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



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  previ-
       ous 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       The sa_restorer element is obsolete and should not be used.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.

       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  wait-
                  pid(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, non-standard 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 estab-
                  lishing 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, non-standard syn-
                  onym 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 */
               int      si_band;     /* Band event */
               int      si_fd;       /* File descriptor */
           }

       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:

       * POSIX.1b signals and SIGCHLD fill in si_pid and si_uid.

       * 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).

       * SIGCHLD fills in si_status, si_utime and si_stime.  The si_utime and si_stime 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)).


       * si_int and si_ptr are specified by the sender of the POSIX.1b signal.  See sigqueue(2) for more details.

       * SIGILL,  SIGFPE, SIGSEGV, and SIGBUS fill in si_addr with the address of the fault.  SIGPOLL fills in si_band
         and si_fd.

       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(2)

           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

       The following values can be placed in si_code for a SIGTRAP signal:

           TRAP_BRKPT     process breakpoint

           TRAP_TRACE     process trace trap

       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 dis-
       positions  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 archi-
       tectures 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 histori-
       cal  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  (effec-
       tively 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 argu-
       ments.

       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 sig-
       nal 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),
       sigqueue(2), sigsuspend(2), wait(2), raise(3), siginterrupt(3), sigsetops(3), sigvec(3), core(5), signal(7)

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2009-07-25                      SIGACTION(2)