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diff --git a/share/doc/psd/05.sysman/1.3.t b/share/doc/psd/05.sysman/1.3.t new file mode 100644 index 00000000000..f81a1850117 --- /dev/null +++ b/share/doc/psd/05.sysman/1.3.t @@ -0,0 +1,254 @@ +.\" Copyright (c) 1983, 1993 +.\" The Regents of the University of California. All rights reserved. +.\" +.\" Redistribution and use in source and binary forms, with or without +.\" modification, are permitted provided that the following conditions +.\" are met: +.\" 1. Redistributions of source code must retain the above copyright +.\" notice, this list of conditions and the following disclaimer. +.\" 2. Redistributions in binary form must reproduce the above copyright +.\" notice, this list of conditions and the following disclaimer in the +.\" documentation and/or other materials provided with the distribution. +.\" 3. All advertising materials mentioning features or use of this software +.\" must display the following acknowledgement: +.\" This product includes software developed by the University of +.\" California, Berkeley and its contributors. +.\" 4. Neither the name of the University nor the names of its contributors +.\" may be used to endorse or promote products derived from this software +.\" without specific prior written permission. +.\" +.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND +.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE +.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE +.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE +.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL +.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS +.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) +.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT +.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY +.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF +.\" SUCH DAMAGE. +.\" +.\" @(#)1.3.t 8.1 (Berkeley) 6/8/93 +.\" +.sh "Signals +.PP +.NH 3 +Overview +.PP +The system defines a set of \fIsignals\fP 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 +the \fIhandler\fP to which a signal is delivered, or specify that +the signal is to be \fIblocked\fP or \fIignored\fP. A process may +also specify that a +\fIdefault\fP action is to be taken when signals occur. +.PP +Some signals +will cause a process to exit when they are not caught. This +may be accompanied by creation of a \fIcore\fP image file, containing +the current memory image of the process for use in post-mortem debugging. +A process may choose to have signals delivered on a special +stack, so that sophisticated software stack manipulations are possible. +.PP +All signals have the same \fIpriority\fP. If multiple signals +are pending simultaneously, the order in which they are delivered +to a process is implementation specific. Signal routines execute +with the signal that caused their invocation \fIblocked\fP, but other +signals may yet occur. Mechanisms are provided whereby critical sections +of code may protect themselves against the occurrence of specified signals. +.NH 3 +Signal types +.PP +The signals defined by the system fall into one of +five classes: hardware conditions, +software conditions, input/output notification, process control, or +resource control. +The set of signals is defined in the file \fI<signal.h>\fP. +.PP +Hardware signals are derived from exceptional conditions which +may occur during +execution. Such signals include SIGFPE representing floating +point and other arithmetic exceptions, SIGILL for illegal instruction +execution, SIGSEGV for addresses outside the currently assigned +area of memory, and SIGBUS for accesses that violate memory +protection constraints. +Other, more cpu-specific hardware signals exist, +such as those for the various customer-reserved instructions on +the VAX (SIGIOT, SIGEMT, and SIGTRAP). +.PP +Software signals reflect interrupts generated by user request: +SIGINT for the normal interrupt signal; SIGQUIT for the more +powerful \fIquit\fP signal, that normally causes a core image +to be generated; SIGHUP and SIGTERM that cause graceful +process termination, either because a user has ``hung up'', or +by user or program request; and SIGKILL, a more powerful termination +signal which a process cannot catch or ignore. +Programs may define their own asynchronous events using SIGUSR1 +and SIGUSR2. +Other software signals (SIGALRM, SIGVTALRM, SIGPROF) +indicate the expiration of interval timers. +.PP +A process can request notification via a SIGIO signal +when input or output is possible +on a descriptor, or when a \fInon-blocking\fP operation completes. +A process may request to receive a SIGURG signal when an +urgent condition arises. +.PP +A process may be \fIstopped\fP by a signal sent to it or the members +of its process group. The SIGSTOP signal is a powerful stop +signal, because it cannot be caught. Other stop signals +SIGTSTP, SIGTTIN, and SIGTTOU are used when a user request, input +request, or output request respectively is the reason for stopping the process. +A SIGCONT signal is sent to a process when it is +continued from a stopped state. +Processes may receive notification with a SIGCHLD signal when +a child process changes state, either by stopping or by terminating. +.PP +Exceeding resource limits may cause signals to be generated. +SIGXCPU occurs when a process nears its CPU time limit and SIGXFSZ +warns that the limit on file size creation has been reached. +.NH 3 +Signal handlers +.PP +A process has a handler associated with each signal. +The handler controls the way the signal is delivered. +The call +.DS +#include <signal.h> + +._f +struct sigvec { + int (*sv_handler)(); + int sv_mask; + int sv_flags; +}; + +sigvec(signo, sv, osv) +int signo; struct sigvec *sv; result struct sigvec *osv; +.DE +assigns interrupt handler address \fIsv_handler\fP to signal \fIsigno\fP. +Each handler address +specifies either an interrupt routine for the signal, that the +signal is to be ignored, +or that a default action (usually process termination) is to occur +if the signal occurs. +The constants +SIG_IGN and SIG_DEF used as values for \fIsv_handler\fP +cause ignoring or defaulting of a condition. +The \fIsv_mask\fP value specifies the +signal mask to be used when the handler is invoked; it implicitly includes +the signal which invoked the handler. +Signal masks include one bit for each signal; +the mask for a signal \fIsigno\fP is provided by the macro +\fIsigmask\fP(\fIsigno\fP), from \fI<signal.h>\fP. +\fISv_flags\fP specifies whether system calls should be +restarted if the signal handler returns and +whether the handler should operate on the normal run-time +stack or a special signal stack (see below). If \fIosv\fP +is non-zero, the previous signal vector is returned. +.PP +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 \fIblocked\fP by the process +then it will be delivered. The process of signal delivery +adds the signal to be delivered and those signals +specified in the associated signal +handler's \fIsv_mask\fP to a set of those \fImasked\fP +for the process, saves the current process context, +and places the process in the context of the signal +handling routine. The call is arranged so that if the signal +handling routine exits normally the signal mask will be restored +and the process will resume execution in the original context. +If the process wishes to resume in a different context, then +it must arrange to restore the signal mask itself. +.PP +The mask of \fIblocked\fP signals is independent of handlers for +signals. It delays signals from being delivered much as a +raised hardware interrupt priority level delays hardware interrupts. +Preventing an interrupt from occurring by changing the handler is analogous to +disabling a device from further interrupts. +.PP +The signal handling routine \fIsv_handler\fP is called by a C call +of the form +.DS +(*sv_handler)(signo, code, scp); +int signo; long code; struct sigcontext *scp; +.DE +The \fIsigno\fP gives the number of the signal that occurred, and +the \fIcode\fP, a word of information supplied by the hardware. +The \fIscp\fP parameter is a pointer to a machine-dependent +structure containing the information for restoring the +context before the signal. +.NH 3 +Sending signals +.PP +A process can send a signal to another process or group of processes +with the calls: +.DS +kill(pid, signo) +int pid, signo; + +killpgrp(pgrp, signo) +int pgrp, signo; +.DE +Unless the process sending the signal is privileged, +it must have the same effective user id as the process receiving the signal. +.PP +Signals are also sent implicitly from a terminal device to the +process group associated with the terminal when certain input characters +are typed. +.NH 3 +Protecting critical sections +.PP +To block a section of code against one or more signals, a \fIsigblock\fP +call may be used to add a set of signals to the existing mask, returning +the old mask: +.DS +oldmask = sigblock(mask); +result long oldmask; long mask; +.DE +The old mask can then be restored later with \fIsigsetmask\fP\|, +.DS +oldmask = sigsetmask(mask); +result long oldmask; long mask; +.DE +The \fIsigblock\fP call can be used to read the current mask +by specifying an empty \fImask\fP\|. +.PP +It is possible to check conditions with some signals blocked, +and then to pause waiting for a signal and restoring the mask, by using: +.DS +sigpause(mask); +long mask; +.DE +.NH 3 +Signal stacks +.PP +Applications that maintain complex or fixed size stacks can use +the call +.DS +._f +struct sigstack { + caddr_t ss_sp; + int ss_onstack; +}; + +sigstack(ss, oss) +struct sigstack *ss; result struct sigstack *oss; +.DE +to provide the system with a stack based at \fIss_sp\fP for delivery +of signals. The value \fIss_onstack\fP indicates whether the +process is currently on the signal stack, +a notion maintained in software by the system. +.PP +When a signal is to be delivered, the system checks whether +the process is on a signal stack. If not, then the process is switched +to the signal stack for delivery, with the return from the signal +arranged to restore the previous stack. +.PP +If the process wishes to take a non-local exit from the signal routine, +or run code from the signal stack that uses a different stack, +a \fIsigstack\fP call should be used to reset the signal stack. |