SIGACTION(2) UNIX Programmer's Manual SIGACTION(2) NAME sigaction - software signal facilities SYNOPSIS #include struct sigaction { int (*sa_handler)(); sigset_t sa_mask; int sa_flags; }; sigaction(sig, act, oact) _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; DESCRIPTION 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 process 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 handlers 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 glo- bal _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 (normally empty). It may be changed with a sigprocmask(2) call, or when a signal 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 signals are ready to be delivered at the same time, any sig- nals that could be caused by traps are delivered first. Additional signals may be processed at the same time, with each appearing to interrupt the handlers for the previous signals before their first instructions. The set of pending Printed 11/26/99 September 3, 1997 1 SIGACTION(2) UNIX Programmer's Manual SIGACTION(2) 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 handling routine returns normally the process will resume execution in the context from before the signal's delivery. If the process wishes to resume 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 sig- nal to be delivered, and the signal mask associated with the handler to be invoked. Sigaction 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 han- dling information for the signal is returned to the user. Once a signal handler is installed, it remains installed until another sigaction call is made, or an execve(2) is performed. A signal-specific default action may be reset by setting _s_a__h_a_n_d_l_e_r to SIG_DFL. The defaults are process termination, possibly with core dump; no action; stopping 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 pending, 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 Printed 11/26/99 September 3, 1997 2 SIGACTION(2) UNIX Programmer's Manual SIGACTION(2) (such as a terminal, but not a regular file) and during a wait(2) or ioctl(2). However, calls that have already com- mitted are not restarted, but instead return a partial suc- cess (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 inher- ited 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>: NAME Action Description SIGHUP terminate terminal line hangup SIGINT terminate interrupt program SIGQUIT core quit program SIGILL core illegal instruction SIGTRAP core trace trap SIGIOT core abort(2) call (same as SIGABRT) SIGEMT core emulate instruction executed SIGFPE core floating-point exception SIGKILL terminate kill program SIGBUS core bus error SIGSEGV core segmentation violation SIGSYS core system call given invalid argu- ment SIGPIPE terminate write on a pipe with no reader SIGALRM terminate real-time timer expired SIGTERM terminate software termination signal SIGURG discard urgent condition present on socket SIGSTOP stop stop (cannot be caught or ignored) SIGTSTP stop stop generated from keyboard SIGCONT discard continue after stop SIGCHLD discard child status has changed SIGTTIN stop background read attempted on control terminal SIGTTOU stop background write attemped to control terminal SIGIO discard I/O is possible on a descriptor (see fcntl(2)) SIGXCPU terminate cpu time limit exceeded (see setrlimit(2)) SIGXFSZ terminate file size limit exceeded (see setrlimit(2)) Printed 11/26/99 September 3, 1997 3 SIGACTION(2) UNIX Programmer's Manual SIGACTION(2) SIGVTALRM terminate virtual time alarm (see setiti- mer(2)) SIGPROF terminate profiling timer alarm (see seti- timer(2)) SIGWINCH discard Window size change SIGINFO discard status request from keyboard SIGUSR1 terminate User defined signal 1 SIGUSR2 terminate User defined signal 2 NOTE The mask specified in _a_c_t is not allowed to block SIGKILL or SIGSTOP. This is done silently by the system. RETURN VALUES 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 indi- cated the reason. EXAMPLE The handler routine can be declared: int 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. ERRORS Sigaction 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. STANDARDS The sigaction function is defined by IEEE Std1003.1-1988 (``POSIX''). The SA_ONSTACK and SA_RESTART flags are Berke- ley extensions, as are the signals, SIGTRAP, SIGEMT, SIGBUS, SIGSYS, SIGURG, SIGIO, SIGXCPU, SIGXFSZ, SIGVTALRM, SIGPROF, SIGWINCH, and SIGINFO. Those signals are available on most BSD-derived systems. Printed 11/26/99 September 3, 1997 4 SIGACTION(2) UNIX Programmer's Manual SIGACTION(2) BUGS The networking related syscalls are not properly restarted in 2.11BSD. The SIGINFO signal is not implemented in 2.11BSD. SEE ALSO kill(1), fcntl(2), ptrace(2), kill(2), setitimer(2), setrlimit(2), sigaction(2), sigprocmask(2), sigsuspend(2), sigblock(2), sigsetmask(2), sigpause(2), sigstack(2), sigvec(2), setjmp(3), siginterrupt(3), sigsetops(3), tty(4) Printed 11/26/99 September 3, 1997 5