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NAME

       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 >= 1 || _XOPEN_SOURCE || _POSIX_SOURCE

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       The sa_restorer element is obsolete and should not be used.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)
       specifies 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(2) 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:

           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 only meaningful  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   only   meaningful   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 only meaningful  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 only meaningful when establishing a signal handler.

           SA_RESETHAND
                  Restore the signal action to the default state once the signal handler has been
                  called.  This flag is only  meaningful  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  only   meaningful   when
                  establishing  a  signal handler.  See signal(7) for a discussion of system call
                  restarting.

           SA_SIGINFO (since Linux 2.2)
                  The signal handler takes 3 arguments, not  one.   In  this  case,  sa_sigaction
                  should  be  set  instead  of  sa_handler.   This  flag  is only meaningful when
                  establishing 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 kernel 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 only read 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
         time(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  filed.   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.

       * SIGPOLL/SIGIO  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.

       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) or raise(3)

           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

           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
                          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)
                          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 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

RETURN VALUE

       sigaction() returns 0 on success and -1 on 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, 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  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
       similar.

       POSIX.1-1990  only specified 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.

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

COLOPHON

       This  page  is  part of release 3.35 of the Linux man-pages project.  A description of the
       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.