X-Git-Url: http://repo.macrolet.net/gitweb/?a=blobdiff_plain;f=doc%2Fbeyond-ansi.sgml;h=b1c0a39a32b301bf7aa883f11018530091b0faed;hb=df679ed627975948b1cee190f4d79c397588c43e;hp=3ede78f9909fec04351def9d2682618a8fdc3396;hpb=b624a686af5426145841a597bdb96b27d5bd042e;p=sbcl.git diff --git a/doc/beyond-ansi.sgml b/doc/beyond-ansi.sgml index 3ede78f..b1c0a39 100644 --- a/doc/beyond-ansi.sgml +++ b/doc/beyond-ansi.sgml @@ -1,3 +1,4 @@ + Beyond The &ANSI; Standard</> <para>&SBCL; is mostly an implementation of the &ANSI; standard for @@ -116,8 +117,134 @@ GCed.</para> <!-- FIXME: Actually documenting these would be good.:-| --> whose instances can be used as Lisp streams (e.g. passed as the first argument to <function>format</>).</para> +<para>&SBCL; supports a MetaObject Protocol which is intended to be +compatible with &AMOP;; present exceptions to this (as distinct from +current bugs) are: +<itemizedlist> + <listitem><para>the abstract <classname>metaobject</> class is not + present in the class hierarchy;</para></listitem> + <listitem><para>the <classname>standard-object</> and + <classname>funcallable-standard-object</> classes are + disjoint;</para></listitem> + <listitem><para><function>compute-effective-method</> only returns + one value, not two;</para></listitem> + <listitem><para>the system-supplied <property>:around</> method for + <function>compute-slots</> specialized on + <classname>funcallable-standard-class</> does not respect the + requested order from a user-supplied primary method. +</itemizedlist> + </sect2> +<sect2><title>Threading (a.k.a Multiprocessing)</> + +<para>&SBCL; (as of version 0.x.y, on Linux x86 only) supports a +fairly low-level threading interface that maps onto the host operating +system's concept of threads or lightweight processes. + +<sect3><title>Lisp-level view + +A rudimentary interface to creating and managing multiple threads +can be found in the sb-thread package. This is +intended for public consumption, so look at the exported symbols and +their documentation strings. + +Dynamic bindings to symbols are per-thread. Signal handlers +are per-thread. + +sb-ext:quit exits the current thread, not +necessarily the whole environment. The environment will be shut down +when the last thread exits. + +Threads arbitrate between themselves for the user's attention. +A thread may be in one of three notional states: foreground, +background, or stopped. When a background process attempts to print a +repl prompt or to enter the debugger, it will stop and print a message +saying that it has stopped. The user at his leisure may switch to +that thread to find out what it needs. If a background thread enters +the debugger, selecting any restart will put it back into the +background before it resumes. + +If the user has multiple views onto the same Lisp image (for +example, using multiple terminals, or a windowing system, or network +access) they are typically set up as multiple `sessions' such that each +view has its own collection of foreground/background/stopped threads. +sb-thread:make-listener-thread can be used to +start a new thread in its own `session'. + +Mutexes and condition variables are available for +managing access to shared data: see + + + +(apropos "mutex" :sb-thread) + +(apropos "condition" :sb-thread) + and the waitqueue structure + + + + +and poke around in their documentation strings. + +Implementation (Linux x86) + +On Linux x86, this is implemented using +clone() and does not involve pthreads. This is +not because there is anything wrong with pthreads per +se, but there is plenty wrong (from our perspective) with +LinuxThreads. &SBCL; threads are mapped 1:1 onto Linux tasks which +share a VM but nothing else - each has its own process id and can be +seen in e.g. ps output. + +Per-thread local bindings for special variables is achieved +using the %fs segment register to point to a per-thread storage area. +This may cause interesting results if you link to foreign code that +expects threading or creates new threads, and the thread library in +question uses %fs in an incompatible way. + +Threads waiting on queues (e.g. for locks or condition +variables) are put to sleep using sigtimedwait() +and woken with SIGCONT. + +&SBCL; at present will alway have at least two tasks running as +seen from Linux: when the first process has done startup +initialization (mapping files in place, installing signal handlers +etc) it creates a new thread to run the Lisp startup and initial listener. +The original thread is then used to run GC and to reap dead subthreads +when they exit. + +Garbage collection is done with the existing Conservative +Generational GC. Allocation is done in small (typically 8k) regions : +each thread has its own region so this involves no stopping. However, +when a region fills, a lock must be obtained while another is +allocated, and when a collection is required, all processes are +stopped. This is achieved using ptrace(), so you +should be very careful if you wish to examine an &SBCL; worker thread +using strace, truss, +gdb or similar. It may be prudent to disable GC +before doing so. + +Large amounts of the &SBCL; library have not been inspected for +thread-safety. Some of the obviously unsafe areas have large locks +around them, so compilation and fasl loading, for example, cannot be +parallelized. Work is ongoing in this area. + +A new thread by default is created in the same POSIX process +group and session as the thread it was created by. This has an impact +on keyboard interrupt handling: pressing your terminal's intr key +(typically Control-C) will interrupt all processes in the foreground +process group, including Lisp threads that &SBCL; considers to be +notionally `background'. This is undesirable, so background threads +are set to ignore the SIGINT signal. Arbitration for the input stream +is managed by locking on sb-thread::*session-lock* + +A thread can be created in a new Lisp 'session' (new terminal or +window) using sb-thread:make-listener-thread. +These sessions map directly onto POSIX sessions, so that pressing +Control-C in the wrong window will not interrupt them - this has been +found to be embarrassing. + Support For Unix</> <para>The UNIX command line can be read from the variable @@ -131,6 +258,17 @@ is also supported.</para> </sect2> +<sect2><title>Customization Hooks for Users + +The behaviour of require when called with only +one argument is implementation-defined. In &SBCL; it calls functions +on the user-settable list sb-ext:*module-provider-functions* +- see the require documentation string for details. + +The toplevel repl prompt may be customized, and the function +that reads user input may be replaced completely. + Tools To Help Developers &SBCL; provides a profiler and other extensions to the &ANSI;