sigaction(2) — Linux manual page


sigaction(2)               System Calls Manual              sigaction(2)

NAME         top

       sigaction, rt_sigaction - examine and change a signal action

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <signal.h>

       int sigaction(int signum,
                     const struct sigaction *_Nullable restrict act,
                     struct sigaction *_Nullable restrict oldact);

   Feature Test Macro Requirements for glibc (see


           _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

       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

       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

       sa_handler specifies the action to be associated with signum and
       can 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

       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 terminate.  See also waitpid(2).  This
              flag is meaningful only when establishing a handler for
              SIGCHLD, or when setting that signal's disposition to

              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

              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 handler.

              SA_ONESHOT is an obsolete, nonstandard synonym for this

              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

              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.

       SA_UNSUPPORTED (since Linux 5.11)
              Used to dynamically probe for flag bit support.

              If an attempt to register a handler succeeds with this
              flag set in act->sa_flags alongside other flags that are
              potentially unsupported by the kernel, and an immediately
              subsequent sigaction() call specifying the same signal
              number and with a non-NULL oldact argument yields
              SA_UNSUPPORTED clear in oldact->sa_flags, then
              oldact->sa_flags may be used as a bitmask describing which
              of the potentially unsupported flags are, in fact,
              supported.  See the section "Dynamically probing for flag
              bit support" below for more details.

       SA_EXPOSE_TAGBITS (since Linux 5.11)
              Normally, when delivering a signal, an architecture-
              specific set of tag bits are cleared from the si_addr
              field of siginfo_t.  If this flag is set, an architecture-
              specific subset of the tag bits will be preserved in

              Programs that need to be compatible with Linux versions
              older than 5.11 must use SA_UNSUPPORTED to probe for

   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

       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 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)).  Up to
          Linux 2.6, and since Linux 2.6.27, these fields report CPU
          time in units of sysconf(_SC_CLK_TCK).  In Linux 2.6 kernels
          before Linux 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)).  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

          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

   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 follows:

       •  _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 up to Linux 2.2; from Linux 2.4
                  onward SIGIO/SIGPOLL fills in si_code as described

           SI_TKILL (since Linux 2.4.19)
                  tkill(2) or tgkill(2).

       The following values can be placed in si_code for a SIGILL

                  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

                  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

                  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

                  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 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

                  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

                  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

                  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.

   Dynamically probing for flag bit support
       The sigaction() call on Linux accepts unknown bits set in
       act->sa_flags without error.  The behavior of the kernel starting
       with Linux 5.11 is that a second sigaction() will clear unknown
       bits from oldact->sa_flags.  However, historically, a second
       sigaction() call would typically leave those bits set in

       This means that support for new flags cannot be detected simply
       by testing for a flag in sa_flags, and a program must test that
       SA_UNSUPPORTED has been cleared before relying on the contents of

       Since the behavior of the signal handler cannot be guaranteed
       unless the check passes, it is wise to either block the affected
       signal while registering the handler and performing the check in
       this case, or where this is not possible, for example if the
       signal is synchronous, to issue the second sigaction() in the
       signal handler itself.

       In kernels that do not support a specific flag, the kernel's
       behavior is as if the flag was not set, even if the flag was set
       in act->sa_flags.

       SA_RESTART, SA_NODEFER, SA_RESETHAND, and, if defined by the
       architecture, SA_RESTORER may not be reliably probed for using
       this mechanism, because they were introduced before Linux 5.11.
       However, in general, programs may assume that these flags are
       supported, since they have all been supported since Linux 2.6,
       which was released in the year 2003.

       See EXAMPLES below for a demonstration of the use of

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.

VERSIONS         top

   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 details.

       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

       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.

STANDARDS         top


HISTORY         top

       POSIX.1-2001, SVr4.

       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

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.

       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

       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.

BUGS         top

       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.

       Up to and including Linux 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 Linux 2.6.14.

EXAMPLES         top

       See mprotect(2).

   Probing for flag support
       The following example program exits with status EXIT_SUCCESS if
       SA_EXPOSE_TAGBITS is determined to be supported, and EXIT_FAILURE

       #include <signal.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <unistd.h>

       handler(int signo, siginfo_t *info, void *context)
           struct sigaction oldact;

           if (sigaction(SIGSEGV, NULL, &oldact) == -1
               || (oldact.sa_flags & SA_UNSUPPORTED)
               || !(oldact.sa_flags & SA_EXPOSE_TAGBITS))

           struct sigaction act = { 0 };

           act.sa_flags = SA_SIGINFO | SA_UNSUPPORTED | SA_EXPOSE_TAGBITS;
           act.sa_sigaction = &handler;
           if (sigaction(SIGSEGV, &act, NULL) == -1) {


SEE ALSO         top

       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), signal(7)

Linux man-pages (unreleased)     (date)                     sigaction(2)

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