sigaction(2)



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

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  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:

           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  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.

           SA_RESETHAND
                  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.

           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 han-
                  dler.   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 han-
                  dler.

   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.

       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 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
              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 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
         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 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-
         tensions.

         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-
       lows:

       *  _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 be-
                  low).

           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  sig-
       nal:

           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 dis-
       positions.  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 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-
       STOP, 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  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-
       tails.

       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
       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  ar-
       gument  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 dur-
       ing 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)  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)

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/.

Linux                             2017-09-15                      SIGACTION(2)

Man(1) output converted with man2html
list of all man pages