On this page:
14.2.1 Locating Paths
14.2.2 Files
14.2.3 Directories
14.2.4 Declaring Paths Needed at Run Time
14.2.5 More File and Directory Utilities
copy-directory/ files
delete-directory/ files
call-with-file-lock/ timeout

14.2 Filesystem

14.2.1 Locating Paths

(find-system-path kind)  path?
  kind : symbol?
Returns a machine-specific path for a standard type of path specified by kind, which must be one of the following:

(path-list-string->path-list str 
  (listof path?)
  str : (or/c string? bytes?)
  default-path-list : (listof path?)
Parses a string or byte string containing a list of paths, and returns a list of path strings. On Unix and Mac OS X, paths in a path list are separated by a :; on Windows, paths are separated by a ;, and all "s in the string are discarded. Whenever the path list contains an empty path, the list default-path-list is spliced into the returned list of paths. Parts of str that do not form a valid path are not included in the returned list.

(find-executable-path program-sub    
  deepest?])  (or/c path? #f)
  program-sub : path-string?
  related-sub : (or/c path-string? #f) = #f
  deepest? : any/c = #f
Finds a path for the executable program-sub, returning #f if the path cannot be found.

If related-sub is not #f, then it must be a relative path string, and the path found for program-sub must be such that the file or directory related-sub exists in the same directory as the executable. The result is then the full path for the found related-sub, instead of the path for the executable.

This procedure is used by the Racket executable to find the standard library collection directory (see Libraries and Collections). In this case, program is the name used to start Racket and related is "collects". The related-sub argument is used because, on Unix and Mac OS X, program-sub may involve to a sequence of soft links; in this case, related-sub determines which link in the chain is relevant.

If related-sub is not #f, then when find-executable-path does not finds a program-sub that is a link to another file path, the search can continue with the destination of the link. Further links are inspected until related-sub is found or the end of the chain of links is reached. If deepest? is #f (the default), then the result corresponds to the first path in a chain of links for which related-sub is found (and further links are not actually explored); otherwise, the result corresponds to the last link in the chain for which related-sub is found.

If program-sub is a pathless name, find-executable-path gets the value of the PATH environment variable; if this environment variable is defined, find-executable-path tries each path in PATH as a prefix for program-sub using the search algorithm described above for path-containing program-subs. If the PATH environment variable is not defined, program-sub is prefixed with the current directory and used in the search algorithm above. (On Windows, the current directory is always implicitly the first item in PATH, so find-executable-path checks the current directory first on Windows.)

14.2.2 Files

(file-exists? path)  boolean?
  path : path-string?
Returns #t if a file (not a directory) path exists, #f otherwise.

On Windows, file-exists? reports #t for all variations of the special filenames (e.g., "LPT1", "x:/baddir/LPT1").

(link-exists? path)  boolean?
  path : path-string?
Returns #t if a link path exists (Unix and Mac OS X), #f otherwise.

The predicates file-exists? or directory-exists? work on the final destination of a link or series of links, while link-exists? only follows links to resolve the base part of path (i.e., everything except the last name in the path).

This procedure never raises the exn:fail:filesystem exception.

(delete-file path)  void?
  path : path-string?
Deletes the file with path path if it exists, otherwise the exn:fail:filesystem exception is raised. If path is a link, the link is deleted rather than the destination of the link.

(rename-file-or-directory old    
  [exists-ok?])  void?
  old : path-string?
  new : path-string?
  exists-ok? : any/c = #f
Renames the file or directory with path oldif it exists—to the path new. If the file or directory is not renamed successfully, the exn:fail:filesystem exception is raised.

This procedure can be used to move a file/directory to a different directory (on the same disk) as well as rename a file/directory within a directory. Unless exists-ok? is provided as a true value, new cannot refer to an existing file or directory. Even if exists-ok? is true, new cannot refer to an existing file when old is a directory, and vice versa.

If new exists and is replaced, the replacement is atomic on Unix and Mac OS X, but it is not guaranteed to be atomic on Windows. Furthermore, if new exists and is opened by any process for reading or writing, then attempting to replace it will typically fail on Windows.

If old is a link, the link is renamed rather than the destination of the link, and it counts as a file for replacing any existing new.

(file-or-directory-modify-seconds path    
  fail-thunk])  any
  path : path-string?
  secs-n : (or/c exact-integer? #f) = #f
  fail-thunk : (-> any)
   = (lambda () (raise (make-exn:fail:filesystem ....)))
Returns the file or directory’s last modification date as platform-specific seconds (see also Time) when secs-n is not provided or is #f. (For FAT filesystems on Windows, directories do not have modification dates. Therefore, the creation date is returned for a directory, but the modification date is returned for a file.)

If secs-n is provided and not #f, the access and modification times of path are set to the given time.

On error (e.g., if no such file exists), fail-thunk is called, and the default fail-thunk raises exn:fail:filesystem.

(file-or-directory-permissions path [mode])
  (listof (or/c 'read 'write 'execute))
  path : path-string?
  mode : #f = #f
(file-or-directory-permissions path mode)  (integer-in 0 65535)
  path : path-string?
  mode : 'bits
(file-or-directory-permissions path mode)  void
  path : path-string?
  mode : (integer-in 0 65535)
When given one argument or #f as the second argument, returns a list containing 'read, 'write, and/or 'execute to indicate permission the given file or directory path by the current user and group. On Unix and Mac OS X, permissions are checked for the current effective user instead of the real user.

If 'bits is supplied as the second argument, the result is a platform-specific integer encoding of the file or directory properties (mostly permissions), and the result is independent of the current user and group. The lowest nine bits of the encoding are somewhat portable, reflecting permissions for the file or directory’s owner, members of the file or directory’s group, or other users:

See also user-read-bit, etc. On Windows, permissions from all three (owner, group, and others) are always the same, and read and execute permission are always available. On Unix and Mac OS X, higher bits have a platform-specific meaning.

If an integer is supplied as the second argument, its is used as an encoding of properties (mostly permissions) to install for the file.

In all modes, the exn:fail:filesystem exception is raised on error (e.g., if no such file exists).

(file-or-directory-identity path [as-link?])
  path : path-string?
  as-link? : any/c = #f
Returns a number that represents the identity of path in terms of the device and file or directory that it accesses. This function can be used to check whether two paths correspond to the same filesystem entity under the assumption that the path’s entity selection does not change.

If as-link? is a true value, then if path refers to a filesystem link, the identity of the link is returned instead of the identity of the referenced file or directory (if any).

Returns the (logical) size of the specified file in bytes. On Mac OS X, this size excludes the resource-fork size. On error (e.g., if no such file exists), the exn:fail:filesystem exception is raised.

(copy-file src dest)  void?
  src : path-string?
  dest : path-string?
Creates the file dest as a copy of src. If the file is not successfully copied, the exn:fail:filesystem exception is raised. If dest already exists, the copy will fail. File permissions are preserved in the copy. On Mac OS X, the resource fork is also preserved in the copy. If src refers to a link, the target of the link is copied, rather than the link itself.

(make-file-or-directory-link to path)  void?
  to : path-string?
  path : path-string?
Creates a link path to to on Unix and Mac OS X. The creation will fail if path already exists. The to need not refer to an existing file or directory, and to is not expanded before writing the link. If the link is not created successfully,the exn:fail:filesystem exception is raised. On Windows, the exn:fail:unsupported exception is raised always.

14.2.3 Directories

See also: rename-file-or-directory, file-or-directory-modify-seconds, file-or-directory-permissions.

A parameter that determines the current directory for resolving relative paths.

When the parameter procedure is called to set the current directory, the path argument is cleansed using cleanse-path, simplified using simplify-path, and then converted to a directory path with path->directory-path; cleansing and simplification raise an exception if the path is ill-formed. Thus, the current value of current-directory is always a cleansed, simplified, complete, directory path.

The path is not checked for existence when the parameter is set.

Returns the current drive name Windows. For other platforms, the exn:fail:unsupported exception is raised. The current drive is always the drive of the current directory.

(directory-exists? path)  boolean?
  path : path-string?
Returns #t if path refers to a directory, #f otherwise.

(make-directory path)  void?
  path : path-string?
Creates a new directory with the path path. If the directory is not created successfully, the exn:fail:filesystem exception is raised.

(delete-directory path)  void?
  path : path-string?
Deletes an existing directory with the path path. If the directory is not deleted successfully, the exn:fail:filesystem exception is raised.

(directory-list [path])  (listof path?)
  path : path-string? = (current-directory)

See also the in-directory sequence constructor.

Returns a list of all files and directories in the directory specified by path. On Windows, an element of the list may start with \\?\REL\\.

Returns a list of all current root directories. Obtaining this list can be particularly slow on Windows.

14.2.4 Declaring Paths Needed at Run Time

The bindings documented in this section are provided by the racket/runtime-path library, not racket/base or racket.

The racket/runtime-path library provides forms for accessing files and directories at run time using a path that are usually relative to an enclosing source file. Unlike using collection-path, define-runtime-path exposes each run-time path to tools like the executable and distribution creators, so that files and directories needed at run time are carried along in a distribution.

In addition to the bindings described below, racket/runtime-path provides #%datum in phase level 1, since string constants are often used as compile-time expressions with define-runtime-path.

Uses expr as both a compile-time (i.e., phase 1) expression and a run-time (i.e., phase 0) expression. In either context, expr should produce a path, a string that represents a path, a list of the form (list 'lib str ...+), or a list of the form (list 'so str).

For run time, id is bound to a path that is based on the result of expr. The path is normally computed by taking a relative path result from expr and adding it to a path for the enclosing file (which is computed as described below). However, tools like the executable creator can also arrange (by colluding with racket/runtime-path) to have a different base path substituted in a generated executable. If expr produces an absolute path, it is normally returned directly, but again may be replaced by an executable creator. In all cases, the executable creator preserves the relative locations of all paths. When expr produces a relative or absolute path, then the path bound to id is always an absolute path.

If expr produces a list of the form (list 'lib str ...+), the value bound to id is an absolute path. The path refers to a collection-based file similar to using the value as a module path.

If expr produces a list of the form (list 'so str), the value bound to id can be either str or an absolute path; it is an absolute path when adding the platform-specific shared-library extension — as produced by (system-type 'so-suffix) and then searching in the Racket-specific shared-object library directories (as determined by get-lib-search-dirs) locates the path. In this way, shared-object libraries that are installed specifically for Racket get carried along in distributions.

If expr produces a list of the form (list 'module module-path var-ref), the value bound to id is a module path index, where module-path is treated as relative (if it is relative) to the module that is the home of the variable reference var-ref, where var-ref can be #f if module-path is absolute. In an executable, the corresponding module is carried along, including all of its dependencies.

For compile-time, the expr result is used by an executable creator—but not the result when the containing module is compiled. Instead, expr is preserved in the module as a compile-time expression (in the sense of begin-for-syntax). Later, at the time that an executable is created, the compile-time portion of the module is executed (again), and the result of expr is the file to be included with the executable. The reason for the extra compile-time execution is that the result of expr might be platform-dependent, so the result should not be stored in the (platform-independent) bytecode form of the module; the platform at executable-creation time, however, is the same as at run time for the executable. Note that expr is still evaluated at run-time; consequently, avoid procedures like collection-path, which depends on the source installation, and instead use relative paths and forms like (list 'lib str ...+).

If a path is needed only on some platforms and not on others, use define-runtime-path-list with an expr that produces an empty list on platforms where the path is not needed.

The enclosing path for a define-runtime-path is determined as follows from the define-runtime-path syntactic form:

In the latter two cases, the path is normally preserved in (platform-specific) byte form. If it is is within the result of find-collects-dir, however, it the path is recorded relative to (find-collects-dir), and it is reconstructed using (find-collects-dir) at run time.


; Access a file "data.txt" at run-time that is originally
; located in the same directory as the module source file:
(define-runtime-path data-file "data.txt")
(define (read-data)
  (with-input-from-file data-file
    (lambda ()
      (read-bytes (file-size data-file)))))
; Load a platform-specific shared object (using ffi-lib)
; that is located in a platform-specific sub-directory of the
; module's source directory:
(define-runtime-path libfit-path
  (build-path "compiled" "native" (system-library-subpath #f)
              (path-replace-suffix "libfit"
                                   (system-type 'so-suffix))))
(define libfit (ffi-lib libfit-path))
; Load a platform-specific shared object that might be installed
; as part of the operating system, or might be installed
; specifically for Racket:
(define-runtime-path libssl-so
  (case (system-type)
    [(windows) '(so "ssleay32")]
    [else '(so "libssl")]))
(define libssl (ffi-lib libssl-so))

(define-runtime-paths (id ...) expr)
Like define-runtime-path, but declares and binds multiple paths at once. The expr should produce as many values as ids.

Like define-runtime-path, but expr should produce a list of paths.

(define-runtime-module-path-index id module-path-expr)
Similar to define-runtime-path, but id is bound to a module path index that encapsulates the result of module-path-expr relative to the enclosing module.

Use define-runtime-module-path to bind a module path that is passed to a reflective function like dynamic-require while also creating a module dependency for building and distributing executables.

(define-runtime-module-path id module-path)
Similar to define-runtime-path, but id is bound to a resolved module path. The resolved module path for id corresponds to module-path (with the same syntax as a module path for require), which can be relative to the enclosing module.

The define-runtime-module-path-index form is usually preferred, because it creates a weaker link to the referenced module. Unlike define-runtime-module-path-index, the define-runtime-module-path form creates a for-label dependency from an enclosing module to module-path. Since the dependency is merely for-label, module-path is not instantiated or visited when the enclosing module is instantiated or visited (unless such a dependency is created by other requires), but the code for the referenced module is loaded when the enclosing module is loaded.

(runtime-paths module-path)
This form is mainly for use by tools such as executable builders. It expands to a quoted list containing the run-time paths declared by module-path, returning the compile-time results of the declaration exprs, except that paths are converted to byte strings. The enclosing module must require (directly or indirectly) the module specified by module-path, which is an unquoted module path. The resulting list does not include module paths bound through define-runtime-module-path.

14.2.5 More File and Directory Utilities

The bindings documented in this section are provided by the racket/file and racket libraries, but not racket/base.

(file->string path [#:mode mode-flag])  string?
  path : path-string?
  mode-flag : (or/c 'binary 'text) = 'binary
Reads all characters from path and returns them as a string. The mode-flag argument is the same as for open-input-file.

(file->bytes path [#:mode mode-flag])  bytes?
  path : path-string?
  mode-flag : (or/c 'binary 'text) = 'binary
Reads all characters from path and returns them as a byte string. The mode-flag argument is the same as for open-input-file.

(file->value path [#:mode mode-flag])  any
  path : path-string?
  mode-flag : (or/c 'binary 'text) = 'binary
Reads a single S-expression from path using read. The mode-flag argument is the same as for open-input-file.

(file->list path [proc #:mode mode-flag])  (listof any/c)
  path : path-string?
  proc : (input-port? . -> . any/c) = read
  mode-flag : (or/c 'binary 'text) = 'binary
Repeatedly calls proc to consume the contents of path, until eof is produced. The mode-flag argument is the same as for open-input-file.

(file->lines path    
  [#:mode mode-flag    
  #:line-mode line-mode])  (listof string?)
  path : path-string?
  mode-flag : (or/c 'binary 'text) = 'binary
  line-mode : (or/c 'linefeed 'return 'return-linefeed 'any 'any-one)
   = 'any
Read all characters from path, breaking them into lines. The line-mode argument is the same as the second argument to read-line, but the default is 'any instead of 'linefeed. The mode-flag argument is the same as for open-input-file.

(file->bytes-lines path    
  [#:mode mode-flag    
  #:line-mode line-mode])  (listof bytes?)
  path : path-string?
  mode-flag : (or/c 'binary 'text) = 'binary
  line-mode : (or/c 'linefeed 'return 'return-linefeed 'any 'any-one)
   = 'any
Like file->lines, but reading bytes and collecting them into lines like read-bytes-line.

(display-to-file v    
  [#:mode mode-flag    
  #:exists exists-flag])  void?
  v : any/c
  path : path-string?
  mode-flag : (or/c 'binary 'text) = 'binary
  exists-flag : 
(or/c 'error 'append 'update
      'replace 'truncate 'truncate/replace)
   = 'error
Uses display to print v to path. The mode-flag and exists-flag arguments are the same as for open-output-file.

(write-to-file v    
  [#:mode mode-flag    
  #:exists exists-flag])  void?
  v : any/c
  path : path-string?
  mode-flag : (or/c 'binary 'text) = 'binary
  exists-flag : 
(or/c 'error 'append 'update
      'replace 'truncate 'truncate/replace)
   = 'error
Like display-to-file, but using write instead of display.

(display-lines-to-file lst    
  [#:separator separator    
  #:mode mode-flag    
  #:exists exists-flag])  void?
  lst : list?
  path : path-string?
  separator : any/c = #"\n"
  mode-flag : (or/c 'binary 'text) = 'binary
  exists-flag : 
(or/c 'error 'append 'update
      'replace 'truncate 'truncate/replace)
   = 'error
Displays each element of lst to path, adding separator after each element. The mode-flag and exists-flag arguments are the same as for open-output-file.

(copy-directory/files src dest)  void?
  src : path-string?
  dest : path-string?
Copies the file or directory src to dest, raising exn:fail:filesystem if the file or directory cannot be copied, possibly because dest exists already. If src is a directory, the copy applies recursively to the directory’s content. If a source is a link, the target of the link is copied rather than the link itself.

(delete-directory/files path)  void?
  path : path-string?
Deletes the file or directory specified by path, raising exn:fail:filesystem if the file or directory cannot be deleted. If path is a directory, then delete-directory/files is first applied to each file and directory in path before the directory is deleted.

(find-files predicate [start-path])  (listof path?)
  predicate : (path? . -> . any/c)
  start-path : (or/c path-string? #f) = #f
Traverses the filesystem starting at start-path and creates a list of all files and directories for which predicate returns true. If start-path is #f, then the traversal starts from (current-directory). In the resulting list, each directory precedes its content.

The predicate procedure is called with a single argument for each file or directory. If start-path is #f, the argument is a pathname string that is relative to the current directory. Otherwise, it is a path building on start-path. Consequently, supplying (current-directory) for start-path is different from supplying #f, because predicate receives complete paths in the former case and relative paths in the latter. Another difference is that predicate is not called for the current directory when start-path is #f.

The find-files traversal follows soft links. To avoid following links, use the more general fold-files procedure.

If start-path does not refer to an existing file or directory, then predicate will be called exactly once with start-path as the argument.

The find-files procedure raises and exception if it encounters a directory for which directory-list fails.

(pathlist-closure path-list)  (listof path?)
  path-list : (listof path-string?)
Given a list of paths, either absolute or relative to the current directory, returns a list such that

(fold-files proc    
  follow-links?])  any
  proc : 
(or/c (path? (or/c 'file 'dir 'link) any/c
        . -> . any/c)
      (path? (or/c 'file 'dir 'link) any/c
        . -> . (values any/c any/c)))
  init-val : any/c
  start-path : (or/c path-string? #f) = #f
  follow-links? : any/c = #t
Traverses the filesystem starting at start-path, calling proc on each discovered file, directory, and link. If start-path is #f, then the traversal starts from (current-directory).

The proc procedure is called with three arguments for each file, directory, or link:

The proc argument is used in an analogous way to the procedure argument of foldl, where its result is used as the new accumulated result. There is an exception for the case of a directory (when the second argument is 'dir): in this case the procedure may return two values, the second indicating whether the recursive scan should include the given directory or not. If it returns a single value, the directory is scanned. In the cases of files or links (when the second argument is 'file or 'link), a second value is permitted but ignored.

If the start-path is provided but no such path exists, or if paths disappear during the scan, then an exception is raised.

(make-directory* path)  void?
  path : path-string?
Creates directory specified by path, creating intermediate directories as necessary.

(make-temporary-file [template    
  directory])  path?
  template : string? = "mztmp~a"
  copy-from-filename : (or/c path-string? #f 'directory) = #f
  directory : (or/c path-string? #f) = #f
Creates a new temporary file and returns a pathname string for the file. Instead of merely generating a fresh file name, the file is actually created; this prevents other threads or processes from picking the same temporary name.

The template argument must be a format string suitable for use with format and one additional string argument (where the string contains only digits). If the resulting string is a relative path, it is combined with the result of (find-system-path 'temp-dir), unless directory is provided and non-#f, in which case the file name generated from template is combined with directory to obtain a full path.

If copy-from-filename is provided as path, the temporary file is created as a copy of the named file (using copy-file). If copy-from-filename is #f, the temporary file is created as empty. If copy-from-filename is 'directory, then the temporary “file” is created as a directory.

When a temporary file is created, it is not opened for reading or writing when the pathname is returned. The client program calling make-temporary-file is expected to open the file with the desired access and flags (probably using the 'truncate flag; see open-output-file) and to delete it when it is no longer needed.

(get-preference name    
  #:use-lock? use-lock?    
  #:timeout-lock-there timeout-lock-there    
  #:lock-there lock-there])  any
  name : symbol?
  failure-thunk : (-> any) = (lambda () #f)
  flush-mode : any/c = 'timestamp
  filename : (or/c string-path? #f) = #f
  use-lock? : any/c = #t
  timeout-lock-there : (or/c (path? . -> . any) #f) = #f
  lock-there : (or/c (path? . -> . any) #f)
 0.01 name failure-thunk flush-mode filename
 #:lock-there timeout-lock-there)
Extracts a preference value from the file designated by (find-system-path 'pref-file), or by filename if it is provided and is not #f. In the former case, if the preference file doesn’t exist, get-preferences attempts to read an old preferences file, and then a "racket-prefs.rktd" file in the "defaults" collection, instead. If none of those files exists, the preference set is empty.

The preference file should contain a symbol-keyed association list (written to the file with the default parameter settings). Keys starting with racket:, mzscheme:, mred:, and plt: in any letter case are reserved for use by Racket implementers.

The result of get-preference is the value associated with name if it exists in the association list, or the result of calling failure-thunk otherwise.

Preference settings are cached (weakly) across calls to get-preference, using (path->complete-path filename) as a cache key. If flush-mode is provided as #f, the cache is used instead of the re-consulting the preferences file. If flush-mode is provided as 'timestamp (the default), then the cache is used only if the file has a timestamp that is the same as the last time the file was read. Otherwise, the file is re-consulted.

On platforms for which preferences-lock-file-mode returns 'file-lock and when use-lock? is true, preference-file reading is guarded by a lock; multiple readers can share the lock, but writers take the lock exclusively. If the preferences file cannot be read because the lock is unavailable, lock-there is called on the path of the lock file; if lock-there is #f, an exception is raised. The default lock-there handler retries about 5 times (with increasing delays between each attempt) before trying timeout-lock-there, and the default timeout-lock-there triggers an exception.

See also put-preferences. For a more elaborate preference system, see preferences:get.

Old preferences files: When a filename is not provided and the file indicated by (find-system-path 'pref-file) does not exist, the following paths are checked for compatibility with old versions of Racket:

(put-preferences names    
  filename])  void?
  names : (listof symbol?)
  vals : list?
  locked-proc : (path? . -> . any) = (lambda (p) (error ....))
  filename : (or/c #f path-string?) = #f
Installs a set of preference values and writes all current values to the preference file designated by (find-system-path 'pref-file), or filename if it is supplied and not #f.

The names argument supplies the preference names, and vals must have the same length as names. Each element of vals must be an instance of a built-in data type whose write output is readable (i.e., the print-unreadable parameter is set to #f while writing preferences).

Current preference values are read from the preference file before updating, and a write lock is held starting before the file read, and lasting until after the preferences file is updated. The lock is implemented by the existence of a file in the same directory as the preference file; see preferences-lock-file-mode for more information. If the directory of the preferences file does not already exist, it is created.

If the write lock is already held, then locked-proc is called with a single argument: the path of the lock file. The default locked-proc reports an error; an alternative thunk might wait a while and try again, or give the user the choice to delete the lock file (in case a previous update attempt encountered disaster and locks are implemented by the presence of the lock file).

If filename is #f or not supplied, and the preference file does not already exist, then values read from the "defaults" collection (if any) are written for preferences that are not mentioned in names.

(preferences-lock-file-mode)  (or/c 'exists 'file-lock)
Reports the way that the lock file is used to implement preference-file locking on the current platform.

The 'exists mode is currently used on all platforms except Windows. In 'exists mode, the existence of the lock file indicates that a write lock is held, and readers need no lock (because the preferences file is atomically updated via rename-file-or-directory).

The 'file-lock mode is currently used on Windows. In 'file-lock mode, shared and exclusive locks (in the sense of port-try-file-lock?) on the lock file reflect reader and writer locks on the preference-file content. (The preference file itself is not locked, because a lock would interfere with replacing the file via rename-file-or-directory.)

(make-handle-get-preference-locked delay 
  #:lock-there lock-there 
  #:max-delay max-delay]) 
  (path-string? . -> . any)
  delay : real?
  name : symbol?
  failure-thunk : (-> any) = (lambda () #f)
  flush-mode : any/c = 'timestamp
  filename : (or/c path-string? #f) = #f
  lock-there : (or/c (path? . -> . any) #f) = #f
  max-delay : real? = 0.2
Creates a procedure suitable for use as the #:lock-there argument to get-preference, where the name, failure-thunk, flush-mode, and filename are all passed on to get-preference by the result procedure to retry the preferences lookup.

Before calling get-preference, the result procedure uses (sleep delay) to pause. Then, if (* 2 delay) is less than max-delay, the result procedure calls

make-handle-get-preference-locked to generate a new retry procedure to pass to get-preference, but with a delay of (* 2 delay). If (* 2 delay) is not less than max-delay, then get-preference is called with the given lock-there, instead.

(call-with-file-lock/timeout filename 
  [#:get-lock-file get-lock-file 
  #:delay delay 
  #:max-delay max-delay]) 
  filename : (or/c path-string? #f)
  kind : (or/c 'shared 'exclusive)
  thunk : (-> any)
  failure-thunk : (-> any)
  get-lock-file : (-> path-string?)
   = (lambda () (make-lock-filename filename))
  delay : real? = 0.01
  max-delay : real? = 0.2
Obtains a lock for the filename returned from (get-lock-file) and then calls thunk. When thunk returns, call-with-file-lock releases the lock, returning the result of thunk. The call-with-file-lock/timeout function will retry after #:delay seconds and continue retrying with exponential backoff until delay reaches #:max-delay. If call-with-file-lock/timeout fails to obtain the lock, failure-thunk is called in tail position. The kind argument specifies whether the lock is 'shared or 'exclusive

The filename argument specifies a file path prefix that is only used to generate the lock filename, when #:get-lock-file is not present. The call-with-file-lock/timeout function uses a separate lock file to prevent race conditions on filename, when filename has not yet been created. On the Windows platfom, the call-with-file-lock/timeout function uses a separate lock file ("_LOCKfilename"), because a lock on filename would interfere with replacing filename via rename-file-or-directory.


> (call-with-file-lock/timeout filename 'exclusive
    (lambda () (printf "File is locked\n"))
    (lambda () (printf "Failed to obtain lock for file\n")))

File is locked

> (call-with-file-lock/timeout #f 'exclusive
    (lambda ()
      (call-with-file-lock/timeout filename 'shared
        (lambda () (printf "Shouldn't get here\n"))
        (lambda () (printf "Failed to obtain lock for file\n"))))
    (lambda () (printf "Shouldn't ger here eithere\n"))
    #:get-lock-file (lambda () (make-lock-file-name filename)))

Failed to obtain lock for file

(make-lock-file-name path)  path-string?
  path : path-string?
(make-lock-file-name dir name)  path-string?
  dir : path-string?
  name : path-string?
Creates a lock filename by prepending "_LOCK" on windows or ".LOCK" on all other platforms to the file portion of the path.


> (make-lock-file-name "/home/george/project/important-file")


user-read-bit : #o400
user-write-bit : #o200
user-execute-bit : #o100
group-read-bit : #o040
group-write-bit : #o020
group-execute-bit : #o010
other-read-bit : #o004
other-write-bit : #o002
other-execute-bit : #o001
Constants that are useful with file-or-directory-permissions and bitwise operations such as bitwise-ior, and bitwise-and.