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SIGACTION(2) UNIX Programmer's Manual SIGACTION(2) NNAAMMEE ssiiggaaccttiioonn - software signal facilities SSYYNNOOPPSSIISS ##iinncclluuddee <<ssiiggnnaall..hh>> struct sigaction { void (*sa_handler)(); sigset_t sa_mask; int sa_flags; }; _i_n_t ssiiggaaccttiioonn(_i_n_t _s_i_g, _s_t_r_u_c_t _s_i_g_a_c_t_i_o_n _*_a_c_t, _s_t_r_u_c_t _s_i_g_a_c_t_i_o_n _*_o_a_c_t) DDEESSCCRRIIPPTTIIOONN The system defines a set of signals that may be delivered to a process. Signal delivery resembles the occurrence of a hardware interrupt: the signal is blocked from further occurrence, the current process context is saved, and a new one is built. A process may specify a _h_a_n_d_l_e_r to which a signal is delivered, or specify that a signal is to be _i_g_n_o_r_e_d. A pro- cess may also specify that a default action is to be taken by the system when a signal occurs. A signal may also be _b_l_o_c_k_e_d, in which case its delivery is postponed until it is _u_n_b_l_o_c_k_e_d. The action to be taken on delivery is determined at the time of delivery. Normally, signal han- dlers execute on the current stack of the process. This may be changed, on a per-handler basis, so that signals are taken on a special _s_i_g_n_a_l _s_t_a_c_k. Signal routines execute with the signal that caused their invocation _b_l_o_c_k_e_d, but other signals may yet occur. A global _s_i_g_n_a_l _m_a_s_k defines the set of signals currently blocked from delivery to a process. The signal mask for a process is initialized from that of its parent (normal- ly empty). It may be changed with a sigprocmask(2) call, or when a sig- nal is delivered to the process. When a signal condition arises for a process, the signal is added to a set of signals pending for the process. If the signal is not currently _b_l_o_c_k_e_d by the process then it is delivered to the process. Signals may be delivered any time a process enters the operating system (e.g., during a system call, page fault or trap, or clock interrupt). If multiple sig- nals are ready to be delivered at the same time, any signals that could be caused by traps are delivered first. Additional signals may be pro- cessed at the same time, with each appearing to interrupt the handlers for the previous signals before their first instructions. The set of pending signals is returned by the sigpending(2) function. When a caught signal is delivered, the current state of the process is saved, a new signal mask is calculated (as described below), and the signal handler is invoked. The call to the handler is arranged so that if the signal han- dling routine returns normally the process will resume execution in the context from before the signal's delivery. If the process wishes to re- sume in a different context, then it must arrange to restore the previous context itself. When a signal is delivered to a process a new signal mask is installed for the duration of the process' signal handler (or until a sigprocmask call is made). This mask is formed by taking the union of the current signal mask set, the signal to be delivered, and the signal mask associ- ated with the handler to be invoked. SSiiggaaccttiioonn() assigns an action for a specific signal. If _a_c_t is non-zero, it specifies an action (SIG_DFL, SIG_IGN, or a handler routine) and mask to be used when delivering the specified signal. If _o_a_c_t is non-zero, the previous handling information for the signal is returned to the user. Once a signal handler is installed, it remains installed until another ssiiggaaccttiioonn() call is made, or an execve(2) is performed. A signal-specif- ic default action may be reset by setting _s_a___h_a_n_d_l_e_r to SIG_DFL. The de- faults are process termination, possibly with core dump; no action; stop- ping the process; or continuing the process. See the signal list below for each signal's default action. If _s_a___h_a_n_d_l_e_r is SIG_DFL, the default action for the signal is to discard the signal, and if a signal is pend- ing, the pending signal is discarded even if the signal is masked. If _s_a___h_a_n_d_l_e_r is set to SIG_IGN current and pending instances of the signal are ignored and discarded. Options may be specified by setting _s_a___f_l_a_g_s. If the SA_NOCLDSTOP bit is set when installing a catching function for the SIGCHLD signal, the SIGCHLD signal will be generated only when a child process exits, not when a child process stops. Further, if the SA_ONSTACK bit is set in _s_a___f_l_a_g_s, the system will deliver the signal to the process on a _s_i_g_n_a_l _s_t_a_c_k, specified with sigstack(2). If a signal is caught during the system calls listed below, the call may be forced to terminate with the error EINTR, the call may return with a data transfer shorter than requested, or the call may be restarted. Restart of pending calls is requested by setting the SA_RESTART bit in _s_a___f_l_a_g_s. The affected system calls include open(2), read(2), write(2), sendto(2), recvfrom(2), sendmsg(2) and recvmsg(2) on a communications channel or a slow device (such as a terminal, but not a regular file) and during a wait(2) or ioctl(2). However, calls that have already committed are not restarted, but instead return a partial success (for example, a short read count). After a fork(2) or vfork(2) all signals, the signal mask, the signal stack, and the restart/interrupt flags are inherited by the child. Execve(2) reinstates the default action for all signals which were caught and resets all signals to be caught on the user stack. Ignored signals remain ignored; the signal mask remains the same; signals that restart pending system calls continue to do so. The following is a list of all signals with names as in the include file <_s_i_g_n_a_l_._h>: NNAAMMEE DDeeffaauulltt AAccttiioonn DDeessccrriippttiioonn SIGHUP terminate process terminal line hangup SIGINT terminate process interrupt program SIGQUIT create core image quit program SIGILL create core image illegal instruction SIGTRAP create core image trace trap SIGABRT create core image abort(2) call (formerly SIGIOT) SIGEMT create core image emulate instruction executed SIGFPE create core image floating-point exception SIGKILL terminate process kill program SIGBUS create core image bus error SIGSEGV create core image segmentation violation SIGSYS create core image system call given invalid argument SIGPIPE terminate process write on a pipe with no reader SIGALRM terminate process real-time timer expired SIGTERM terminate process software termination signal SIGURG discard signal urgent condition present on socket SIGSTOP stop process stop (cannot be caught or ignored) SIGTSTP stop process stop signal generated from keyboard SIGCONT discard signal continue after stop SIGCHLD discard signal child status has changed SIGTTIN stop process background read attempted from control terminal SIGTTOU stop process background write attempted to control terminal SIGIO discard signal I/O is possible on a descriptor (see fcntl(2)) SIGXCPU terminate process cpu time limit exceeded (see setrlimit(2)) SIGXFSZ terminate process file size limit exceeded (see setrlimit(2)) SIGVTALRM terminate process virtual time alarm (see setitimer(2)) SIGPROF terminate process profiling timer alarm (see setitimer(2)) SIGWINCH discard signal Window size change SIGINFO discard signal status request from keyboard SIGUSR1 terminate process User defined signal 1 SIGUSR2 terminate process User defined signal 2 NNOOTTEE The mask specified in _a_c_t is not allowed to block SIGKILL or SIGSTOP. This is done silently by the system. RREETTUURRNN VVAALLUUEESS A 0 value indicated that the call succeeded. A -1 return value indicates an error occurred and _e_r_r_n_o is set to indicated the reason. EEXXAAMMPPLLEE The handler routine can be declared: void handler(sig, code, scp) int sig, code; struct sigcontext *scp; Here _s_i_g is the signal number, into which the hardware faults and traps are mapped. _C_o_d_e is a parameter that is either a constant or the code provided by the hardware. _S_c_p is a pointer to the _s_i_g_c_o_n_t_e_x_t structure (defined in <_s_i_g_n_a_l_._h>), used to restore the context from before the sig- nal. EERRRROORRSS SSiiggaaccttiioonn() will fail and no new signal handler will be installed if one of the following occurs: [EFAULT] Either _a_c_t or _o_a_c_t points to memory that is not a valid part of the process address space. [EINVAL] _S_i_g is not a valid signal number. [EINVAL] An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP. SSTTAANNDDAARRDDSS The ssiiggaaccttiioonn function is defined by IEEE Std1003.1-1988 (``POSIX''). The SA_ONSTACK and SA_RESTART flags are Berkeley extensions, as are the sig- nals, SIGTRAP, SIGEMT, SIGBUS, SIGSYS, SIGURG, SIGIO, SIGXCPU, SIGXFSZ, SIGVTALRM, SIGPROF, SIGWINCH, and SIGINFO. Those signals are available on most BSD-derived systems. SSEEEE AALLSSOO kill(1), ptrace(2), kill(2), sigaction(2), sigprocmask(2), sigsuspend(2), sigblock(2), sigsetmask(2), sigpause(2), sigstack(2), sigvec(3), setjmp(3), siginterrupt(3), sigsetops(3), tty(4)