Chapter 2  Design Choices and Extensions

• Data Types
  • Integers
  • Floats
    • IEEE Special Values
    • Negative Zero
    • Denormalized Floats
    • Floating Point Exceptions
    • Floating Point Rounding Mode
      • Warning:
    • Precision Control
      • Warning:
    • Accessing the Floating Point Modes
  • Characters
  • Array Initialization
  • Hash tables
• Default Interrupts for Lisp
• Implementation-specific Packages
• Hierarchical Packages
  • Introduction
  • Relative package names
  • Compatibility with ANSI Common Lisp
    • Changes to cl:find-package
    • Using hierarchical packages without modifying cl:find-package
• Package locks
  • Rationale
  • Disabling package locks
• The Editor
• Garbage Collection
  • GC Parameters
  • Generational GC
  • Weak Pointers
  • Finalization
• Describe
• The Inspector
  • The Graphical Interface
  • The TTY Inspector
• Load
• The Reader
  • Reader Extensions
  • Reader Parameters
• Stream Extensions
• Simple Streams
• Running Programs from Lisp
  • Process Accessors
• Saving a Core Image
• Pathnames
  • Unix Pathnames
  • Wildcard Pathnames
  • Logical Pathnames
  • Search Lists
  • Predefined Search-Lists
  • Search-List Operations
  • Search List Example
• Filesystem Operations
  • Wildcard Matching
  • File Name Completion
  • Miscellaneous Filesystem Operations
• Time Parsing and Formatting
• Random Number Generation
  • Original Generator
  • New Generator
  • MT-19987 Generator
• Lisp Threads
• Lisp Library
• Generalized Function Names
• CLOS
  • Primary Method Errors
  • Slot Type Checking
  • Slot Access Optimization
    • slot-boundp Declaration
    • inline Declaration
    • Automatic Method Recompilation
  • Inlining Methods in Effective Methods
  • Effective Method Precomputation
  • Sealing
  • Method Tracing and Profiling
  • Misc
• Differences from ANSI Common Lisp
  • Extensions
• Function Wrappers
• Dynamic-Extent Declarations
  • &rest argument lists
  • Closures
  • list, list*, and cons
• Modular Arithmetic
• Extension to REQUIRE

Several design choices in Common Lisp are left to the individual implementation, and some essential parts of the programming environment are left undefined. This chapter discusses the most important design choices and extensions.

2.1  Data Types

2.1.1  Integers

The

type is equivalent to

. Integers outside this range are represented as a

or a word integer (see section 5.11.6.) Almost all integers that appear in programs can be represented as a

, so integer number consing is rare.

2.1.2  Floats

CMUCL supports two floating point formats:

and

. These are implemented with IEEE single and double float arithmetic, respectively.

is a synonym for

, and

is a synonym for

. The initial value of

is

.

Both

and

are represented with a pointer descriptor, so float operations can cause number consing. Number consing is greatly reduced if programs are written to allow the use of non-descriptor representations (see section 5.11.)

2.1.2.1  IEEE Special Values

CMUCL supports the IEEE infinity and NaN special values. These non-numeric values will only be generated when trapping is disabled for some floating point exception (see section 2.1.2.4), so users of the default configuration need not concern themselves with special values.

[Constant]
extensions:short-float-negative-infinity    

[Constant]
extensions:single-float-positive-infinity    

[Constant]
extensions:single-float-negative-infinity    

[Constant]
extensions:double-float-positive-infinity    

[Constant]
extensions:double-float-negative-infinity    

[Constant]
extensions:long-float-positive-infinity    

[Constant]
extensions:long-float-negative-infinity    

The values of these constants are the IEEE positive and negative infinity objects for each float format.

This function returns true if

x

is an IEEE float infinity (of either sign.)

x

must be a float.

[Function]
extensions:float-trapping-nan-p x    

float-nan-p

returns true if

x

is an IEEE NaN (Not A Number) object.

float-trapping-nan-p

returns true only if

x

is a trapping NaN. With either function,

x

must be a float.

2.1.2.2  Negative Zero

The IEEE float format provides for distinct positive and negative zeros. To test the sign on zero (or any other float), use the Common Lisp

function. Negative zero prints as

or

.

2.1.2.3  Denormalized Floats

CMUCL supports IEEE denormalized floats. Denormalized floats provide a mechanism for gradual underflow. The Common Lisp

function returns the actual precision of a denormalized float, which will be less than

. Note that in order to generate (or even print) denormalized floats, trapping must be disabled for the underflow exception (see section 2.1.2.4.) The Common Lisp

-

constants are denormalized.

This function returns true if

x

is a denormalized float.

x

must be a float.

2.1.2.4  Floating Point Exceptions

The IEEE floating point standard defines several exceptions that occur when the result of a floating point operation is unclear or undesirable. Exceptions can be ignored, in which case some default action is taken, such as returning a special value. When trapping is enabled for an exception, a error is signalled whenever that exception occurs. These are the possible floating point exceptions:

:underflow

This exception occurs when the result of an operation is too small to be represented as a normalized float in its format. If trapping is enabled, the floating-point-underflow condition is signalled. Otherwise, the operation results in a denormalized float or zero.

:overflow

This exception occurs when the result of an operation is too large to be represented as a float in its format. If trapping is enabled, the floating-point-overflow exception is signalled. Otherwise, the operation results in the appropriate infinity.

:inexact

This exception occurs when the result of a floating point operation is not exact, i.e. the result was rounded. If trapping is enabled, the extensions:floating-point-inexact condition is signalled. Otherwise, the rounded result is returned.

:invalid

This exception occurs when the result of an operation is ill-defined, such as (/ 0.0 0.0). If trapping is enabled, the extensions:floating-point-invalid condition is signalled. Otherwise, a quiet NaN is returned.

:divide-by-zero

This exception occurs when a float is divided by zero. If trapping is enabled, the divide-by-zero condition is signalled. Otherwise, the appropriate infinity is returned.

2.1.2.5  Floating Point Rounding Mode

IEEE floating point specifies four possible rounding modes:

:nearest

In this mode, the inexact results are rounded to the nearer of the two possible result values. If the neither possibility is nearer, then the even alternative is chosen. This form of rounding is also called “round to even”, and is the form of rounding specified for the Common Lisp round function.

:positive-infinity

This mode rounds inexact results to the possible value closer to positive infinity. This is analogous to the Common Lisp ceiling function.

:negative-infinity

This mode rounds inexact results to the possible value closer to negative infinity. This is analogous to the Common Lisp floor function.

:zero

This mode rounds inexact results to the possible value closer to zero. This is analogous to the Common Lisp truncate function.

Warning:

Although the rounding mode can be changed with

, use of any value other than the default (

) can cause unusual behavior, since it will affect rounding done by Common Lisp system code as well as rounding in user code. In particular, the unary

function will stop doing round-to-nearest on floats, and instead do the selected form of rounding.

2.1.2.6  Precision Control

The floating-point unit for the Intel IA-32 architecture supports a precision control mechanism. The floating-point unit consists of an IEEE extended double-float unit and all operations are always done using his format, and this includes rounding. However, by setting the precision control mode, the user can control how rounding is done for each basic arithmetic operation like addition, subtraction, multiplication, and division. The extra instructions for trigonometric, exponential, and logarithmic operations are not affected. We refer the reader to Intel documentation for more information.

The possible modes are:

:24-bit

In this mode, all basic arithmetic operations like addition, subtraction, multiplication, and division, are rounded after each operation as if both the operands were IEEE single precision numbers.

:53-bit

In this mode, rounding is performed as if the operands and results were IEEE double precision numbers.

:64-bit

In this mode, the default, rounding is performed on the full IEEE extended double precision format.

Warning:

Although the precision mode can be changed with

, use of anything other than

or

can cause unexpected results, especially if external functions or libraries are called. A setting of

also causes

to return

instead of

.

2.1.2.7  Accessing the Floating Point Modes

These functions can be used to modify or read the floating point modes:

[Function]
extensions:get-floating-point-modes    

The keyword arguments to

set-floating-point-modes

set various modes controlling how floating point arithmetic is done:

:traps

A list of the exception conditions that should cause traps. Possible exceptions are :underflow, :overflow, :inexact, :invalid and :divide-by-zero. Initially all traps except :inexact are enabled. See section 2.1.2.4.

:rounding-mode

The rounding mode to use when the result is not exact. Possible values are :nearest, :positive-infinity, :negative-infinity and :zero. Initially, the rounding mode is :nearest. See the warning in section 2.1.2.5 about use of other rounding modes.

:current-exceptions, :accrued-exceptions

Lists of exception keywords used to set the exception flags. The current-exceptions are the exceptions for the previous operation, so setting it is not very useful. The accrued-exceptions are a cumulative record of the exceptions that occurred since the last time these flags were cleared. Specifying () will clear any accrued exceptions.

:fast-mode

Set the hardware’s “fast mode” flag, if any. When set, IEEE conformance or debuggability may be impaired. Some machines may not have this feature, in which case the value is always nil. Sparc platforms support a fast mode where denormal numbers are silently truncated to zero.

:precision-control

On the x86 architecture, you can set the precision of the arithmetic to :24-bit, :53-bit, or :64-bit mode, corresponding to IEEE single precision, double precision, and extended double precision.

If a keyword argument is not supplied, then the associated state is not changed.

get-floating-point-modes

returns a list representing the state of the floating point modes. The list is in the same format as the keyword arguments to

set-floating-point-modes

, so

apply

could be used with

set-floating-point-modes

to restore the modes in effect at the time of the call to

get-floating-point-modes

.

To make handling control of floating-point exceptions, the following macro is useful.

body is executed with the selected floating-point exceptions given by traps masked out (disabled). traps should be a list of possible floating-point exceptions that should be ignored. Possible values are :underflow, :overflow, :inexact, :invalid and :divide-by-zero.

This is equivalent to saving the current traps from

get-floating-point-modes

, setting the floating-point modes to the desired exceptions, running the

body

, and restoring the saved floating-point modes. The advantage of this macro is that it causes less consing to occur.

Some points about the with-float-traps-masked:

• Two approaches are available for detecting FP exceptions:
  1. enabling the traps and handling the exceptions
  2. disabling the traps and either handling the return values or checking the accrued exceptions.
  Of these the latter is the most portable because on the alpha port it is not possible to enable some traps at run-time.
• To assist the checking of the exceptions within the body any accrued exceptions matching the given traps are cleared at the start of the body when the traps are masked.
• To allow the macros to be nested these accrued exceptions are restored at the end of the body to their values at the start of the body. Thus any exceptions that occurred within the body will not affect the accrued exceptions outside the macro.
• Note that only the given exceptions are restored at the end of the body so other exception will be visible in the accrued exceptions outside the body.
• On the x86, setting the accrued exceptions of an unmasked exception would cause a FP trap. The macro behaviour of restoring the accrued exceptions ensures than if an accrued exception is initially not flagged and occurs within the body it will be restored/cleared at the exit of the body and thus not cause a trap.
• On the x86, and, perhaps, the hppa, the FP exceptions may be delivered at the next FP instruction which requires a FP wait instruction (x86::float-wait) if using the lisp conditions to catch trap within a handler-bind. The handler-bind macro does the right thing and inserts a float-wait (at the end of its body on the x86). The masking and noting of exceptions is also safe here.
• The setting of the FP flags uses the (floating-point-modes) and the (set (floating-point-modes)…) VOPs. These VOPs blindly update the flags which may include other state. We assume this state hasn’t changed in between getting and setting the state. For example, if you used the FP unit between the above calls, the state may be incorrectly restored! The with-float-traps-masked macro keeps the intervening code to a minimum and uses only integer operations.

2.1.3  Characters

CMUCL implements characters according to Common Lisp: The Language II. The main difference from the first version is that character bits and font have been eliminated, and the names of the types have been changed.

is the new equivalent of the old

. In this implementation, all characters are base characters (there are no extended characters.) Character codes range between

and

, using the ASCII encoding. Table 2.1 tbl:chars shows characters recognized by CMUCL.

---

ASCII		Lisp
Name	Code	Name	1.5exAlternatives
nul	0	#\NULL	#\NUL
bel	7	#\BELL
bs	8	#\BACKSPACE	#\BS
tab	9	#\TAB
lf	10	#\NEWLINE	#\NL	#\LINEFEED	#\LF
ff	11	#\VT	#\PAGE	#\FORM
cr	13	#\RETURN	#\CR
esc	27	#\ESCAPE	#\ESC	#\ALTMODE	#\ALT
sp	32	#\SPACE	#\SP
del	127	#\DELETE	#\RUBOUT

Table 2.1: Characters recognized by CMUCL

---

2.1.4  Array Initialization

If no

is specified, arrays are initialized to zero.

2.1.5  Hash tables

The

defined by Common Lisp have limited utility because they are limited to testing their keys using the equality predicates provided by (pre-CLOS) Common Lisp. CMUCL overcomes this limitation by allowing its users to specify new hash table tests and hashing methods. The hashing method must also be specified, since the compiler is unable to determine a good hashing function for an arbitrary equality (equivalence) predicate.

The

hash-table-test-name

must be a symbol. The

test-function

takes two objects and returns true iff they are the same. The

hash-function

takes one object and returns two values: the (positive fixnum) hash value and true if the hashing depends on pointer values and will have to be redone if the object moves.

To create a hash-table using this new “test” (really, a test/hash-function pair), use

(make-hash-table :test hash-table-test-name …)

.

Note that it is the

hash-table-test-name

that will be returned by the function

hash-table-test

, when applied to a hash-table created using this function.

This function updates

*hash-table-tests*

, which is now internal.

2.2  Default Interrupts for Lisp

CMUCL has several interrupt handlers defined when it starts up, as follows:

SIGINT (Ctrl-c)

causes Lisp to enter a break loop. This puts you into the debugger which allows you to look at the current state of the computation. If you proceed from the break loop, the computation will proceed from where it was interrupted.

SIGQUIT (Ctrl-L)

causes Lisp to do a throw to the top-level. This causes the current computation to be aborted, and control returned to the top-level read-eval-print loop.

SIGTSTP (Ctrl-z)

causes Lisp to suspend execution and return to the Unix shell. If control is returned to Lisp, the computation will proceed from where it was interrupted.

SIGILL, SIGBUS, SIGSEGV, and SIGFPE

cause Lisp to signal an error.

For keyboard interrupt signals, the standard interrupt character is in parentheses. Your

may set up different interrupt characters. When a signal is generated, there may be some delay before it is processed since Lisp cannot be interrupted safely in an arbitrary place. The computation will continue until a safe point is reached and then the interrupt will be processed. See section 6.8.1 to define your own signal handlers.

2.3  Implementation-specific Packages

When CMUCL is first started up, the default package is the

package. The

package uses the

and

packages. The symbols exported from these three packages can be referenced without package qualifiers. This section describes packages which have exported interfaces that may concern users. The numerous internal packages which implement parts of the system are not described here. Package nicknames are in parenthesis after the full name.

alien, c-call

Export the features of the Alien foreign data structure facility (see section 8.)

pcl

This package contains PCL (Portable CommonLoops), which is a portable implementation of CLOS (the Common Lisp Object System.) This implements most (but not all) of the features in the CLOS chapter of Common Lisp: The Language II.

clos-mop (mop)

This package contains an implementation of the CLOS Metaobject Protocol, as per the book The Art of the Metaobject Protocol.

debug

The debug package contains the command-line oriented debugger. It exports utility various functions and switches.

debug-internals

The debug-internals package exports the primitives used to write debuggers. See section 11.

extensions (ext)

The extensions packages exports local extensions to Common Lisp that are documented in this manual. Examples include the save-lisp function and time parsing.

hemlock (ed)

The hemlock package contains all the code to implement Hemlock commands. The hemlock package currently exports no symbols.

hemlock-internals (hi)

The hemlock-internals package contains code that implements low level primitives and exports those symbols used to write Hemlock commands.

keyword

The keyword package contains keywords (e.g., :start). All symbols in the keyword package are exported and evaluate to themselves (i.e., the value of the symbol is the symbol itself).

profile

The profile package exports a simple run-time profiling facility (see section 5.14).

common-lisp (cl)

The common-lisp package exports all the symbols defined by Common Lisp: The Language and only those symbols. Strictly portable Lisp code will depend only on the symbols exported from the common-lisp package.

unix

This package exports system call interfaces to Unix (see section 6).

system (sys)

The system package contains functions and information necessary for system interfacing. This package is used by the lisp package and exports several symbols that are necessary to interface to system code.

xlib

The xlib package contains the Common Lisp X interface (CLX) to the X11 protocol. This is mostly Lisp code with a couple of functions that are defined in C to connect to the server.

wire

The wire package exports a remote procedure call facility (see section 9).

stream

The stream package exports the public interface to the simple-streams implementation (see section 2.13).

2.4  Hierarchical Packages

2.4.1  Introduction

The Common Lisp package system, designed and standardized several years ago, is not hierarchical. Since Common Lisp was standardized, other languages, including Java and Perl, have evolved namespaces which are hierarchical. This document describes a hierarchical package naming scheme for Common Lisp. The scheme was proposed by Franz Inc and implemented in their Allegro Common Lisp product; a compatible implementation of the naming scheme is implemented in CMUCL. This documentation is based on the Franz Inc. documentation, and is included with permission.

The goals of hierarchical packages in Common Lisp are:

• Reduce collisions with user-defined packages: it is a well-known problem that package names used by the Lisp implementation and those defined by users can easily conflict. The intent of hierarchical packages is to reduce such conflicts to a minimum.
• Improve modularity: the current organization of packages in various implementations has grown over the years and appears somewhat random. Organizing future packages into a hierarchy will help make the intention of the implementation more clear.
• Foster growth in Common Lisp programs, or modules, available to the CL community: the Perl and Java communities are able to contribute code to repositories, with minimal fear of collision, because of the hierarchical nature of the name spaces used by the contributed code. We want the Lisp community to benefit from shared modules in the same way.

In a nutshell, a dot (.) is used to separate levels in package names, and a leading dot signifies a relative package name. The choice of dot follows Java. Perl, another language with hierarchical packages, uses a colon (:) as a delimiter, but the colon is already reserved in Common Lisp. Absolute package names require no modifications to the underlying Common Lisp implementation. Relative package names require only small and simple modifications.

2.4.2  Relative package names

Relative package names are needed for the same reason as relative pathnames, for brevity and to reduce the brittleness of absolute names. A relative package name is one that begins with one or more dots. A single dot means the current package, two dots mean the parent of the current package, and so on.

Table 2.2 presents a number of examples, assuming that packages named foo, foo.bar, mypack, mypack.foo, mypack.foo.bar, mypack.foo.baz, mypack.bar, and mypack.bar.baz, have all been created.

---

relative name	current package	absolute name of referenced package
foo	any	foo
foo.bar	any	foo.bar
.foo	mypack	mypack.foo
.foo.bar	mypack	mypack.foo.bar
..foo	mypack.bar	mypack.foo
..foo.baz	mypack.bar	mypack.foo.baz
...foo	mypack.bar.baz	mypack.foo
.	mypack.bar.baz	mypack.bar.baz
..	mypack.bar.baz	mypack.bar
...	mypack.bar.baz	mypack

Table 2.2: Examples of hierarchical packages

---

Additional notes:

1. All packages in the hierarchy must exist.
2. Warning about nicknames: Unless you provide nicknames for your hierarchical packages (and we recommend against doing so because the number gets quite large), you can only use the names supplied. You cannot mix in nicknames or alternate names. cl-user is nickname of the common-lisp-user package. Consider the following:

      (defpackage :cl-user.foo)
   

   When the current package (the value of the variable

   *package*

   ) is

   common-lisp-user

   , you might expect .foo to refer to cl-user.foo, but it does not. It refers to the non-existent package common-lisp-user.foo. Note that the purpose of nicknames is to provide shorter names in place of the longer names that are designed to be fully descriptive. The hope is that hierarchical packages makes longer names unnecessary and thus makes nicknames unnecessary.

3. Multiple dots can only appear at the beginning of a package name. For example, foo.bar..baz does not mean foo.baz – it is invalid. (Of course, it is perfectly legal to name a package foo.bar..baz, but cl:find-package will not process such a name to find foo.baz in the package hierarchy.)

2.4.3  Compatibility with ANSI Common Lisp

The implementation of hierarchical packages modifies the

function, and provides certain auxiliary functions,

,

, and

, as described in this section. The function

itself requires no modification.

While the changes to

are small and described below, it is an important consideration for authors who would like their programs to run on a variety of implementations that using hierarchical packages will work in an implementation without the modifications discussed in this document. We show why after describing the changes to

.

Absolute hierarchical package names require no changes in the underlying Common Lisp implementation.

2.4.3.1  Changes to cl:find-package

Using relative hierarchical package names requires a simple modification of

.

In ANSI Common Lisp,

, if passed a package object, returns it; if passed a string,

looks for a package with that string as its name or nickname, and returns the package if it finds one, or returns nil if it does not; if passed a symbol, the symbol name (a string) is extracted and

proceeds as it does with a string.

For implementing hierarchical packages, the behavior when the argument is a package object (return it) does not change. But when the argument is a string starting with one or more dots not directly naming a package,

will, instead of returning nil, check whether the string can be resolved as naming a relative package, and if so, return the associated absolute package object. (If the argument is a symbol, the symbol name is extracted and

proceeds as it does with a string argument.)

Note that you should not use leading dots in package names when using hierarchical packages.

2.4.3.2  Using hierarchical packages without modifying cl:find-package

Even without the modifications to

, authors need not avoid using relative package names, but the ability to reuse relative package names is restricted. Consider for example a module foo which is composed of the my.foo.bar and my.foo.baz packages. In the code for each of the these packages there are relative package references, ..bar and ..baz.

Implementations that have the new

would carry the keyword :relative-package-names on their

list (this is the case of CMUCL releases starting from 18d). Then, in the foo module, there would be definitions of the my.foo.bar and my.foo.baz packages like so:

   (defpackage :my.foo.bar
     #-relative-package-names (:nicknames #:..bar)
     ...)

   (defpackage :my.foo.baz
     #-relative-package-names (:nicknames #:..baz)
     ...)

Then, in a #-relative-package-names implementation, the symbol my.foo.bar:blam would be visible from my.foo.baz as ..bar:blam, just as it would from a #+relative-package-names implementation.

So, even without the implementation of the augmented

, one can still write Common Lisp code that will work in both types of implementations, but ..bar and ..baz are now used, so you cannot also have otherpack.foo.bar and otherpack.foo.baz and use ..bar and ..baz as relative names. (The point of hierarchical packages, of course, is to allow reusing relative package names.)

2.5  Package locks

CMUCL provides two types of package locks, as an extension to the ANSI Common Lisp standard. The package-lock protects a package from changes in its structure (the set of exported symbols, its use list, etc). The package-definition-lock protects the symbols in the package from being redefined due to the execution of a

,

,

,

or

form.

2.5.1  Rationale

Package locks are an aid to program development, by helping to detect inadvertent name collisions and function redefinitions. They are consistent with the principle that a package “belongs to” its implementor, and that noone other than the package’s developer should be making or modifying definitions on symbols in that package. Package locks are compatible with the ANSI Common Lisp standard, which states that the consequences of redefining functions in the

package are undefined.

Violation of a package lock leads to a continuable error of type

being signaled. The user may choose to ignore the lock and proceed, or to abort the computation. Two other restarts are available, one which disables all locks on all packages, and one to disable only the package-lock or package-definition-lock that was tripped.

The following transcript illustrates the behaviour seen when attempting to redefine a standard macro in the

package, or to redefine a function in one of CMUCL’s implementation-defined packages:

CL-USER> (defmacro 1+ (x) (* x 2))
Attempt to modify the locked package COMMON-LISP, by defining macro 1+
   [Condition of type LISP::PACKAGE-LOCKED-ERROR]

Restarts:
  0: [continue      ] Ignore the lock and continue
  1: [unlock-package] Disable the package's definition-lock then continue
  2: [unlock-all    ] Unlock all packages, then continue
  3: [abort         ] Return to Top-Level.

CL-USER> (defun ext:gc () t)
Attempt to modify the locked package EXTENSIONS, by redefining function GC
   [Condition of type LISP::PACKAGE-LOCKED-ERROR]

Restarts:
  0: [continue      ] Ignore the lock and continue
  1: [unlock-package] Disable package's definition-lock, then continue
  2: [unlock-all    ] Disable all package locks, then continue
  3: [abort         ] Return to Top-Level.

The following transcript illustrates the behaviour seen when an attempt to modify the structure of a package is made:

CL-USER> (unexport 'load-foreign :ext)
Attempt to modify the locked package EXTENSIONS, by unexporting symbols LOAD-FOREIGN
   [Condition of type lisp::package-locked-error]

Restarts:
  0: [continue      ] Ignore the lock and continue
  1: [unlock-package] Disable package's lock then continue
  2: [unlock-all    ] Unlock all packages, then continue
  3: [abort         ] Return to Top-Level.

The

package and the CMUCL-specific implementation packages are locked on startup. Users can lock their own packages by using the

and

accessors.

2.5.2  Disabling package locks

A package’s locks can be enabled or disabled by using the

and

accessors, as follows:

   (setf (ext:package-lock (find-package "UNIX")) nil)
   (setf (ext:package-definition-lock (find-package "UNIX")) nil)

This function is an accessor for a package’s structural lock, which protects it against modifications to its list of exported symbols.

This function is an accessor for a package’s definition-lock, which protects symbols in that package from redefinition. As well as protecting the symbol’s fdefinition from change, attempts to change the symbol’s definition using defstruct, defclass or deftype will be trapped.

This macro can be used to execute forms with all package locks (both structure and definition locks) disabled.

This function disables both structure and definition locks on all currently defined packages. Note that package locks are reset when CMUCL is restarted, so the effect of this function is limited to the current session.

2.6  The Editor

The

function invokes the Hemlock editor which is described in Hemlock User’s Manual and Hemlock Command Implementor’s Manual. Most users at CMU prefer to use Hemlock’s slave Common Lisp mechanism which provides an interactive buffer for the

loop and editor commands for evaluating and compiling text from a buffer into the slave Common Lisp. Since the editor runs in the Common Lisp, using slaves keeps users from trashing their editor by developing in the same Common Lisp with Hemlock.

2.7  Garbage Collection

CMUCL uses either a stop-and-copy garbage collector or a generational, mostly copying garbage collector. Which collector is available depends on the platform and the features of the platform. The stop-and-copy GC is available on all RISC platforms. The x86 platform supports a conservative stop-and-copy collector, which is now rarely used, and a generational conservative collector. On the Sparc platform, both the stop-and-copy GC and the generational GC are available, but the stop-and-copy GC is deprecated in favor of the generational GC.

The generational GC is available if

contains

.

The following functions invoke the garbage collector or control whether automatic garbage collection is in effect:

heney]extensions:gc&optional verbose-p

This function runs the garbage collector. If

ext:*gc-verbose*

is non-

nil

, then it invokes

ext:*gc-notify-before*

before GC’ing and

ext:*gc-notify-after*

afterwards.

verbose-p

indicates whether GC statistics are printed or not.

This function inhibits automatic garbage collection. After calling it, the system will not GC unless you call

ext:gc

or

ext:gc-on

.

This function reinstates automatic garbage collection. If the system would have GC’ed while automatic GC was inhibited, then this will call

ext:gc

.

2.7.1  GC Parameters

The following variables control the behavior of the garbage collector:

CMUCL automatically GC’s whenever the amount of memory allocated to dynamic objects exceeds the value of an internal variable. After each GC, the system sets this internal variable to the amount of dynamic space in use at that point plus the value of the variable

ext:*bytes-consed-between-gcs*

. The default value is 2000000.

This variable controls whether

ext:gc

invokes the functions in

ext:*gc-notify-before*

and

ext:*gc-notify-after*

. If

*gc-verbose*

is

nil

,

ext:gc

foregoes printing any messages. The default value is

T

.

This variable’s value is a function that should notify the user that the system is about to GC. It takes one argument, the amount of dynamic space in use before the GC measured in bytes. The default value of this variable is a function that prints a message similar to the following:

   [GC threshold exceeded with 2,107,124 bytes in use.  Commencing GC.]

This variable’s value is a function that should notify the user when a GC finishes. The function must take three arguments, the amount of dynamic spaced retained by the GC, the amount of dynamic space freed, and the new threshold which is the minimum amount of space in use before the next GC will occur. All values are byte quantities. The default value of this variable is a function that prints a message similar to the following:

    [GC completed with 25,680 bytes retained and 2,096,808 bytes freed.]
    [GC will next occur when at least 2,025,680 bytes are in use.]
  

Note that a garbage collection will not happen at exactly the new threshold printed by the default

function. The system periodically checks whether this threshold has been exceeded, and only then does a garbage collection.

This variable’s value is either a function of one argument or

nil

. When the system has triggered an automatic GC, if this variable is a function, then the system calls the function with the amount of dynamic space currently in use (measured in bytes). If the function returns

nil

, then the GC occurs; otherwise, the system inhibits automatic GC as if you had called

ext:gc-off

. The writer of this hook is responsible for knowing when automatic GC has been turned off and for calling or providing a way to call

ext:gc-on

. The default value of this variable is

nil

.

[Variable]
extensions:*after-gc-hooks*    

These variables’ values are lists of functions to call before or after any GC occurs. The system provides these purely for side-effect, and the functions take no arguments.

2.7.2  Generational GC

Generational GC also supports some additional functions and variables to control it.

encgc]extensions:gc&key :verbose :gen :full

This function runs the garbage collector. If

ext:*gc-verbose*

is non-

nil

, then it invokes

ext:*gc-notify-before*

before GC’ing and

ext:*gc-notify-after*

afterwards.

verbose

Print GC statistics if non-NIL.

gen

The number of generations to be collected.

full

If non-NIL, a full collection of all generations is performed.

Returns statistics about the generation, as multiple values:

1. Bytes allocated in this generation
2. The GC trigger for this generation. When this many bytes have been allocated, a GC is started automatically.
3. The number of bytes consed between GCs.
4. The number of GCs that have been done on this generation. This is reset to zero when the generation is raised.
5. The trigger age, which is the maximum number of GCs to perform before this generation is raised.
6. The total number of bytes allocated to this generation.
7. Average age of the objects in this generations. The average age is the cumulative bytes allocated divided by current number of bytes allocated.

Sets the GC trigger value for the specified generation.

Sets the GC trigger age for the specified generation.

Sets the minimum average memory age for the specified generation. If the computed memory age is below this, GC is not performed, which helps prevent a GC when a large number of new live objects have been added in which case a GC would usually be a waste of time.

2.7.3  Weak Pointers

A weak pointer provides a way to maintain a reference to an object without preventing an object from being garbage collected. If the garbage collector discovers that the only pointers to an object are weak pointers, then it breaks the weak pointers and deallocates the object.

[Function]
extensions:weak-pointer-value weak-pointer    

make-weak-pointer

returns a weak pointer to an object.

weak-pointer-value

follows a weak pointer, returning the two values: the object pointed to (or

nil

if broken) and a boolean value which is

nil

if the pointer has been broken, and true otherwise.

2.7.4  Finalization

Finalization provides a “hook” that is triggered when the garbage collector reclaims an object. It is usually used to recover non-Lisp resources that were allocated to implement the finalized Lisp object. For example, when a unix file-descriptor stream is collected, finalization is used to close the underlying file descriptor.

This function registers

object

for finalization.

function

is called with no arguments when

object

is reclaimed. Normally

function

will be a closure over the underlying state that needs to be freed, e.g. the unix file descriptor in the fd-stream case. Note that

function

must not close over

object

itself, as this prevents the object from ever becoming garbage.

This function cancel any finalization request for

object

.

2.8  Describe

The

describe

function prints useful information about

object

on

stream

, which defaults to

*standard-output*

. For any object,

describe

will print out the type. Then it prints other information based on the type of

object

. The types which are presently handled are:

hash-table

describe prints the number of entries currently in the hash table and the number of buckets currently allocated.

function

describe prints a list of the function’s name (if any) and its formal parameters. If the name has function documentation, then it will be printed. If the function is compiled, then the file where it is defined will be printed as well.

fixnum

describe prints whether the integer is prime or not.

symbol

The symbol’s value, properties, and documentation are printed. If the symbol has a function definition, then the function is described.

If there is anything interesting to be said about some component of the object, describe will invoke itself recursively to describe that object. The level of recursion is indicated by indenting output.

A number of switches can be used to control

’s behavior.

The maximum level of recursive description allowed. Initially two.

The number of spaces to indent for each level of recursive description, initially three.

[Variable]
extensions:*describe-print-length*    

The values of

*print-level*

and

*print-length*

during description. Initially two and five.

2.9  The Inspector

CMUCL has both a graphical inspector that uses the X Window System, and a simple terminal-based inspector.

inspect

calls the inspector on the optional argument

object

. If

object

is unsupplied,

inspect

immediately returns

nil

. Otherwise, the behavior of inspect depends on whether Lisp is running under X. When

inspect

is eventually exited, it returns some selected Lisp object.

2.9.1  The Graphical Interface

CMUCL has an interface to Motif which is functionally similar to CLM, but works better in CMUCL. This interface is documented in separate manuals CMUCL Motif Toolkit and Design Notes on the Motif Toolkit, which are distributed with CMUCL.

This motif interface has been used to write the inspector and graphical debugger. There is also a Lisp control panel with a simple file management facility, apropos and inspector dialogs, and controls for setting global options. See the

and

packages.

This function creates a control panel for the Lisp process.

When the graphical interface is loaded, this variable controls whether it is used by

inspect

and the error system. If the value is

:graphics

(the default) and the

DISPLAY

environment variable is defined, the graphical inspector and debugger will be invoked by

inspect

or when an error is signalled. Possible values are

:graphics

and tty. If the value is

:graphics

, but there is no X display, then we quietly use the TTY interface.

2.9.2  The TTY Inspector

If X is unavailable, a terminal inspector is invoked. The TTY inspector is a crude interface to

which allows objects to be traversed and maintains a history. This inspector prints information about and object and a numbered list of the components of the object. The command-line based interface is a normal

–

–

loop, but an integer

descends into the

’th component of the current object, and symbols with these special names are interpreted as commands:

U

Move back to the enclosing object. As you descend into the components of an object, a stack of all the objects previously seen is kept. This command pops you up one level of this stack.

Q, E

Return the current object from inspect.

R

Recompute object display, and print again. Useful if the object may have changed.

D

Display again without recomputing.

H, ?

Show help message.

2.10  Load

As in standard Common Lisp, this function loads a file containing source or object code into the running Lisp. Several CMU extensions have been made to

load

to conveniently support a variety of program file organizations.

filename

may be a wildcard pathname such as

*.lisp

, in which case all matching files are loaded.

If

filename

has a

pathname-type

(or extension), then that exact file is loaded. If the file has no extension, then this tells

load

to use a heuristic to load the “right” file. The

*load-source-types*

and

*load-object-types*

variables below are used to determine the default source and object file types. If only the source or the object file exists (but not both), then that file is quietly loaded. Similarly, if both the source and object file exist, and the object file is newer than the source file, then the object file is loaded. The value of the

if-source-newer

argument is used to determine what action to take when both the source and object files exist, but the object file is out of date:

:load-object

The object file is loaded even though the source file is newer.

:load-source

The source file is loaded instead of the older object file.

:compile

The source file is compiled and then the new object file is loaded.

:query

The user is asked a yes or no question to determine whether the source or object file is loaded.

This argument defaults to the value of

ext:*load-if-source-newer*

(initially

:load-object

.)

The

contents

argument can be used to override the heuristic (based on the file extension) that normally determines whether to load the file as a source file or an object file. If non-null, this argument must be either

:source

or

:binary

, which forces loading in source and binary mode, respectively. You really shouldn’t ever need to use this argument.

[Variable]
extensions:*load-object-types*    

These variables are lists of possible

pathname-type

values for source and object files to be passed to

load

. These variables are only used when the file passed to

load

has no type; in this case, the possible source and object types are used to default the type in order to determine the names of the source and object files.

This variable determines the default value of the

if-source-newer

argument to

load

. Its initial value is

:load-object

.

2.11  The Reader

2.11.1  Reader Extensions

CMUCL supports an ANSI-compatible extension to enable reading of specialized arrays. Thus

  * (setf *print-readably* nil)
  NIL
  * (make-array ’(2 2) :element-type ’(signed-byte 8))
  #2A((0 0) (0 0))
  * (setf *print-readably* t)
  T
  * (make-array ’(2 2) :element-type ’(signed-byte 8))
  #A((SIGNED-BYTE 8) (2 2) ((0 0) (0 0)))
  * (type-of (read-from-string "#A((SIGNED-BYTE 8) (2 2) ((0 0) (0 0)))"))
  (SIMPLE-ARRAY (SIGNED-BYTE 8) (2 2))
  * (setf *print-readably* nil)
  NIL
  * (type-of (read-from-string "#A((SIGNED-BYTE 8) (2 2) ((0 0) (0 0)))"))
  (SIMPLE-ARRAY (SIGNED-BYTE 8) (2 2))

2.11.2  Reader Parameters

If this variable is

t

(the default), then the reader merely prints a warning when an extra close parenthesis is detected (instead of signalling an error.)

2.12  Stream Extensions

On streams that support it, this function reads multiple bytes of data into a buffer. The buffer must be a

simple-string

or

(simple-array (unsigned-byte 8) (*))

. The argument

nbytes

specifies the desired number of bytes, and the return value is the number of bytes actually read.

• If eof-error-p is true, an end-of-file condition is signalled if end-of-file is encountered before count bytes have been read.
• If eof-error-p is false, read-n-bytes reads as much data is currently available (up to count bytes.) On pipes or similar devices, this function returns as soon as any data is available, even if the amount read is less than count and eof has not been hit. See also make-fd-stream.

2.13  Simple Streams

CMUCL includes a partial implementation of Simple Streams, a protocol that allows user-extensible streams. The protocol was proposed by Franz, Inc. and is intended to replace the Gray Streams method of extending streams. Simple streams are distributed as a CMUCL subsystem, that can be loaded into the image by saying

   (require :simple-streams)

Note that CMUCL’s implementation of simple streams is incomplete, and in particular is currently missing support for the functions

and

. Please consult the Allegro Common Lisp documentation for more information on simple streams.

2.14  Running Programs from Lisp

It is possible to run programs from Lisp by using the following function.

run-program

runs

program

in a child process.

Program

should be a pathname or string naming the program.

Args

should be a list of strings which this passes to

program

as normal Unix parameters. For no arguments, specify

args

as

nil

. The value returned is either a process structure or

nil

. The process interface follows the description of

run-program

. If

run-program

fails to fork the child process, it returns

nil

.

Except for sharing file descriptors as explained in keyword argument descriptions,

run-program

closes all file descriptors in the child process before running the program. When you are done using a process, call

process-close

to reclaim system resources. You only need to do this when you supply

:stream

for one of

:input

,

:output

, or

:error

, or you supply

:pty

non-

nil

. You can call

process-close

regardless of whether you must to reclaim resources without penalty if you feel safer.

run-program

accepts the following keyword arguments:

:env

This is an a-list mapping keywords and simple-strings. The default is ext:*environment-list*. If :env is specified, run-program uses the value given and does not combine the environment passed to Lisp with the one specified.

:wait

If non-nil (the default), wait until the child process terminates. If nil, continue running Lisp while the child process runs.

:pty

This should be one of t, nil, or a stream. If specified non-nil, the subprocess executes under a Unix PTY. If specified as a stream, the system collects all output to this pty and writes it to this stream. If specified as t, the process-pty slot contains a stream from which you can read the program’s output and to which you can write input for the program. The default is nil.

:input

This specifies how the program gets its input. If specified as a string, it is the name of a file that contains input for the child process. run-program opens the file as standard input. If specified as nil (the default), then standard input is the file /dev/null. If specified as t, the program uses the current standard input. This may cause some confusion if :wait is nil since two processes may use the terminal at the same time. If specified as :stream, then the process-input slot contains an output stream. Anything written to this stream goes to the program as input. :input may also be an input stream that already contains all the input for the process. In this case run-program reads all the input from this stream before returning, so this cannot be used to interact with the process.

:if-input-does-not-exist

This specifies what to do if the input file does not exist. The following values are valid: nil (the default) causes run-program to return nil without doing anything; :create creates the named file; and :error signals an error.

:output

This specifies what happens with the program’s output. If specified as a pathname, it is the name of a file that contains output the program writes to its standard output. If specified as nil (the default), all output goes to /dev/null. If specified as t, the program writes to the Lisp process’s standard output. This may cause confusion if :wait is nil since two processes may write to the terminal at the same time. If specified as :stream, then the process-output slot contains an input stream from which you can read the program’s output.

:if-output-exists

This specifies what to do if the output file already exists. The following values are valid: nil causes run-program to return nil without doing anything; :error (the default) signals an error; :supersede overwrites the current file; and :append appends all output to the file.

:error

This is similar to :output, except the file becomes the program’s standard error. Additionally, :error can be :output in which case the program’s error output is routed to the same place specified for :output. If specified as :stream, the process-error contains a stream similar to the process-output slot when specifying the :output argument.

:if-error-exists

This specifies what to do if the error output file already exists. It accepts the same values as :if-output-exists.

:status-hook

This specifies a function to call whenever the process changes status. This is especially useful when specifying :wait as nil. The function takes the process as a required argument.

:before-execve

This specifies a function to run in the child process before it becomes the program to run. This is useful for actions such as authenticating the child process without modifying the parent Lisp process.

2.14.1  Process Accessors

The following functions interface the process returned by

:

This function returns

t

if

thing

is a process. Otherwise it returns

nil

This function returns the process ID, an integer, for the

process

.

This function returns the current status of

process

, which is one of

:running

,

:stopped

,

:exited

, or

:signaled

.

This function returns either the exit code for

process

, if it is

:exited

, or the termination signal

process

if it is

:signaled

. The result is undefined for processes that are still alive.

This function returns

t

if someone used a Unix signal to terminate the

process

and caused it to dump a Unix core image.

This function returns either the two-way stream connected to

process

’s Unix PTY connection or

nil

if there is none.

[Function]
extensions:process-output process    

[Function]
extensions:process-error process    

If the corresponding stream was created, these functions return the input, output or error fd-stream.

nil

is returned if there is no stream.

This function returns the current function to call whenever

process

’s status changes. This function takes the

process

as a required argument.

process-status-hook

is

setf

’able.

This function returns annotations supplied by users, and it is

setf

’able. This is available solely for users to associate information with

process

without having to build a-lists or hash tables of process structures.

This function waits for

process

to finish. If

check-for-stopped

is non-

nil

, this also returns when

process

stops.

This function sends the Unix

signal

to

process

.

Signal

should be the number of the signal or a keyword with the Unix name (for example,

:sigsegv

).

Whom

should be one of the following:

:pid

This is the default, and it indicates sending the signal to process only.

:process-group

This indicates sending the signal to process’s group.

:pty-process-group

This indicates sending the signal to the process group currently in the foreground on the Unix PTY connected to process. This last option is useful if the running program is a shell, and you wish to signal the program running under the shell, not the shell itself. If process-pty of process is nil, using this option is an error.

This function returns

t

if

process

’s status is either

:running

or

:stopped

.

This function closes all the streams associated with

process

. When you are done using a process, call this to reclaim system resources.

2.15  Saving a Core Image

A mechanism has been provided to save a running Lisp core image and to later restore it. This is convenient if you don’t want to load several files into a Lisp when you first start it up. The main problem is the large size of each saved Lisp image, typically at least 20 megabytes.

The

save-lisp

function saves the state of the currently running Lisp core image in

file

. The keyword arguments have the following meaning:

:purify

If non-nil (the default), the core image is purified before it is saved (see purify.) This reduces the amount of work the garbage collector must do when the resulting core image is being run. Also, if more than one Lisp is running on the same machine, this maximizes the amount of memory that can be shared between the two processes.

:root-structures

This should be a list of the main entry points in any newly loaded systems. This need not be supplied, but locality and/or GC performance will be better if they are. Meaningless if :purify is nil. See purify.

:init-function

This is the function that starts running when the created core file is resumed. The default function simply invokes the top level read-eval-print loop. If the function returns the lisp will exit.

:load-init-file

If non-NIL, then load an init file; either the one specified on the command line or “init.fasl-type”, or, if “init.fasl-type” does not exist, init.lisp from the user’s home directory. If the init file is found, it is loaded into the resumed core file before the read-eval-print loop is entered.

:site-init

If non-NIL, the name of the site init file to quietly load. The default is library:site-init. No error is signalled if the file does not exist.

:print-herald

If non-NIL (the default), then print out the standard Lisp herald when starting.

:process-command-line

If non-NIL (the default), processes the command line switches and performs the appropriate actions.

:batch-mode

If NIL (the default), then the presence of the -batch command-line switch will invoke batch-mode processing upon resuming the saved core. If non-NIL, the produced core will always be in batch-mode, regardless of any command-line switches.

To resume a saved file, type:

lisp -core file

This function optimizes garbage collection by moving all currently live objects into non-collected storage. Once statically allocated, the objects can never be reclaimed, even if all pointers to them are dropped. This function should generally be called after a large system has been loaded and initialized.

:root-structures

is an optional list of objects which should be copied first to maximize locality. This should be a list of the main entry points for the resulting core image. The purification process tries to localize symbols, functions, etc., in the core image so that paging performance is improved. The default value is NIL which means that Lisp objects will still be localized but probably not as optimally as they could be.defstruct

structures defined with the

(:pure t)

option are moved into read-only storage, further reducing GC cost. List and vector slots of pure structures are also moved into read-only storage.

:environment-name

is gratuitous documentation for the compacted version of the current global environment (as seen in c::*info-environment*.) If nil is supplied, then environment compaction is inhibited.

2.16  Pathnames

In Common Lisp quite a few aspects of

semantics are left to the implementation.

2.16.1  Unix Pathnames

Unix pathnames are always parsed with a

object as the host and

as the device. The last two dots (

) in the namestring mark the type and version, however if the first character is a dot, it is considered part of the name. If the last character is a dot, then the pathname has the empty-string as its type. The type defaults to

and the version defaults to

.

(defun parse (x)
  (values (pathname-name x) (pathname-type x) (pathname-version x)))

(parse "foo") ==> "foo", NIL, :NEWEST
(parse "foo.bar") ==> "foo", "bar", :NEWEST
(parse ".foo") ==> ".foo", NIL, :NEWEST
(parse ".foo.bar") ==> ".foo", "bar", :NEWEST
(parse "..") ==> ".", "", :NEWEST
(parse "foo.") ==> "foo", "", :NEWEST
(parse "foo.bar.1") ==> "foo", "bar", 1
(parse "foo.bar.baz") ==> "foo.bar", "baz", :NEWEST

The directory of pathnames beginning with a slash (or a search-list, see section 2.16.4) is starts

, others start with

. The

directory is parsed as

; there is no namestring for

:

(pathname-directory "/usr/foo/bar.baz") ==> (:ABSOLUTE "usr" "foo")
(pathname-directory "../foo/bar.baz") ==> (:RELATIVE :UP "foo")

2.16.2  Wildcard Pathnames

Wildcards are supported in Unix pathnames. If ‘

’ is specified for a part of a pathname, that is parsed as

. ‘

’ can be used as a directory name to indicate

. Filesystem operations treat

the same as

, but pathname pattern matching (e.g. for logical pathname translation, see section 2.16.3) matches any number of directory parts with ‘

’ (see see section 2.17.1.)

‘

’ embedded in a pathname part matches any number of characters. Similarly, ‘

’ matches exactly one character, and ‘

’ matches the characters ‘

’ or ‘

’. These pathname parts are parsed as

objects.

Backslash can be used as an escape character in namestring parsing to prevent the next character from being treated as a wildcard. Note that if typed in a string constant, the backslash must be doubled, since the string reader also uses backslash as a quote:

(pathname-name "foo\\*bar") => "foo*bar"

2.16.3  Logical Pathnames

If a namestring begins with the name of a defined logical pathname host followed by a colon, then it will be parsed as a logical pathname. Both ‘

’ and ‘

’ wildcards are implemented.

on

looks for a logical host definition file in

. Note that

designates the search list (see section 2.16.4) initialized to the CMUCL

directory, not a logical pathname. The format of the file is a single list of two-lists of the from and to patterns:

(("foo;*.text" "/usr/ram/foo/*.txt")
 ("foo;*.lisp" "/usr/ram/foo/*.l"))

2.16.4  Search Lists

Search lists are an extension to Common Lisp pathnames. They serve a function somewhat similar to Common Lisp logical pathnames, but work more like Unix PATH variables. Search lists are used for two purposes:

• They provide a convenient shorthand for commonly used directory names, and
• They allow the abstract (directory structure independent) specification of file locations in program pathname constants (similar to logical pathnames.)

Each search list has an associated list of directories (represented as pathnames with no name or type component.) The namestring for any relative pathname may be prefixed with “

”, indicating that the pathname is relative to the search list

(instead of to the current working directory.) Once qualified with a search list, the pathname is no longer considered to be relative.

When a search list qualified pathname is passed to a file-system operation such as

,

or

, each directory in the search list is successively used as the root of the pathname until the file is located. When a file is written to a search list directory, the file is always written to the first directory in the list.

2.16.5  Predefined Search-Lists

These search-lists are initialized from the Unix environment or when Lisp was built:

default:

The current directory at startup.

home:

The user’s home directory.

library:

The CMUCL lib/ directory (CMUCLLIB environment variable.)

path:

The Unix command path (PATH environment variable.)

target:

The root of the tree where CMUCL was compiled.

It can be useful to redefine these search-lists, for example,

can be augmented to allow logical pathname translations to be located, and

can be redefined to point to where CMUCL system sources are locally installed.

2.16.6  Search-List Operations

These operations define and access search-list definitions. A search-list name may be parsed into a pathname before the search-list is actually defined, but the search-list must be defined before it can actually be used in a filesystem operation.

This function returns the list of directories associated with the search list

name

. If

name

is not a defined search list, then an error is signaled. When set with

setf

, the list of directories is changed to the new value. If the new value is just a namestring or pathname, then it is interpreted as a one-element list. Note that (unlike Unix pathnames), search list names are case-insensitive.

[Function]
extensions:clear-search-list name    

search-list-defined-p

returns

t

if

name

is a defined search list name,

nil

otherwise.

clear-search-list

make the search list

name

undefined.

This macro provides an interface to search list resolution. The body

forms

are executed with

var

bound to each successive possible expansion for

name

. If

name

does not contain a search-list, then the body is executed exactly once. Everything is wrapped in a block named

nil

, so

return

can be used to terminate early. The

result

form (default

nil

) is evaluated to determine the result of the iteration.

2.16.7  Search List Example

The search list

can be defined as follows:

(setf (ext:search-list "code:") ’("/usr/lisp/code/"))

It is now possible to use

as an abbreviation for the directory

in all file operations. For example, you can now specify

to refer to the file

.

To obtain the value of a search-list name, use the function search-list as follows:

(ext:search-list name)

Where

is the name of a search list as described above. For example, calling

on

as follows:

(ext:search-list "code:")

returns the list

.

2.17  Filesystem Operations

CMUCL provides a number of extensions and optional features beyond those required by the Common Lisp specification.

2.17.1  Wildcard Matching

Unix filesystem operations such as

will accept wildcard pathnames that match a single file (of course,

allows any number of matches.) Filesystem operations treat

the same as

.

The keyword arguments to this Common Lisp function are a CMUCL extension. The arguments (all default to

t

) have the following functions:

:all

Include files beginning with dot such as .login, similar to “ls -a”.

:check-for-subdirs

Test whether files are directories, similar to “ls -F”.

:truenamep

Call truename on each file, which expands out all symbolic links. Note that this option can easily result in pathnames being returned which have a different directory from the one in the wildname argument.

:follow-links

Follow symbolic links when searching for matching directories.

Print a directory of

wildname

listing to

stream

(default

*standard-output*

.)

:all

and

:verbose

both default to

nil

and correspond to the “

-a

” and “

-l

” options of

ls

. Normally this function returns

nil

, but if

:return-list

is true, a list of the matched pathnames are returned.

2.17.2  File Name Completion

Attempt to complete a file name to the longest unambiguous prefix. If supplied, directory from

:defaults

is used as the “working directory” when doing completion.

:ignore-types

is a list of strings of the pathname types (a.k.a. extensions) that should be disregarded as possible matches (binary file names, etc.)

Return a list of pathnames for all the possible completions of

pathname

with respect to

defaults

.

2.17.3  Miscellaneous Filesystem Operations

Return the current working directory as a pathname. If set with

setf

, set the working directory.

This function accepts a pathname and returns

t

if the current process can write it, and

nil

otherwise.

This function converts

pathname

into a string that can be used with UNIX system calls. Search-lists and wildcards are expanded.

for-input

controls the treatment of search-lists: when true (the default) and the file exists anywhere on the search-list, then that absolute pathname is returned; otherwise the first element of the search-list is used as the directory.

2.18  Time Parsing and Formatting

Functions are provided to allow parsing strings containing time information and printing time in various formats are available.

parse-time

accepts a string containing a time (e.g., "

Jan 12, 1952

") and returns the universal time if it is successful. If it is unsuccessful and the keyword argument

:error-on-mismatch

is non-

nil

, it signals an error. Otherwise it returns

nil

. The other keyword arguments have the following meaning:

:default-seconds

specifies the default value for the seconds value if one is not provided by time-string. The default value is 0.

:default-minutes

specifies the default value for the minutes value if one is not provided by time-string. The default value is 0.

:default-hours

specifies the default value for the hours value if one is not provided by time-string. The default value is 0.

:default-day

specifies the default value for the day value if one is not provided by time-string. The default value is the current day.

:default-month

specifies the default value for the month value if one is not provided by time-string. The default value is the current month.

:default-year

specifies the default value for the year value if one is not provided by time-string. The default value is the current year.

:default-zone

specifies the default value for the time zone value if one is not provided by time-string. The default value is the current time zone.

:default-weekday

specifies the default value for the day of the week if one is not provided by time-string. The default value is the current day of the week.

Any of the above keywords can be given the value

:current

which means to use the current value as determined by a call to the operating system.

[Function]
extensions:format-decoded-time dest seconds minutes hours day month year
&key :timezone
:style :date-first
:print-seconds :print-meridian
:print-timezone :print-weekday    

format-universal-time

formats the time specified by

universal-time

.

format-decoded-time

formats the time specified by

seconds

,

minutes

,

hours

,

day

,

month

, and

year

.

Dest

is any destination accepted by the

format

function. The keyword arguments have the following meaning:

:timezone

is an integer specifying the hours west of Greenwich. :timezone defaults to the current time zone.

:style

specifies the style to use in formatting the time. The legal values are:

:short

specifies to use a numeric date.

:long

specifies to format months and weekdays as words instead of numbers.

:abbreviated

is similar to long except the words are abbreviated.

:government

is similar to abbreviated, except the date is of the form “day month year” instead of “month day, year”.

:date-first

if non-nil (default) will place the date first. Otherwise, the time is placed first.

:print-seconds

if non-nil (default) will format the seconds as part of the time. Otherwise, the seconds will be omitted.

:print-meridian

if non-nil (default) will format “AM” or “PM” as part of the time. Otherwise, the “AM” or “PM” will be omitted.

:print-timezone

if non-nil (default) will format the time zone as part of the time. Otherwise, the time zone will be omitted.

:print-weekday

if non-nil (default) will format the weekday as part of date. Otherwise, the weekday will be omitted.

2.19  Random Number Generation

Common Lisp includes a random number generator as a standard part of the language; however, the implementation of the generator is not specified. Two random number generators are available in CMUCL, depending on the version.

2.19.1  Original Generator

The default random number generator uses a lagged Fibonacci generator given by

where z[i] is the i’th random number. This generator produces small integer-valued numbers. For larger integer, the small random integers are concatenated to produce larger integers. For floating-point numbers, the bits from this generator are used as the bits of the floating-point significand.

2.19.2  New Generator

In some versions of CMUCL, the original generator above has been replaced with a subtract-with-borrow generator combined with a Weyl generator.¹ The reason for the change was to use a documented generator which has passed tests for randomness.

The subtract-with-borrow generator is described by the following equation

where z[i] is the i’th random number, which is a

. All of the indices in this equation are interpreted modulo 32. The quantity b is carried over from the previous iteration and is either 0 or

. If z[i] is positive, b is set to zero. Otherwise, b is set to

.

To increase the randomness of this generator, this generator is combined with a Weyl generator defined by

where y = 7097293079245107 × 2⁻⁵³. Thus, the resulting random number r[i] is

This generator has been tested by Peter VanEynde using Marsaglia’s diehard test suite for random number generators; this generator passes the test suite.

This generator is designed for generating floating-point random numbers. To obtain integers, the bits from the significand of the floating-point number are used as the bits of the integer. As many floating-point numbers as needed are generated to obtain the desired number of bits in the random integer.

For floating-point numbers, this generator can by significantly faster than the original generator.

2.19.3  MT-19987 Generator

On all platforms, this is the preferred generator as indicated by

being in

. This is a Lisp implementation of the MT-19987 generator of Makoto Matsumoto and T. Nishimura. We refer the reader to their paper² or to their website.

2.20  Lisp Threads

CMUCL supports Lisp threads for the x86 platform.

2.21  Lisp Library

The CMUCL project maintains a collection of useful or interesting programs written by users of our system. The library is in

. Two files there that users should read are:

CATALOG.TXT

This file contains a page for each entry in the library. It contains information such as the author, portability or dependency issues, how to load the entry, etc.

READ-ME.TXT

This file describes the library’s organization and all the possible pieces of information an entry’s catalog description could contain.

Hemlock has a command

that displays a list of the current library entries in an editor buffer. There are mode specific commands that display catalog descriptions and load entries. This is a simple and convenient way to browse the library.

2.22  Generalized Function Names

Define lists starting with the symbol name as a new extended function name syntax.body

is executed with

var

bound to an actual function name of that form, and should return two values:

• A generalized boolean that is true if var is a valid function name.
• A symbol that can be used as a block name in functions whose name is var. (For some sorts of function names it might make sense to return nil for the block name, or just return one value.)

Users should not define function names starting with a symbol that CMUCL might be using internally. It is therefore advisable to only define new function names starting with a symbol from a user-defined package.

Returns two values:

• True if name is a valid function name.
• A symbol that can be used as a block name in functions whose name is name. This can be nil for some function names.

2.23  CLOS

2.23.1  Primary Method Errors

The standard requires that an error is signaled when a generic function is called and

• no primary method is applicable to the generic function’s actual arguments, and
• the generic function’s method combination is either the standard method combination or a method combination defined with the short form of define-method-combination. The latter includes the standardized method combinations like progn, and, etc.

In CMUCL, this generic function is called in the above erroneous cases. The parameter

is the generic function being called, and

is a list of actual arguments in the generic function call.

This method signals a continuable error of type

.

2.23.2  Slot Type Checking

Declared slot types are used when

• reading slot values with slot-value in methods, or
• setting slots with (setf slot-value) in methods, or
• creating instances with make-instance, when slots are initialized from initforms. This currently depends on PCL being able to use its internal make-instance optimization, which it usually can.

Example:

(defclass foo ()
  ((a :type fixnum)))

(defmethod bar ((object foo) value)
  (with-slots (a) object
    (setf a value)))

(defmethod baz ((object foo))
  (< (slot-value object ’a) 10))

In method

, and with a suitable safety setting, a type error will occur if

is not a

. In method

, a

comparison can be used by the compiler.

Slot type checking can be turned off by setting this variable to nil, which can be useful for compiling code containing incorrect slot type declarations.

2.23.3  Slot Access Optimization

The declaration

is used for optimizing slot access in methods.

declare (ext:slots specifier*)

specifier   ::= (quality class-entry*)
quality     ::= SLOT-BOUNDP | INLINE
class-entry ::= class | (class slot-name*)
class       ::= the name of a class
slot-name   ::= the name of a slot

The

quality specifies that all or some slots of a class are always bound.

The

quality specifies that access to all or some slots of a class should be inlined, using compile-time knowledge of class layouts.

2.23.3.1  slot-boundp Declaration

Example:

(defclass foo ()
  (a b))

(defmethod bar ((x foo))
  (declare (ext:slots (slot-boundp foo)))
  (list (slot-value x ’a) (slot-value x ’b)))

The

declaration in method

specifies that the slots

and

accessed through parameter

in the scope of the declaration are always bound, because parameter

is specialized on class

to which the

declaration applies. The PCL-generated code for the

forms will thus not contain tests for the slots being bound or not. The consequences are undefined should one of the accessed slots not be bound.

2.23.3.2  inline Declaration

Example:

(defclass foo ()
  (a b))

(defmethod bar ((x foo))
  (declare (ext:slots (inline (foo a))))
  (list (slot-value x ’a) (slot-value x ’b)))

The

declaration in method

tells PCL to use compile-time knowledge of slot locations for accessing slot

of class

, in the scope of the declaration.

Class

must be known at compile time for this optimization to be possible. PCL prints a warning and uses normal slot access If the class is not defined at compile time.

If a class is

ed to use inline slot access before it is defined, the class is defined at compile time. Example:

(declaim (ext:slots (inline (foo slot-a))))
(defclass foo () ...)
(defclass bar (foo) ...)

Class

will be defined at compile time because it is declared to use inline slot access; methods accessing slot

of

will use inline slot access if otherwise possible. Class

will be defined at compile time because its superclass

is declared to use inline slot access. PCL uses compile-time information from subclasses to warn about situations where using inline slot access is not possible.

Normal slot access will be used if PCL finds, at method compilation time, that

• class foo has a subclass in which slot a is at a different location, or
• there exists a slot-value-using-class method for foo or a subclass of foo.

When the declaration is used to optimize calls to slot accessor generic functions in methods, as opposed to

or

, the optimization is additionally not used if

• there exist, at compile time, applicable methods on the reader/writer generic function that are not standard accessor methods (for instance, there exist around-methods), or
• applicable reader/writer methods access different slots in a class accessed inline, and one of its subclasses.

The consequences are undefined if the compile-time environment is not the same as the run-time environment in these respects, or if the definition of class

or any subclass of

is changed in an incompatible way, that is, if slot locations change.

The effect of the

optimization combined with the

optimization is that CLOS slot access becomes as fast as structure slot access, which is an order of magnitude faster than normal CLOS slot access.

This variable controls if inline slot access optimizations are performed. It is true by default.

2.23.3.3  Automatic Method Recompilation

Methods using inline slot access can be automatically recompiled after class changes. Two declarations control which methods are automatically recompiled.

declaim (ext:auto-compile specifier*)
declaim (ext:not-auto-compile specifier*)

specifier   ::= gf-name | (gf-name qualifier* (specializer*))
gf-name     ::= the name of a generic function
qualifier   ::= a method qualifier
specializer ::= a method specializer

If no specifier is given, auto-compilation is by default done/not done for all methods of all generic functions using inline slot access; current default is that it is not done. This global policy can be overridden on a generic function and method basis. If

is a generic function name, it applies to all methods of that generic function.

Examples:

(declaim (ext:auto-compile foo))
(defmethod foo :around ((x bar)) ...)

The around-method

will be automatically recompiled because the declamation applies to all methods with name

.

(declaim (ext:auto-compile (foo (bar))))
(defmethod foo :around ((x bar)) ...)
(defmethod foo ((x bar)) ...)

The around-method will not be automatically recompiled, but the primary method will.

(declaim (ext:auto-compile foo))
(declaim (ext:not-auto-compile (foo :around (bar)))  
(defmethod foo :around ((x bar)) ...)
(defmethod foo ((x bar)) ...)

The around-method will not be automatically recompiled, because it is explicitly declaimed not to be. The primary method will be automatically recompiled because the first declamation applies to it.

Auto-recompilation works by recording method bodies using inline slot access. When PCL determines that a recompilation is necessary, a

form is constructed and evaluated.

Auto-compilation can only be done for methods defined in a null lexical environment. PCL prints a warning and doesn’t record the method body if a method using inline slot access is defined in a non-null lexical environment. Instead of doing a recompilation on itself, PCL will then print a warning that the method must be recompiled manually when classes are changed.

2.23.4  Inlining Methods in Effective Methods

When a generic function is called, an effective method is constructed from applicable methods. The effective method is called with the original arguments, and itself calls applicable methods according to the generic function’s method combination. Some of the function call overhead in effective methods can be removed by inlining methods in effective methods, at the expense of increased code size.

Inlining of methods is controlled by the usual

declaration. In the following example, both

methods shown will be inlined in effective methods:

(declaim (inline (method foo (foo))
                 (method foo :before (foo))))
(defmethod foo ((x foo)) ...)
(defmethod foo :before ((x foo)) ...)

Please note that this form of inlining has no noticeable effect for effective methods that consist of a primary method only, which doesn’t have keyword arguments. In such cases, PCL uses the primary method directly for the effective method.

When the definition of an inlined method is changed, effective methods are not automatically updated to reflect the change. This is just as it is when inlining normal functions. Different from the normal case is that users do not have direct access to effective methods, as it would be the case when a function is inlined somewhere else. Because of this, the function

is provided for forcing such an update of effective methods.

Flush cached effective method functions. If gf is supplied, it should be a generic function metaobject or the name of a generic function, and this function flushes all cached effective methods for the given generic function. If gf is not supplied, all cached effective methods are flushed.

If true, the default, perform method inlining as described above. If false, don’t.

2.23.5  Effective Method Precomputation

When a generic function is called, the generic function’s discriminating function computes the set of methods applicable to actual arguments and constructs an effective method function from applicable methods, using the generic function’s method combination.

Effective methods can be precomputed at method load time instead of when the generic function is called depending on the value of

.

If nonzero, the default value is 100, precompute effective methods when methods are loaded, and the method’s generic function has less than the specified number of methods.

If zero, compute effective methods only when the generic function is called.

2.23.6  Sealing

Support for sealing classes and generic functions have been implemented. Please note that this interface is subject to change.

Seal name with respect to the given specifiers; name can be the name of a class or generic-function.

Supported specifiers are

:subclasses

for classes, which prevents changing subclasses of a class, and

:methods

which prevents changing the methods of a generic function.

Sealing violations signal an error of type

pcl:sealed-error

.

Remove seals from name-or-object.

2.23.7  Method Tracing and Profiling

Methods can be traced with

, using function names of the form

. Example:

(defmethod foo ((x integer)) x)
(defmethod foo :before ((x integer)) x)

(trace (method foo (integer)))
(trace (method foo :before (integer)))
(untrace (method foo :before (integer)))

and

also allow a name specifier

for tracing all methods of a generic function:

(trace :methods ’foo)
(untrace :methods ’foo)

Methods can also be specified for the

option to

. Because this option is a name or a list of names, methods must be specified as a list. Thus, to trace all calls of

from the method

specialized on integer argument, use

  (trace foo :wherein ((method bar (integer))))

Before and after methods are supported as well:

  (trace foo :wherein ((method bar :before (integer))))

Method profiling is done analogously to

:

(defmethod foo ((x integer)) x)
(defmethod foo :before ((x integer)) x)

(profile:profile (method foo (integer)))
(profile:profile (method foo :before (integer)))
(profile:unprofile (method foo :before (integer)))

(profile:profile :methods ’foo)
(profile:unprofile :methods ’foo)

(profile:profile-all :methods t)

2.23.8  Misc

This variable controls compilation of interpreted method functions, e.g. for methods defined interactively at the REPL. Default is true, that is, method functions are compiled.

2.24  Differences from ANSI Common Lisp

This section describes some of the known differences between CMUCL and ANSI Common Lisp. Some may be non-compliance issues; same may be extensions.

2.24.1  Extensions

As an extension, CMUCL allows constantly to accept more than one value which are returned as multiple values.

2.25  Function Wrappers

Function wrappers, fwrappers for short, are a facility for efficiently encapsulating functions.

Functions in CMUCL are represented by

objects. Each

object contains a reference to its function’s actual code, which we call the function’s primary function.

A function wrapper replaces the primary function in the

object with a function of its own, and records the original function in an fwrapper object, a funcallable instance. Thus, when the function is called, the fwrapper gets called, which in turn might call the primary function, or a previously installed fwrapper that was found in the

object when the second fwrapper was installed.

Example:

(use-package :fwrappers)

(define-fwrapper foo (x y)
  (format t "x = ~s, y = ~s, user-data = ~s~%"
          x y (fwrapper-user-data fwrapper))
  (let ((value (call-next-function)))
    (format t "value = ~s~%" value)
    value))

(defun bar (x y)
  (+ x y))

(fwrap ’bar #’foo :type ’foo :user-data 42)

(bar 1 2)
 =>
 x = 1, y = 2, user-data = 42
 value = 3
 3   

Fwrappers are used in the implementation of

and

.

Please note that

always returns the primary definition of a function; if a function is fwrapped,

returns the primary function stored in the innermost fwrapper object. Likewise, if a function is fwrapped,

will set the primary function in the innermost fwrapper.

This macro is like defun, but defines a function named name that can be used as an fwrapper definition.

In

body

, the symbol

fwrapper

is bound to the current fwrapper object.

The macro

call-next-function

can be used to invoke the next fwrapper, or the primary function that is being fwrapped. When called with no arguments,

call-next-function

invokes the next function with the original arguments passed to the fwrapper, unless you modify one of the parameters. When called with arguments,

call-next-function

invokes the next function with the given arguments.

This function wraps function function-name in an fwrapper fwrapper which was defined with define-fwrapper.

The value of

type

, if supplied, is used as an identifying tag that can be used in various other operations.

The value of

user-data

is stored as user-supplied data in the fwrapper object that is created for the function encapsulation. User-data is accessible in the body of fwrappers defined with

define-fwrapper

as

(fwrapper-user-data fwrapper)

.

Value is the fwrapper object created.

Remove fwrappers from the function named function-name. If type is supplied, remove fwrappers whose type is equal to type. If test is supplied, remove fwrappers satisfying test.

Find an fwrapper of function-name. If type is supplied, find an fwrapper whose type is equal to type. If test is supplied, find an fwrapper satisfying test.

Update the funcallable instance function of the fwrapper object fwrapper from the definition of its function that was defined with define-fwrapper. This can be used to update fwrappers after changing a define-fwrapper.

Update fwrappers of function-name; see update-fwrapper. If type is supplied, update fwrappers whose type is equal to type. If test is supplied, update fwrappers satisfying test.

Set function-names’s fwrappers to elements of the list fwrappers, which is assumed to be ordered from outermost to innermost. fwrappers null means remove all fwrappers.

Return a list of all fwrappers of function-name, ordered from outermost to innermost.

Prepend fwrapper fwrapper to the definition of function-name. Signal an error if function-name is an undefined function.

Remove fwrapper fwrapper from the definition of function-name. Signal an error if function-name is an undefined function.

Evaluate body with var bound to consecutive fwrappers of fdefn. Return result at the end. Note that fdefn must be an fdefn object. You can use kernel:fdefn-or-lose, for instance, to get the fdefn object from a function name.

2.26  Dynamic-Extent Declarations

Note: As of the 19a release,

is unfortunately disabled by default. It is known to cause some issues with CLX and Hemlock. The cause is not known, but causes random errors and brokeness. Enable at your own risk. However, it is safe enough to build all of CMUCL without problems.

On x86 and sparc, CMUCL can exploit

declarations by allocating objects on the stack instead of the heap.

You can tell CMUCL to trust or not trust

declarations by setting the variable

.

If the value of *trust-dynamic-extent-declarations* is NIL, dynamic-extent declarations are effectively ignored.

If the value of this variable is a function, the function is called with four arguments to determine if a

dynamic-extent

declaration should be trusted. The arguments are the safety, space, speed, and debug settings at the point where the

dynamic-extent

declaration is used. If the function returns true, the declaration is trusted, otherwise it is not trusted.

In all other cases,

dynamic-extent

declarations are trusted.

Please note that stack-allocation is inherently unsafe. If you make a mistake, and a stack-allocated object or part of it escapes, CMUCL is likely to crash, or format your hard disk.

2.26.1  &rest argument lists

Rest argument lists can be allocated on the stack by declaring the rest argument variable

. Examples:

(defun foo (x &rest rest)
  (declare (dynamic-extent rest))
  ...)

(defun bar ()
  (lambda (&rest rest)
    (declare (dynamic-extent rest))
    ...))

2.26.2  Closures

Closures for local functions can be allocated on the stack if the local function is declared

, and the closure appears as an argument in the call of a named function. In the example:

(defun foo (x)
  (flet ((bar () x))
    (declare (dynamic-extent #’bar))
    (baz #’bar)))

the closure passed to function

is allocated on the stack. Likewise in the example:

(defun foo (x)
  (flet ((bar () x))
    (baz #’bar)
    (locally (declare (dynamic-extent #’bar))
      (baz #’bar))))

Stack-allocation of closures can also automatically take place when calling certain known CL functions taking function arguments, for example

or

.

2.26.3  list, list*, and cons

New conses allocated by

,

, or

which are used to initialize variables can be allocated from the stack if the variables are declared

. In the case of

, only the outermost cons cell is allocated from the stack; this is an arbitrary restriction.

(let ((x (list 1 2))
      (y (list* 1 2 x))
      (z (cons 1 (cons 2 nil))))
  (declare (dynamic-extent x y z))
  ...
  (setq x (list 2 3))
  ...)

Please note that the

of

in the example program assigns to

a list that is allocated from the heap. This is another arbitrary restriction that exists because other Lisps behave that way.

2.27  Modular Arithmetic

This section is mostly taken, with permission, from the documentation for SBCL.

Some numeric functions have a property:

lower bits of the result depend only on

lower bits of (all or some) arguments. If the compiler sees an expression of form

, where

is a tree of such “good” functions and

is known to be of type

, where

is a "good" width, all intermediate results will be cut to

bits (but it is not done for variables and constants!). This often results in an ability to use simple machine instructions for the functions.

Consider an example.

(defun i (x y)
  (declare (type (unsigned-byte 32) x y))
  (ldb (byte 32 0) (logxor x (lognot y))))

The result of

will be negative and of type

, so a naive implementation on a 32-bit platform is unable to use 32-bit arithmetic here. But modular arithmetic optimizer is able to do it: because the result is cut down to 32 bits, the compiler will replace

and

with versions cutting results to 32 bits, and because terminals (here—expressions

and

) are also of type

, 32-bit machine arithmetic can be used.

Currently “good” functions are

,

,

;

,

,

,

and their combinations; and

with the positive second argument. “Good” widths are 32 on HPPA, MIPS, PPC, Sparc and X86 and 64 on Alpha. While it is possible to support smaller widths as well, currently it is not implemented.

A more extensive description of modular arithmetic can be found in the paper “Efficient Hardware Arithmetic in Common Lisp” by Alexey Dejneka, and Christophe Rhodes, to be published.

2.28  Extension to REQUIRE

The behavior of

when called with only one argument is implementation-defined. In CMUCL, functions from the list

are called in order with the stringified module name as the argument. The first function to return non-

is assumed to have loaded the module.

By default the functions

and

are on this list of functions, in that order.

This is a list of functions taking a single argument. require calls each function in turn with the stringified module name. The first function to return non-NIL indicates that the module has been loaded. The remaining functions, if any, are not called.

To add new providers, push the new provider function onto the beginning of this list.

Defines a module by registering the files that need to be loaded when the module is required. If name is a symbol, its print name is used after downcasing it.

This function is the module-provider for modules registered by a ext:defmodule form.

This function is the module-provider for CMUCL’s libraries, including Gray streams, simple streams, CLX, CLM, Hemlock, etc.

This function causes a file to be loaded whose name is formed by merging the search-list “modules:” and the concatenation of module-name with the suffix “-LIBRARY”. Note that both the module-name and the suffix are each, separately, converted from :case :common to :case :local. This merged name will be probed with both a .lisp and .fasl extensions, calling

LOAD

if it exists.

1

The generator described here is available if the feature :new-random is available.

2

“Mersenne Twister: A 623-Dimensionally Equidistributed Uniform Pseudorandom Number Generator,” ACM Trans. on Modeling and Computer Simulation, Vol. 8, No. 1, January 1998, pp.3–30