PLT MzScheme: Language Manual

Chapter 7

Threads

MzScheme supports multiple threads of control within a program. Threads are implemented for all operating systems, even when the operating system does not provide primitive thread support.

(thread thunk) invokes the procedure thunk with no arguments in a new thread of control. The thread procedure returns immediately with a thread descriptor value. When the invocation of thunk returns, the thread created to invoke thunk terminates.

Example:

(thread (lambda () (sleep 2) (display 7) (newline))) ; => a thread descriptor  
 displays 7 after two seconds pass

Each thread has its own parameter settings (see section 7.4), such as the current directory or current exception handler. A newly-created thread inherits the parameter settings of the creating thread, except

the error-escape-handler parameter, which is initialized to the default error escape handler; and
the current-exception-handler parameter, which is initialized to the value of initial-exception-handler.

When a thread is created, it is placed into the management of the current custodian (See section 9.2). A thread that has not terminated can be ``garbage collected'' only if it is unreachable and blocked on an unreachable semaphore.⁸

7.1 Thread Utilities

(current-thread) returns the thread descriptor for the currently executing thread.

(thread? v) returns #t if v is a thread descriptor, #f otherwise.

(sleep [x]) causes the current thread to sleep for at least x seconds, where x is a non-negative real number. The x argument defaults to 0 (allowing other threads to execute when operating system threads are not used). The value of x can be non-integral to request a sleep duration to any precision, but the precision of the actual sleep time is unspecified.

(thread-running? thread) returns #t if thread has not terminated, #f otherwise.

(thread-wait thread) blocks execution of the current thread until thread has terminated. Note that (thread-wait (current-thread)) deadlocks the current thread, but a break can end the deadlock (if breaking is enabled; see section 6.6).

(kill-thread thread) terminates the specified thread immediately. Terminating the main thread exits the application. If thread is already not running, kill-thread does nothing. Otherwise, if the current custodian (see section 9.2) does not manage thread (and none of its subordinates manages thread), the exn:misc exception is raised.

All of the MzScheme (and MrEd) primitives are kill-safe; that is, killing a thread never interferes with the application of primitives in other threads. For example, if a thread is killed while extracting a character from an input port, the character is either completely consumed or not consumed, and other threads can safely use the port.

(break-thread thread) registers a break with the specified thread. If breaking is disabled in thread, the break will be ignored until breaks are re-enabled (see section 6.6).

(call-in-nested-thread thunk [custodian]) creates a nested thread managed by custodian to execute thunk.⁹ The current thread blocks until thunk returns, and the result of the call-in-nested-thread call is the result returned by thunk. The default value of custodian is the current custodian.

The nested thread's exception handler is initialized to a procedure that jumps to the beginning of the thread and transfers the exception to the original thread. The handler thus terminates the nested thread and re-raises the exception in the original thread.

If the thread created by call-in-nested-thread dies before thunk returns, the exn:thread exception is raised in the original thread. If the original thread is killed before thunk returns, a break is queued for the nested thread.

If a break is queued for the original thread (with break-thread) while the nested thread is running, the break is redirected to the nested thread. If a break is already queued on the original thread when the nested thread is created, the break is moved to the nested thread. If a break remains queued on the nested thread when it completes, the break is moved to the original thread.

7.2 Semaphores

A semaphore is a value that is used to synchronize MzScheme threads. Each semaphore has an internal counter; when this counter is zero, the semaphore can block a thread's execution (through semaphore-wait) until another thread increments the counter (using semaphore-post). The maximum value for a semaphore's internal counter is platform-specific, but always at least 10000. A semaphore's counter is updated in a single-threaded manner, of course, so tat semaphore can be used for reliable synchronization.

(make-semaphore [init-k]) creates and returns a new semaphore with the counter initially set to init-k, which defaults to 0. If init-k is larger than a semaphore's maximum internal counter value, the exn:application:mismatch exception is raised.
(semaphore? v) returns #t if v is a semaphore created by make-semaphore, #f otherwise.
(semaphore-post sema) increments the semaphore's internal counter and returns void. If the semaphore's internal counter has already reached its maximum value, the exn:misc exception is raised.
(semaphore-wait sema) blocks until the internal counter for semaphore sema is non-zero. When the counter is non-zero, it is decremented and semaphore-wait returns void.
(semaphore-try-wait? sema) is like semaphore-wait, but semaphore-try-wait? never blocks execution. If sema's internal counter is zero, semaphore-try-wait? returns #f immediately without decrementing the counter. If sema's counter is positive, it is decremented and #t is returned.
(semaphore-wait/enable-break sema) is like semaphore-wait, but breaking is enabled (see section 6.6) while waiting on sema. If breaking is disabled when semaphore-wait/enable-break is called, then either the semaphore's counter is decremented or the exn:break exception is raised, but not both.

See also object-wait-multiple in section 7.3.

7.3 Synchronizing Multiple Objects with Timeout

(object-wait-multiple timeout v ···¹) blocks as long as all of the vs are in a blocking state, as defined below, or until timeout seconds have passed. The timeout argument can be a real number or #f; if timeout is #f, then object-wait-multiple does not return until some v is unblocked.

When at least one v is unblocked, it is returned as the result. If multiple vs are unblocked before the call to object-wait-multiple, the returned v is the earliest unblocked v according to argument order. If multiple vs become unblocked later, any one of the unblocked vs can be returned (but only one).

If the returned v is a semaphore, then the semaphore's internal count is decremented, just as with semaphore-wait. Otherwise, the returned v is unmodified. Any v that is not returned by object-wait-multiple is unmodified.

If object-wait-multiple returns because timeout seconds have passed, the return value is #f. Each v is checked at least once, so a timeout value of 0 can be used for polling.

Only certain kinds of values, listed below, are waitable in stand-alone MzScheme. If any other kind of value is provided to object-wait-multiple, the exn:application:mismatch exception is raised. An extension or embedding application can extend the set of waitable values. In particular, an eventspace in MrEd is waitable.

semaphore -- a semaphore is in a blocking state when semaphore-wait (see section 7.2) would block.
input-port -- an input port is in a blocking state when read-char would block.
output-port -- an output port is in a blocking state if write-string-avail would block (see section 11.2.2), unless the port contains buffered characters and write-string-avail* can flush part of the buffer (although write-string-avail might block).
tcp-listener -- a TCP listener is in a blocking state when tcp-accept (see section 11.4) would block.
thread -- a thread is in a blocking state when thread-wait (see section 7.1) would block.
subprocess -- a subprocess is in a blocking state when subprocess-wait (see section 15.2) would block.
will-executor -- a will executor is in a blocking state when will-execute (see section 13.2) would block.

(object-wait-multiple/enable-break v ···¹) is like object-wait-multiple, but breaking is enabled (see section 6.6) while waiting on the vs. If breaking is disabled when object-wait-multiple/enable-break is called, then either all vs remain unmodified or the exn:break exception is raised, but not both.

(object-waitable? v) returns #t if v is a waitable object, #f otherwise. See object-wait-multiple, above, for the list of waitable object types.

7.4 Parameters

A parameter is a thread-specific setting, such a the current output port or the current directory for resolving relative pathnames. A parameter procedure sets and retrieves the value of a specific parameter. For example, the current-output-port parameter procedure sets and retrieves a port value that is used by display when a specific output port is not provided. Applying a parameter procedure without an argument obtains the current value of a parameter in the current thread, and applying a parameter procedure to a single argument sets the parameter's value in the current thread (and returns void). For example, (current-output-port) returns the current default output port, while (current-output-port p) sets the default output port to p.

7.4.1 Built-in Parameters

MzScheme's built-in parameter procedures are listed in the following sections. The make-parameter procedure, described in section 7.4.2, creates a new parameter and returns a corresponding parameter procedure.

7.4.1.1 Current Directory

(current-directory [path]) gets or sets a string path that determines the current directory. When the parameter procedure is called to set the current directory, the path argument is expanded and then simplified using simplify-path (see section 11.3.1) and converted to an immutable string; expansion and simplification raise an exception if the path is ill-formed. Otherwise, if the given path cannot be made the current directory (e.g., because the path does not exist), the exn:i/o:filesystem exception is raised.

7.4.1.2 Ports

(current-input-port [input-port]) gets or sets an input port used by read, read-char, char-ready?, read-line, read-string, and read-string-avail! when a specific input port is not provided.
(current-output-port [output-port]) gets or sets an output port used by display, write, print, write-char, and printf when a specific output port is not provided.
(current-error-port [output-port]) gets or sets an output port used by the default error display handler.
(global-port-print-handler [proc]) gets or sets a procedure that takes an arbitrary value and an output port. This global port print handler is called by the default port print handler (see section 11.2.5) to print values into a port.
(port-count-lines-enabled [on?]) gets or sets a boolean value that determines whether line counting is enabled automatically for newly created input ports; see also section 11.2.3. The default value is #f.

7.4.1.3 Parsing

(read-case-sensitive [on?]) gets or sets a boolean value that controls parsing input symbols. When this parameter's value is #f, the reader downcases symbols (e.g., hi when the input is any one of hi, Hi, HI, or hI). The parameter also affects the way that write prints symbols containing uppercase characters; if the parameter's value is #f, then symbols are printed with uppercase characters quoted by a backslash (\) or vertical bar (|). The parameter's value is overridden by backslash and vertical-bar quotes and the #cs and #ci prefixes; see section 14.3 for more information. While a module is loaded, the parameter is set to #f (see section 5.8).
(read-square-bracket-as-paren [on?]) gets or sets a boolean value that controls whether square brackets (``['' and ``]'') are treated as parentheses. See section 14.3 for more information.
(read-curly-brace-as-paren [on?]) gets or sets a boolean value that controls whether curly braces (``{'' and ``}'') are treated as parentheses. See section 14.3 for more information.
(read-accept-box [on?]) gets or sets a boolean value that controls parsing #\& input. See section 14.3 for more information.
(read-accept-compiled [on?]) gets or sets a boolean value that controls parsing pre-compiled input. See section 14.3 for more information.
(read-accept-bar-quote [on?]) gets or sets a boolean value that controls parsing and printing a vertical bar (|) in symbols. See section 14.3 and section 14.4 for more information.
(read-accept-graph [on?]) gets or sets a boolean value that controls parsing input S-expressions with sharing. See section 14.5 for more information.
(read-decimal-as-inexact [on?]) gets or sets a boolean value that controls parsing input numbers with a decimal point or exponent (but no explicit exactness tag). See section 14.5 for more information.
(read-accept-dot [on?]) gets or sets a boolean value that controls parsing input with a dot, which is normally used for literal cons cells. See section 14.3 for more information.
(read-accept-quasiquote [on?]) gets or sets a boolean value that controls parsing input with a backquote or comma, which is normally used for quasiquote, unquote, and unquote-splicing abbreviations. See section 14.3 for more information.

7.4.1.4 Printing

(print-graph [on?]) gets or sets a boolean value that controls printing S-expressions with sharing. See section 14.5 for more information.
(print-struct [on?]) gets or sets a boolean value that controls printing structure values. See section 14.4 for more information.
(print-box [on?]) gets or sets a boolean value that controls printing box values. See section 14.4 for more information.
(print-vector-length [on?]) gets or sets a boolean value that controls printing vectors. See section 14.4 for more information.

7.4.1.5 Read-Eval-Print

(current-prompt-read [proc]) gets or sets a procedure that takes no arguments, displays a prompt string, and returns an expression to evaluate. This prompt read handler is called by the read phase of read-eval-print-loop (see section 14.1). The default prompt read handler prints ``> '' and returns the result of (read-syntax name-string), where name-string corresponds to the current input source.
(current-eval [proc]) gets or sets a procedure that takes a syntax object or S-expression and returns its value (or values; see section 2.2). This evaluation handler is called by eval, the default load handler, and read-eval-print-loop to evaluate an expression (see section 14.1). The default evaluation handler compiles and executes the expression in the current namespace (determined by the current-namespace parameter).
(current-namespace [namespace]) gets or sets a namespace value (see section 8) that determines the global variable namespace used to resolve variable references. The current namespace is used by the default evaluation handler, the compile procedure, and other built-in procedures that operate on global variables.
(current-print [proc]) gets or sets a procedure that takes a value to print. This print handler is called by read-eval-print-loop (see section 14.1) to print the result of an evaluation (and the result is ignored). The default print handler prints the value to the current output port (determined by the current-output-port parameter) and then outputs a newline.
(compile-allow-set!-undefined [on?]) gets or sets a boolean value indicating how to compile a set! expression that mutates a global variable. If the value of this parameter is a true value, set! expressions for global variables are compiled so that the global variable is set even if it was not previously defined. Otherwise, set! expressions for global variables are compiled to raise the exn:variable exception if the global variable is not defined at the time the set! is performed. Note that this parameter is used when an expression is compiled, not when it is evaluated.

7.4.1.6 Loading

(current-load [proc]) gets or sets a procedure that loads a file and returns the value (or values; see section 2.2) of the last expression read from the file. This load handler is called by load, load-relative, load/use-compiled, and load/cd.

A load handler procedure takes two arguments: a file path string and an expected module name. The expected module name is either a symbol or #f; see section 5.8 for further information.

The default load handler reads expressions from the file (with compiled expressions enabled and line-counting enabled) and passes each expression to the current evaluation handler. The default load handler also treats a hash mark on the first line of the file as a comment (see section 14.3). The current load directory for loading the file is set before the load handler is called (see section 14.1).
(current-load-extension [proc]) gets or sets a procedure loads a dynamic extension (see section 14.7) and returns the extension's value(s). This load extension handler is called by load-extension, load-relative, and load/use-compiled.

A load extension handler procedure takes two arguments: a file path string and an expected module name. The expected module name is either a symbol or #f; see section 5.8 for further information.

The default load extension handler loads an extension using operating system primitives.
(current-load/use-compiled [proc]) gets or sets a procedure that loads a file or a compiled version of the file; see section 14.1 for more information. A load/use-compiled handler procedure takes the same arguments as a load handler. The handler is expected to call the load handler or the load-extension handler. Unlike a load handler or load-extension handler, a load/use-compiled handler is expected to set the current load-relative directory.
(current-load-relative-directory [path]) gets or sets a complete directory pathname or #f. This current load-relative directory is set by load, load-relative, load/use-compiled, load/cd, load-extension, and load-relative-extension to the directory of the file being loaded. This parameter is used by load-relative, load/use-compiled and load-relative-extension (see section 14.1). When a new pathname is provided to the parameter procedure current-load-relative-directory, it is immediately expanded (see section 11.3.1) and converted to an immutable string; the result must be a complete pathname for an existing directory.
(use-compiled-file-kinds [kind-symbol]) gets or sets a symbol, either 'all or 'none, indicating whether load/used-compiled (and thus require) should recognize compiled files. If the value of this parameter is 'all, then the default handler for load/use-compiled recognizes compiled files as described in section 14.1. If the value is 'none, then the default handler for load/use-compiled ignores compiled files.
(current-library-collection-paths [path-list]) gets or sets a list of complete directory pathnames for library collections used by require. See Chapter 16 for more information. When a new list of pathnames is provided to the parameter procedure, it is converted to an immutable list of immutable strings.
(current-command-line-arguments [string-vector]) gets or sets a vector of strings representing command-line arguments. The stand-alone version of MzScheme (and MrEd) initializes the parameter to contain extra command-line arguments. (The same vector is also installed as the value of the argv global.)

7.4.1.7 Exceptions

(current-exception-handler [proc]) gets or sets a procedure that is invoked to handle an exception. See section 6.1 for more information about exceptions.
(initial-exception-handler [proc]) gets or sets a procedure that is used as the initial current exception handler for a new thread.
(error-escape-handler [proc]) gets or sets a procedure that takes no arguments and escapes from the dynamic context of an exception. The default error escape handler escapes to the start of the current thread, but read-eval-print-loop (see section 14.1) also sets the escape handler. To report a run-time error, use raise (see section 6.1) or error (see section 6.2) instead of calling the error escape procedure directly. If an exception is raised while the error escape handler is executing, an error message is printed using a primitive error printer and the primitive error escape handler is invoked. Unlike all other parameters, the value of the error-escape-handler parameter in a new thread is not inherited from the creating thread; instead, the parameter is always initialized to the default error escape handler.
(error-display-handler [proc]) gets or sets a procedure that takes two arguments: a string to print as an error message, and a value representing a raised exception. This error display handler is called by the default exception handler with an error message and the exception value. The default error display handler displays its first argument to the current error port (determined by the current-error-port parameter), and ignores the second argument. To report a run-time error, use raise (see section 6.1) or error (see section 6.2) instead of calling the error display procedure directly. If an exception is raised while the error display handler is executing, an error message is printed using a primitive error printer and the primitive error escape handler is invoked.
(error-print-width [k]) gets or sets an integer greater than 3. This value is used as the maximum number of characters used to print a Scheme value that is embedded in a primitive error message.
(error-value->string-handler [proc]) gets or sets a procedure that takes an arbitrary Scheme value and an integer and returns a string. This error value conversion handler is used to to print a Scheme value that is embedded in a primitive error message. The integer argument to the handler specifies the maximum number of characters that should be used to represent the value in the resulting string. The default error value conversion handler prints the value into a string;¹⁰ if the printed form is too long, the printed form is truncated and the last three characters of the return string are set to ``...''.

If the string returned by an error value conversion handler is longer than requested, the string is destructively ``truncated'' by setting the first extra position in the string to the null character. If a non-string is returned, then the string "..." is used. If a primitive error string needs to be generated before the handler has returned, the default error value conversion handler is used.
(error-print-source-location [include?]) gets or sets a boolean that controls whether read and syntax error messages include source information, such as the source line and column or the expression. Only the message field of an exn:read or exn:syntax structure is affected by the parameter. The default is #t.

7.4.1.8 Security

(break-enabled [enabled?]) gets or sets a boolean value that controls whether breaks are allowed. See section 6.6 for more information.
(current-security-guard [security-guard]) gets or sets a security guard (see section 9.1) that controls access to the filesystem and network.
(current-custodian [custodian]) gets or sets a custodian (see section 9.2) that assumes responsibility for newly created threads, ports, and TCP listeners.
(current-inspector [inspector]) gets or sets an inspector (see section 4.6) that controls debugging access to newly created structure types.

7.4.1.9 Exiting

(exit-handler [proc]) gets or sets a procedure that takes a single argument. This exit handler is called by exit. The default exit handler takes any argument and shuts down MzScheme; see section 14.2 for information about exit codes.

7.4.1.10 Random Numbers

(current-pseudo-random-generator [generator]) gets or sets a pseudo-random number generator (see section 3.3) used by random and random-seed.

7.4.1.11 Locale

(current-locale [string-or-#f]) gets or sets a string/boolean value that controls the interpretation of characters for functions such as char-locale<?, char-locale-upcase, and string-locale<? (see section 3.4 and section 3.5). When locale sensitivity is disabled by setting the parameter to #f, characters are interpreted in a fully portable manner, which is the same as the standard procedures; otherwise, they are interpreted according to a locale setting (in the sense of the C library's setlocale). The "" locale is always a synonym for the current machine's default locale; other locale names are platform-specific.¹¹ String and character printing with write is affected by the parameter, because unprintable characters are printed with escapes (see section 14.4). Case conversion for case-insensitive symbols and regular expression patterns (see section 10) are not affected. The parameter's default value is "".

7.4.1.12 Modules

(current-module-name-resolver [proc]) gets or sets a procedure used to resolve module paths. See section 5.4 for more information.
(current-module-name-prefix [symbol]) gets or sets a symbol prefixed onto a module declaration when it is evaluated. This parameter is intended for use by a module name resolver; see section 5.4 for more information.

7.4.2 Parameter Utilities

(make-parameter v [guard-proc]) returns a new parameter procedure. The value of the parameter is initialized to v in all threads. If guard-proc is supplied, it is used as the parameter's guard procedure. A guard procedure takes one argument. Whenever the parameter procedure is applied to an argument, the argument is passed on to the guard procedure. The result returned by the guard procedure is used as the new parameter value. A guard procedure can raise an exception to reject a change to the parameter's value.

(parameter? v) returns #t if v is a parameter procedure, #f otherwise.

(parameter-procedure=? a b) returns #t if the parameter procedures a and b always modify the same parameter, #f otherwise.

The parameterize form evaluates an expression with temporarily values installed for a group of parameters. The syntax of parameterize is:

(parameterize ((parameter-expr value-expr) ···) body-expr ···¹)

The result of a parameterize expression is the result of the last body-expr. The parameter-exprs determine the parameters to set, and the value-exprs determine the corresponding values to install before evaluating the body-exprs. All of the parameter-exprs are evaluated first (checked with check-parameter-procedure), then all value-exprs are evaluated, and then the parameters are set.

After the body-exprs are evaluated, each parameter's setting is restored to its original value in the dynamic context of the parameterize expression. More generally, the values specified by the value-exprs determine initial ``remembered'' values, and whenever control jumps into or out of the body-exprs, the value of each parameter is swapped with the corresponding ``remembered'' value.

Examples:

(parameterize ([exit-handler (lambda (x) 'no-exit)])  
  (exit)) ; => 'no-exit 
 
(define p1 (make-parameter 1)) 
(define p2 (make-parameter 2)) 
(parameterize ([p1 3] 
               [p2 (p1)])  
  (cons (p1) (p2))) ; => '(3 . 1) 
 
(let ([k (let/cc out  
           (parameterize ([p1 2])  
             (p1 3)  
             (cons (let/cc k  
                     (out k))  
                   (p1))))])  
  (if (procedure? k)  
      (k (p1)) 
      k)) ; =>  '(1 . 3)

(check-parameter-procedure v) returns v if it is a procedure that can take both 0 arguments and 1 argument, and raises exn:application:type otherwise. The check-parameter-procedure procedure is used in the expansion of parameterize.

⁸ In MrEd, a handler thread for an eventspace is blocked on an internal semaphore when its event queue is empty. Thus, the handler thread is collectable when the eventspace is unreachable and contains no visible windows or running timers.

⁹ The nested thread's current custodian is inherited from the creating thread, independent of the custodian argument.

¹⁰ Using the current global port print handler; see section 7.4.1.2.

¹¹ The "C" locale is also always available; setting the locale to "C" is the same as disabling locale sensitivity with #f only when string and character operations are restricted to the first 128 characters.