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

       sigaction, sigprocmask, sigpending, sigsuspend - POSIX signal handling

       #include <signal.h>

       int sigaction(int signum, const struct sigaction *act, struct sigaction

       int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);

       int sigpending(sigset_t *set);

       int sigsuspend(const sigset_t *mask);

       The sigaction system call is used to change the action taken by a process
       on receipt of a specific signal.

       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       The sa_restorer element is obsolete and should not be used.  POSIX does
       not specify a sa_restorer element.

       sa_handler specifies the action to be associated with signum and may be
       SIG_DFL for the default action, SIG_IGN to ignore this signal, or a
       pointer to a signal handling function.  This function receives the signal
       number as its only argument.

       sa_sigaction also specifies the action to be associated with signum.
       This function receives the signal number as its first argument, a pointer
       to a siginfo_t as its second argument and a pointer to a ucontext_t (cast
       to void *) as its third argument.

       sa_mask gives a mask of signals which should be blocked during execution
       of the signal handler.  In addition, the signal which triggered the
       handler will be blocked, unless the SA_NODEFER or SA_NOMASK flags are

       sa_flags specifies a set of flags which modify the behaviour of the
       signal handling process. 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 child processes receive
                     one of SIGSTOP, SIGTSTP, SIGTTIN or SIGTTOU).

              SA_ONESHOT or SA_RESETHAND
                     Restore the signal action to the default state once the
                     signal handler has been called.

                     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.

                     Provide behaviour compatible with BSD signal semantics by
                     making certain system calls restartable across signals.

              SA_NOMASK or SA_NODEFER
                     Do not prevent the signal from being received from within
                     its own signal handler.

                     The signal handler takes 3 arguments, not one.  In this
                     case, sa_sigaction should be set instead of sa_handler.
                     (The sa_sigaction field was added in Linux 2.1.86.)

       The siginfo_t parameter to sa_sigaction is a struct with the following

              siginfo_t {
                  int      si_signo;  /* Signal number */
                  int      si_errno;  /* An errno value */
                  int      si_code;   /* Signal code */
                  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 */
                  void *   si_addr;   /* Memory location which caused fault */
                  int      si_band;   /* Band event */
                  int      si_fd;     /* File descriptor */

       si_signo, si_errno and si_code are defined for all signals.  The rest of
       the struct may be a union, so that one should only read the fields that
       are meaningful for the given signal.  kill(2), POSIX.1b signals and
       SIGCHLD fill in si_pid and si_uid.   SIGCHLD also fills in si_status,
       si_utime and si_stime.  si_int and si_ptr are specified by the sender of
       the POSIX.1b signal.  SIGILL, SIGFPE, SIGSEGV and SIGBUS fill in si_addr
       with the address of the fault.  SIGPOLL fills in si_band and si_fd.

       si_code indicates why this signal was sent.  It is a value, not a
       bitmask.  The values which are possible for any signal are listed in this

       │              si_code               │
       │Value      │ Signal origin          │
       │SI_USER    │ kill, sigsend or raise │
       │SI_KERNEL  │ The kernel             │
       │SI_QUEUE   │ sigqueue               │
       │SI_TIMER   │ timer expired          │
       │SI_MESGQ   │ mesq state changed     │
       │SI_ASYNCIO │ AIO completed          │
       │SI_SIGIO   │ queued SIGIO           │

       │               SIGILL                │
       │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    │

       │                   SIGFPE                     │
       │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           │

       │                      SIGSEGV                       │
       │SEGV_MAPERR │ address not mapped to object          │
       │SEGV_ACCERR │ invalid permissions for mapped object │

       │                  SIGBUS                    │
       │BUS_ADRALN │ invalid address alignment      │
       │BUS_ADRERR │ non-existent physical address  │
       │BUS_OBJERR │ object specific hardware error │

       │            SIGTRAP             │
       │TRAP_BRKPT │ process breakpoint │
       │TRAP_TRACE │ process trace trap │

       │                  SIGCHLD                   │
       │CLD_EXITED    │ child has exited            │
       │CLD_KILLED    │ child was killed            │
       │CLD_DUMPED    │ child terminated abnormally │
       │CLD_TRAPPED   │ traced child has trapped    │
       │CLD_STOPPED   │ child has stopped           │
       │CLD_CONTINUED │ stopped child has continued │

       │                SIGPOLL                  │
       │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 sigprocmask call is used to change the list of currently blocked
       signals. The behaviour of the call is dependent on the value of how, as

                     The set of blocked signals is the union of the current set
                     and the set argument.

                     The signals in set are removed from the current set of
                     blocked signals.  It is legal to attempt to unblock a
                     signal which is not blocked.

                     The set of blocked signals is set to the argument set.

       If oldset is non-null, the previous value of the signal mask is stored in

       The sigpending call allows the examination of pending signals (ones which
       have been raised while blocked).  The signal mask of pending signals is
       stored in set.

       The sigsuspend call temporarily replaces the signal mask for the process
       with that given by mask and then suspends the process until a signal is

       The functions sigaction, sigprocmask, and sigpending return 0 on success
       and -1 on error.  The function sigsuspend always returns -1, normally
       with the error EINTR.

       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.

       EFAULT act, oldact, set, oldset or mask point to memory which is not a
              valid part of the process address space.

       EINTR  System call was interrupted.

       It is not possible to block SIGKILL or SIGSTOP with the sigprocmask call.
       Attempts to do so will be silently ignored.

       According to POSIX, the behaviour of a process is undefined after it
       ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by the
       kill() or the raise() functions.  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 (B. disallows setting the action for SIGCHLD to SIG_IGN.
       The BSD and SYSV behaviours differ, causing BSD software that sets the
       action for SIGCHLD to SIG_IGN to fail on Linux.

       The POSIX spec only defines SA_NOCLDSTOP.  Use of other sa_flags is

       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).

       The SA_RESETHAND and SA_NODEFER names for SVr4 compatibility are present
       only in library versions 3.0.9 and greater.

       The SA_SIGINFO flag is specified by POSIX.1b.  Support for it was added
       in Linux 2.2.

       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

       See sigsetops(3) for details on manipulating signal sets.

       POSIX, SVr4.  SVr4 does not document the EINTR condition.

       Before the introduction of SA_SIGINFO it was also possible to get some
       additional information, namely by using a sa_handler with second argument
       of type struct sigcontext.  See the relevant kernel sources for details.
       This use is obsolete now.

       kill(1), kill(2), killpg(2), pause(2), sigaltstack(2), raise(3),
       siginterrupt(3), signal(2), signal(7), sigsetops(3), sigvec(2)

Linux 2.4                          2001-12-29                       SIGACTION(2)