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

       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 >= 1 || _XOPEN_SOURCE || _POSIX_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 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_sigac‐

       The sa_restorer element is obsolete and should not be used.   POSIX  does  not  specify  a
       sa_restorer element.

       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) speci‐
       fies the signal-handling function for signum.  This function receives the signal number as
       its  first  argument,  a  pointer to a siginfo_t as its second argument and a pointer to a
       ucontext_t (cast to void *) as  its  third  argument.   (Commonly,  the  handler  function
       doesn't  make  any  use  of the third argument.  See getcontext(3) for further information
       about ucontext_t.)

       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 termi‐
                  nate.  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  han‐
                  dler.   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  sigalt‐
                  stack(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 establish‐
                  ing a signal handler.  See signal(7) for a discussion of system  call  restart‐

           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  estab‐
                  lishing a signal handler.

       The siginfo_t argument to sa_sigaction is a struct with the following elements:

           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) */

       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, sig‐
         nals 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  informa‐
         tion  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

       * 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)).   BUS_MCERR_*  and  si_addr_lsb  are
         Linux-specific extensions.

       * 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

       si_code is a value (not a bit mask) indicating why this signal was  sent.   The  following
       list  shows  the  values  which can be placed in si_code for any signal, along with 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       POSIX message queue state changed (since Linux 2.6.6); 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       tkill(2) or tgkill(2) (since Linux 2.4.19)

       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

       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

       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)
                          process taken branch trap

           TRAP_HWBKPT (since Linux 2.4)
                          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  stopped child has continued (since Linux 2.6.9)

       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

       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, 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  disposi‐
       tions of ignored signals are left unchanged.

       According to POSIX, the behavior of a process is undefined after it ignores a SIGFPE, SIG‐
       ILL, 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 allows
       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

       POSIX.1-1990 specified only SA_NOCLDSTOP.  POSIX.1-2001 added SA_NOCLDWAIT,  SA_RESETHAND,
       SA_NODEFER,  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(7)  for  a  list  of the async-signal-safe functions that can be safely called
       inside from inside a signal handler.

       Before the introduction of SA_SIGINFO it was also possible to get some additional informa‐
       tion,  namely  by  using a sa_handler with 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 during execution of the handler, but also the sig‐
       nals specified in sa_mask.  This bug was fixed in kernel 2.6.14.

       See mprotect(2).

       kill(1), kill(2), killpg(2), pause(2), restart_syscall(2), sigaltstack(2), signal(2), sig‐
       nalfd(2),  sigpending(2),  sigprocmask(2),  sigsuspend(2),  wait(2),  raise(3),  siginter‐
       rupt(3), sigqueue(3), sigsetops(3), sigvec(3), core(5), signal(7)

       This page is part of release 3.74 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 http://www.kernel.org/doc/man-pages/.

Linux                                       2014-04-27                               SIGACTION(2)

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