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. 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. 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. &SBCL; provides a profiler and other extensions to the &ANSI; trace facility. See the online function documentation for trace for more information. The debugger supports a number of options. Its documentation is accessed by typing help at the debugger prompt. Documentation for inspect is accessed by typing help at the inspect prompt. &SBCL; has the ability to save its state as a file for later execution. This functionality is important for its bootstrapping process, and is also provided as an extension to the user See the documentation for sb-ext:save-lisp-and-die for more information. &SBCL; has inherited from &CMUCL; various hooks to allow the user to tweak and monitor the garbage collection process. These are somewhat stale code, and their interface might need to be cleaned up. If you have urgent need of them, look at the code in src/code/gc.lisp and bring it up on the developers' mailing list. &SBCL; has various hooks inherited from &CMUCL;, like sb-ext:float-denormalized-p, to allow a program to take advantage of &IEEE; floating point arithmetic properties which aren't conveniently or efficiently expressible using the &ANSI; standard. These look good, and their interface looks good, but &IEEE; support is slightly broken due to a stupid decision to remove some support for infinities (because it wasn't in the &ANSI; spec and it didn't occur to me that it was in the &IEEE; spec). If you need this stuff, take a look at the code and bring it up on the developers' mailing list. The sb-ext:purify function causes &SBCL; first to collect all garbage, then to mark all uncollected objects as permanent, never again attempting to collect them as garbage. This can cause a large increase in efficiency when using a primitive garbage collector, or a more moderate increase in efficiency when using a more sophisticated garbage collector which is well suited to the program's memory usage pattern. It also allows permanent code to be frozen at fixed addresses, a precondition for using copy-on-write to share code between multiple Lisp processes. is less important with modern generational garbage collectors. The sb-ext:truly-the operator does what the cl:the operator does in a more conventional implementation of &CommonLisp;, declaring the type of its argument without any runtime checks. (Ordinarily in &SBCL;, any type declaration is treated as an assertion and checked at runtime.) The sb-ext:freeze-type declaration declares that a type will never change, which can make type testing (typep, etc.) more efficient for structure types. The sb-ext:constant-function declaration specifies that a function will always return the same value for the same arguments, which may allow the compiler to optimize calls to it. This is appropriate for functions like sqrt, but is not appropriate for functions like aref, which can change their return values when the underlying data are changed.