NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

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

NAME         top

       sigaction - examine and change a signal action

SYNOPSIS         top

       #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

DESCRIPTION         top

       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 field is not intended for application use.  (POSIX
       does not specify a sa_restorer field.)  Some further details of
       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
       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(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:

           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
                  provided 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
                  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 meaningful only when establishing a signal
                  handler.  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 handler.

       The siginfo_t argument to sa_sigaction is a struct 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_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 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 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 delivering the event.
         BUS_MCEERR_* 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
         occurred.

       * 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).

       si_code is a value (not a bit mask) indicating why this signal was
       sent.  For a ptrace(2) event, si_code will contain SIGTRAP and have
       the ptrace event in the high byte:

           (SIGTRAP | PTRACE_EVENT_foo << 8).

       For a regular signal, 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 (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
                  below).

           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.

       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 (since Linux 2.6.9)
                  Stopped child has continued.

       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.

       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         top

       sigaction() returns 0 on success; on error, -1 is returned, and errno
       is set to indicate the error.

ERRORS         top

       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         top

       POSIX.1-2001, SVr4.

NOTES         top

       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 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.

   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 Linux kernel
       sources for details.  This use is obsolete now.

BUGS         top

       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         top

       See mprotect(2).

SEE ALSO         top

       kill(1), kill(2), killpg(2), pause(2), restart_syscall(2), seccomp(2)
       sigaltstack(2), signal(2), signalfd(2), sigpending(2), sigreturn(2),
       sigprocmask(2), sigsuspend(2), wait(2), raise(3), siginterrupt(3),
       sigqueue(3), sigsetops(3), sigvec(3), core(5), signal(7)

COLOPHON         top

       This page is part of release 3.78 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                            2015-01-22                     SIGACTION(2)