERR_*  and  si_addr_lsb  are  Linux-specific
       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_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 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
         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
     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-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
     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,  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.


     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),  restart_syscall(2),  seccomp(2)  sigalt-
     stack(2), signal(2), signalfd(2), sigpending(2), sigprocmask(2), sigre-
     turn(2), sigsuspend(2), wait(2), killpg(3), raise(3),  siginterrupt(3),
     sigqueue(3), sigsetops(3), sigvec(3), core(5), signal(7)


     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

Linux 2017-09-15 SIGACTION(2)

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