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

       sigaction, rt_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

       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_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 is be
       one of the following:

       * SIG_DFL for the default action.

       * SIG_IGN to ignore this signal.

       * 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 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 signal which
       triggered the handler will be blocked, unless the SA_NODEFER flag is

       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 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.

              Do not add the signal to the thread's signal mask while the
              handler is executing, unless the signal is specified in
              act.sa_mask.  Consequently, a further instance of the signal may
              be delivered to the thread while it is executing the handler.
              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
              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.

              Restore the signal action to the default upon entry to the signal
              handler.  This flag is meaningful only when establishing a signal

              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 establishing a signal handler.  See signal(7)
              for a discussion of system call restarting.

              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

       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 a SA_SIGINFO handler
       When the SA_SIGINFO flag is specified in act.sa_flags, the signal handler
       address is passed via the act.sa_sigaction field.  This handler takes
       three arguments, as follows:

           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 further
              information about the signal, as described below.

              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 kernel;
              for details, see sigreturn(2).  Further information about the
              ucontext_t structure can be found in getcontext(3) and signal(7).
              Commonly, the handler function doesn't make any use of the third

       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 */
               union sigval 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

       * 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

       * 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.

         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 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; 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_errno,
         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 SIGTRAP 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
       definitions of most of these symbols are obtained from <signal.h> by
       defining feature test macros (before including any header file) as

       *  _XOPEN_SOURCE with the value 500 or greater;


       *  _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.


                  Sent by the kernel.


                  POSIX timer expired.

           SI_MESGQ (since Linux 2.6.6)
                  POSIX message queue state changed; see mq_notify(3).

                  AIO completed.

                  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:

                  Illegal opcode.

                  Illegal operand.

                  Illegal addressing mode.

                  Illegal trap.

                  Privileged opcode.

                  Privileged register.

                  Coprocessor error.

                  Internal stack error.

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

                  Integer divide by zero.

                  Integer overflow.

                  Floating-point divide by zero.

                  Floating-point overflow.

                  Floating-point underflow.

                  Floating-point inexact result.

                  Floating-point invalid operation.

                  Subscript out of range.

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

                  Address not mapped to object.

                  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:

                  Invalid address alignment.

                  Nonexistent physical address.

                  Object-specific hardware error.

           BUS_MCEERR_AR (since Linux 2.6.32)
                  Hardware memory error consumed on a machine check; action

           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:

                  Process breakpoint.

                  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:

                  Child has exited.

                  Child was killed.

                  Child terminated abnormally.

                  Traced child has trapped.

                  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:

                  Data input available.

                  Output buffers available.

                  Input message available.

                  I/O error.

                  High priority input available.

                  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.

       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 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.

       POSIX.1-2001, POSIX.1-2008, 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 dispositions of ignored signals are left

       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 and later allow 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_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 later.  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-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

       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
       purpose.  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
       currently 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.

       Before the introduction of SA_SIGINFO, it was also possible to get some
       additional information about the signal.  This was done by providing an
       sa_handler signal handler with a second argument of type struct
       sigcontext, which is the same structure as the one that is passed in the
       uc_mcontext field of the ucontext structure that is passed (via a
       pointer) in the third argument of the sa_sigaction handler.  See the
       relevant Linux kernel sources for details.  This use is obsolete now.

       When delivering a signal with a SA_SIGINFO handler, the kernel does not
       always provide meaningful values for all of the fields of the siginfo_t
       that are relevant for that signal.

       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.

       See mprotect(2).

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

       This page is part of release 5.10 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

Linux                              2020-12-21                       SIGACTION(2)