sigaction(2) — Linux manual page

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

NAME         top

       sigaction, rt_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

       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 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 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 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.

       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 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:

           void
           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.

       ucontext
              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
              argument.

       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)).  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 follows:

       *  _XOPEN_SOURCE with the value 500 or greater;

       *  _XOPEN_SOURCE and _XOPEN_SOURCE_EXTENDED; or

       *  _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.

           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.

           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:

           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, 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:

           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, POSIX.1-2008, 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 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_NOCLDSTOP, SA_NOCLDWAIT, SA_NODEFER, SA_ONSTACK, SA_RESETHAND,
       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 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
       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.

   Undocumented
       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.

       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.

EXAMPLES         top

       See mprotect(2).

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)

COLOPHON         top

       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
       https://www.kernel.org/doc/man-pages/.

Linux                          2020-12-21                   SIGACTION(2)

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