MIT/GNU Scheme Reference: Operating-System Interface

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# 15. Operating-System Interface

The Scheme standard provides a simple mechanism for reading and writing files: file ports. MIT/GNU Scheme provides additional tools for dealing with other aspects of the operating system:

- Pathnames are a reasonably operating-system independent tool for manipulating the component parts of file names. This can be useful for implementing defaulting of file name components.

- Control over the current working directory: the place in the file system from which relative file names are interpreted.

- Procedures that rename, copy, delete, and test for the existence of files. Also, procedures that return detailed information about a particular file, such as its type (directory, link, etc.) or length.

- Procedures for reading the contents of a directory.

- Procedures for obtaining times in various formats, converting between the formats, and generating human-readable time strings.

- Procedures to run other programs as subprocesses of Scheme, to read their output, and write input to them.

- A means to determine the operating system Scheme is running under.

> 15.1 Pathnames |  |
> 15.2 Working Directory |  |
> 15.3 File Manipulation |  |
> 15.4 Directory Reader |  |
> 15.5 Date and Time |  |
> 15.6 Machine Time |  |
> 15.7 Subprocesses |  |
> 15.8 TCP Sockets |  |
> 15.9 Miscellaneous OS Facilities |  |

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## 15.1 Pathnames

MIT/GNU Scheme programs need to use names to designate files. The main difficulty in dealing with names of files is that different file systems have different naming formats for files. For example, here is a table of several file systems (actually, operating systems that provide file systems) and what equivalent file names might look like for each one:

    System          File Name
    ------          ---------
    TOPS-20         <LISPIO>FORMAT.FASL.13
    TOPS-10         FORMAT.FAS[1,4]
    ITS             LISPIO;FORMAT FASL
    MULTICS         >udd>LispIO>format.fasl
    TENEX           <LISPIO>FORMAT.FASL;13
    VAX/VMS         [LISPIO]FORMAT.FAS;13
    UNIX            /usr/lispio/format.fasl
    DOS             C:\USR\LISPIO\FORMAT.FAS

It would be impossible for each program that deals with file names to know about each different file name format that exists; a new operating system to which Scheme was ported might use a format different from any of its predecessors. Therefore, MIT/GNU Scheme provides two ways to represent file names: filenames (also called namestrings), which are strings in the implementation-dependent form customary for the file system, and pathnames, which are special abstract data objects that represent file names in an implementation-independent way. Procedures are provided to convert between these two representations, and all manipulations of files can be expressed in machine-independent terms by using pathnames.

In order to allow MIT/GNU Scheme programs to operate in a network environment that may have more than one kind of file system, the pathname facility allows a file name to specify which file system is to be used. In this context, each file system is called a host, in keeping with the usual networking terminology.(20)

Note that the examples given in this section are specific to unix pathnames. Pathnames for other operating systems have different external representations.

> 15.1.1 Filenames and Pathnames |  |
> 15.1.2 Components of Pathnames |  |
> 15.1.3 Operations on Pathnames |  |
> 15.1.4 Miscellaneous Pathname Procedures |  |

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### 15.1.1 Filenames and Pathnames

Pathname objects are usually created by parsing filenames (character strings) into component parts. MIT/GNU Scheme provides operations that convert filenames into pathnames and vice versa.

procedure: ->pathname object
  Returns a pathname that is the equivalent of object. Object must be a pathname or a string. If object is a pathname, it is returned. If object is a string, this procedure returns the pathname that corresponds to the string; in this case it is equivalent to `(parse-namestring object #f #f)`.

      (->pathname "foo")          =>  #[pathname 65 "foo"]
      (->pathname "/usr/morris")  =>  #[pathname 66 "/usr/morris"]

procedure: parse-namestring thing [host [defaults]]
  This turns thing into a pathname. Thing must be a pathname or a string. If thing is a pathname, it is returned. If thing is a string, this procedure returns the pathname that corresponds to the string, parsed according to the syntax of the file system specified by host.

  This procedure does not do defaulting of pathname components.

  The optional arguments are used to determine what syntax should be used for parsing the string. In general this is only really useful if your implementation of MIT/GNU Scheme supports more than one file system, otherwise you would use `->pathname`. If given, host must be a host object or `#f`, and defaults must be a pathname. Host specifies the syntax used to parse the string. If host is not given or `#f`, the host component from defaults is used instead; if defaults is not given, the host component from `*default-pathname-defaults*` is used.

procedure: ->namestring pathname
  `->namestring` returns a newly allocated string that is the filename corresponding to pathname.

      (->namestring (->pathname "/usr/morris/minor.van"))
           =>  "/usr/morris/minor.van"

procedure: pathname-simplify pathname
  Returns a pathname that locates the same file or directory as pathname, but is in some sense simpler. Note that `pathname-simplify` might not always be able to simplify the pathname, e.g. on unix with symbolic links the directory ``/usr/morris/../`' need not be the same as ``/usr/`'. In cases of uncertainty the behavior is conservative, returning the original or a partly simplified pathname.

      (pathname-simplify "/usr/morris/../morris/dance")
           =>  #[pathname "/usr/morris/dance"]

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### 15.1.2 Components of Pathnames

A pathname object always has six components, described below. These components are the common interface that allows programs to work the same way with different file systems; the mapping of the pathname components into the concepts peculiar to each file system is taken care of by the Scheme implementation.

host
  The name of the file system on which the file resides. In the current implementation, this component is always a host object that is filled in automatically by the runtime system. When specifying the host component, use either `#f` or the value of the variable `local-host`.

device
  Corresponds to the "device" or "file structure" concept in many host file systems: the name of a (logical or physical) device containing files. This component is the drive letter for PC file systems, and is unused for unix file systems.

directory
  Corresponds to the "directory" concept in many host file systems: the name of a group of related files (typically those belonging to a single user or project). This component is always used for all file systems.

name
  The name of a group of files that can be thought of as conceptually the "same" file. This component is always used for all file systems.

type
  Corresponds to the "filetype" or "extension" concept in many host file systems. This says what kind of file this is. Files with the same name but different type are usually related in some specific way, such as one being a source file, another the compiled form of that source, and a third the listing of error messages from the compiler. This component is currently used for all file systems, and is formed by taking the characters that follow the last dot in the namestring.

version
  Corresponds to the "version number" concept in many host file systems. Typically this is a number that is incremented every time the file is modified. This component is currently unused for all file systems.

Note that a pathname is not necessarily the name of a specific file. Rather, it is a specification (possibly only a partial specification) of how to access a file. A pathname need not correspond to any file that actually exists, and more than one pathname can refer to the same file. For example, the pathname with a version of `newest` may refer to the same file as a pathname with the same components except a certain number as the version. Indeed, a pathname with version `newest` may refer to different files as time passes, because the meaning of such a pathname depends on the state of the file system. In file systems with such facilities as "links", multiple file names, logical devices, and so on, two pathnames that look quite different may turn out to address the same file. To access a file given a pathname, one must do a file-system operation such as `open-input-file`.

Two important operations involving pathnames are parsing and merging. Parsing is the conversion of a filename (which might be something supplied interactively by the users when asked to supply the name of a file) into a pathname object. This operation is implementation-dependent, because the format of filenames is implementation-dependent. Merging takes a pathname with missing components and supplies values for those components from a source of default values.

Not all of the components of a pathname need to be specified. If a component of a pathname is missing, its value is `#f`. Before the file system interface can do anything interesting with a file, such as opening the file, all the missing components of a pathname must be filled in. Pathnames with missing components are used internally for various purposes; in particular, parsing a namestring that does not specify certain components will result in a pathname with missing components.

Any component of a pathname may be the symbol `unspecific`, meaning that the component simply does not exist, for file systems in which such a value makes no sense. For example, unix, Windows, and OS/2 file systems usually do not support version numbers, so the version component for such a host might be `unspecific`.(21)

In addition to `#f` and `unspecific`, the components of a pathname may take on the following meaningful values:

host
  An implementation-defined type which may be tested for using the `host?` predicate.

device
  On systems that support this component (Windows and OS/2), it may be specified as a string containing a single alphabetic character, for which the alphabetic case is ignored.

directory
  A non-empty list, which represents a directory path: a sequence of directories, each of which has a name in the previous directory, the last of which is the directory specified by the entire path. Each element in such a path specifies the name of the directory relative to the directory specified by the elements to its left. The first element of the list is either the symbol `absolute` or the symbol `relative`. If the first element in the list is the symbol `absolute`, then the directory component (and subsequently the pathname) is absolute; the first component in the sequence is to be found at the "root" of the file system. If the directory is relative then the first component is to be found in some as yet unspecified directory; typically this is later specified to be the current working directory.

  Aside from `absolute` and `relative`, which may only appear as the first element of the list, each subsequent element in the list is either: a string, which is a literal component; the symbol `wild`, meaningful only when used in conjunction with the directory reader; or the symbol `up`, meaning the next directory is the "parent" of the previous one. `up` corresponds to the file ``..`' in unix and PC file systems.

  (The following note does not refer to any file system currently supported by MIT/GNU Scheme, but is included for completeness.) In file systems that do not have "hierarchical" structure, a specified directory component will always be a list whose first element is `absolute`. If the system does not support directories other than a single global directory, the list will have no other elements. If the system supports "flat" directories, i.e. a global set of directories with no subdirectories, then the list will contain a second element, which is either a string or `wild`. In other words, a non-hierarchical file system is treated as if it were hierarchical, but the hierarchical features are unused. This representation is somewhat inconvenient for such file systems, but it discourages programmers from making code depend on the lack of a file hierarchy.

name
  A string, which is a literal component; or the symbol `wild`, meaningful only when used in conjunction with the directory reader.

type
  A string, which is a literal component; or the symbol `wild`, meaningful only when used in conjunction with the directory reader.

version
  An exact positive integer, which is a literal component; the symbol `newest`, which means to choose the largest available version number for that file; the symbol `oldest`, which means to choose the smallest version number; or the symbol `wild`, meaningful only when used in conjunction with the directory reader. In the future some other possible values may be added, e.g. `installed`. Note that currently no file systems support version numbers; thus this component is not used and should be specified as `#f`.

procedure: make-pathname host device directory name type version
  Returns a pathname object whose components are the respective arguments. Each argument must satisfy the restrictions for the corresponding component, which were outlined above.

      (make-pathname #f
                     #f
                     '(absolute "usr" "morris")
                     "foo"
                     "scm"
                     #f)
           =>  #[pathname 67 "/usr/morris/foo.scm"]

procedure: pathname-host pathname
procedure: pathname-device pathname
procedure: pathname-directory pathname
procedure: pathname-name pathname
procedure: pathname-type pathname
procedure: pathname-version pathname
  Returns a particular component of pathname.

      (define x (->pathname "/usr/morris/foo.scm"))
      (pathname-host x)       =>  #[host 1]
      (pathname-device x)     =>  unspecific
      (pathname-directory x)  =>  (absolute "usr" "morris")
      (pathname-name x)       =>  "foo"
      (pathname-type x)       =>  "scm"
      (pathname-version x)    =>  unspecific

procedure: pathname-new-device pathname device
procedure: pathname-new-directory pathname directory
procedure: pathname-new-name pathname name
procedure: pathname-new-type pathname type
procedure: pathname-new-version pathname version
  Returns a new copy of pathname with the respective component replaced by the second argument. Pathname is unchanged. Portable programs should not explicitly replace a component with `unspecific` because this might not be permitted in some situations.

      (define p (->pathname "/usr/blisp/rel15"))
      p
           =>  #[pathname 71 "/usr/blisp/rel15"]
      (pathname-new-name p "rel100")
           =>  #[pathname 72 "/usr/blisp/rel100"]
      (pathname-new-directory p '(relative "test" "morris"))
           =>  #[pathname 73 "test/morris/rel15"]
      p
           =>  #[pathname 71 "/usr/blisp/rel15"]

procedure: pathname-default-device pathname device
procedure: pathname-default-directory pathname directory
procedure: pathname-default-name pathname name
procedure: pathname-default-type pathname type
procedure: pathname-default-version pathname version
  These operations are similar to the `pathname-new-component` operations, except that they only change the specified component if it has the value `#f` in pathname.

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### 15.1.3 Operations on Pathnames

procedure: pathname? object
  Returns `#t` if object is a pathname; otherwise returns `#f`.

procedure: pathname=? pathname1 pathname2
  Returns `#t` if pathname1 is equivalent to pathname2; otherwise returns `#f`. Pathnames are equivalent if all of their components are equivalent, hence two pathnames that are equivalent must identify the same file or equivalent partial pathnames. However, the converse is not true: non-equivalent pathnames may specify the same file (e.g. via absolute and relative directory components), and pathnames that specify no file at all (e.g. name and directory components unspecified) may be equivalent.

procedure: pathname-absolute? pathname
  Returns `#t` if pathname is an absolute rather than relative pathname object; otherwise returns `#f`. Specifically, this procedure returns `#t` when the directory component of pathname is a list starting with the symbol `absolute`, and returns `#f` in all other cases. All pathnames are either absolute or relative, so if this procedure returns `#f`, the argument is a relative pathname.

procedure: directory-pathname? pathname
  Returns `#t` if pathname has only directory components and no file components. This is roughly equivalent to

      (define (directory-pathname? pathname)
        (string-null? (file-namestring pathname)))

  except that it is faster.

procedure: pathname-wild? pathname
  Returns `#t` if pathname contains any wildcard components; otherwise returns `#f`.

procedure: merge-pathnames pathname [defaults [default-version]]
  Returns a pathname whose components are obtained by combining those of pathname and defaults. Defaults defaults to the value of `*default-pathname-defaults*` and default-version defaults to `newest`.

  The pathnames are combined by components: if pathname has a non-missing component, that is the resulting component, otherwise the component from defaults is used. The default version can be `#f` to preserve the information that the component was missing from pathname. The directory component is handled specially: if both pathnames have directory components that are lists, and the directory component from pathname is relative (i.e. starts with `relative`), then the resulting directory component is formed by appending pathname's component to defaults's component. For example:

      (define path1 (->pathname "scheme/foo.scm"))
      (define path2 (->pathname "/usr/morris"))
      path1
           =>  #[pathname 74 "scheme/foo.scm"]
      path2
           =>  #[pathname 75 "/usr/morris"]
      (merge-pathnames path1 path2)
           =>  #[pathname 76 "/usr/scheme/foo.scm"]
      (merge-pathnames path2 path1)
           =>  #[pathname 77 "/usr/morris.scm"]

  The merging rules for the version are more complex and depend on whether pathname specifies a name. If pathname does not specify a name, then the version, if not provided, will come from defaults. However, if pathname does specify a name then the version is not affected by defaults. The reason is that the version "belongs to" some other file name and is unlikely to have anything to do with the new one. Finally, if this process leaves the version missing, then default-version is used.

  The net effect is that if the user supplies just a name, then the host, device, directory and type will come from defaults, but the version will come from default-version. If the user supplies nothing, or just a directory, the name, type and version will come over from defaults together.

variable: *default-pathname-defaults*
  This is the default pathname-defaults pathname; if any pathname primitive that needs a set of defaults is not given one, it uses this one. `set-working-directory-pathname!` sets this variable to a new value, computed by merging the new working directory with the variable's old value.

procedure: pathname-default pathname device directory name type version
  This procedure defaults all of the components of pathname simultaneously. It could have been defined by:

      (define (pathname-default pathname
                                device directory name type version)
        (make-pathname (pathname-host pathname)
                       (or (pathname-device pathname) device)
                       (or (pathname-directory pathname) directory)
                       (or (pathname-name pathname) name)
                       (or (pathname-type pathname) type)
                       (or (pathname-version pathname) version)))

procedure: file-namestring pathname
procedure: directory-namestring pathname
procedure: host-namestring pathname
procedure: enough-namestring pathname [defaults]
  These procedures return a string corresponding to a subset of the pathname information. `file-namestring` returns a string representing just the name, type and version components of pathname; the result of `directory-namestring` represents just the host, device, and directory components; and `host-namestring` returns a string for just the host portion.

  `enough-namestring` takes another argument, defaults. It returns an abbreviated namestring that is just sufficient to identify the file named by pathname when considered relative to the defaults (which defaults to `*default-pathname-defaults*`).

      (file-namestring "/usr/morris/minor.van")
           =>  "minor.van"
      (directory-namestring "/usr/morris/minor.van")
           =>  "/usr/morris/"
      (enough-namestring "/usr/morris/men")
           =>  "men"      ;perhaps

procedure: file-pathname pathname
procedure: directory-pathname pathname
procedure: enough-pathname pathname [defaults]
  These procedures return a pathname corresponding to a subset of the pathname information. `file-pathname` returns a pathname with just the name, type and version components of pathname. The result of `directory-pathname` is a pathname containing the host, device and directory components of pathname.

  `enough-pathname` takes another argument, defaults. It returns an abbreviated pathname that is just sufficient to identify the file named by pathname when considered relative to the defaults (which defaults to `*default-pathname-defaults*`).

  These procedures are similar to `file-namestring`, `directory-namestring` and `enough-namestring`, but they return pathnames instead of strings.

procedure: directory-pathname-as-file pathname
  Returns a pathname that is equivalent to pathname, but in which the directory component is represented as a file. The last directory is removed from the directory component and converted into name and type components. This is the inverse operation to `pathname-as-directory`.

      (directory-pathname-as-file (->pathname "/usr/blisp/"))
           =>  #[pathname "/usr/blisp"]

procedure: pathname-as-directory pathname
  Returns a pathname that is equivalent to pathname, but in which any file components have been converted to a directory component. If pathname does not have name, type, or version components, it is returned without modification. Otherwise, these file components are converted into a string, and the string is added to the end of the list of directory components. This is the inverse operation to `directory-pathname-as-file`.

      (pathname-as-directory (->pathname "/usr/blisp/rel5"))
           =>  #[pathname "/usr/blisp/rel5/"]

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### 15.1.4 Miscellaneous Pathname Procedures

This section gives some standard operations on host objects, and some procedures that return some useful pathnames.

variable: local-host
  This variable has as its value the host object that describes the local host's file system.

procedure: host? object
  Returns `#t` if object is a pathname host; otherwise returns `#f`.

procedure: host=? host1 host2
  Returns `#t` if host1 and host2 denote the same pathname host; otherwise returns `#f`.

procedure: init-file-pathname [host]
  Returns a pathname for the user's initialization file on host. The host argument defaults to the value of `local-host`. If the initialization file does not exist this procedure returns `#f`.

  Under unix, the init file is called ``.scheme.init`'; under Windows and OS/2, the init file is called ``scheme.ini`'. In either case, it is located in the user's home directory, which is computed by `user-homedir-pathname`.

procedure: user-homedir-pathname [host]
  Returns a pathname for the user's "home directory" on host. The host argument defaults to the value of `local-host`. The concept of a "home directory" is itself somewhat implementation-dependent, but it should be the place where the user keeps personal files, such as initialization files and mail.

  Under unix, the user's home directory is specified by the `HOME` environment variable. If this variable is undefined, the user name is computed using the `getlogin` system call, or if that fails, the `getuid` system call. The resulting user name is passed to the `getpwnam` system call to obtain the home directory.

  Under OS/2, several heuristics are tried to find the user's home directory. First, if the environment variable `HOME` is defined, that is the home directory. If `HOME` is undefined, but the `USERDIR` and `USER` environment variables are defined and the directory ``%USERDIR%\%USER%`' exists, then it is used. Failing that, if the directory ``%USER%`' exists on the OS/2 system drive, then it is used. As a last resort, the OS/2 system drive is the home directory.

  Like OS/2, the Windows implementation uses heuristics based on environment variables. The user's home directory is computed by examining several environment variables, in the following order:

  - `HOMEDRIVE` and `HOMEPATH` are both defined and ``%HOMEDRIVE%%HOMEPATH%`' is an existing directory. (These variables are automatically defined by Windows NT.)

  - `HOME` is defined and ``%HOME%`' is an existing directory.

  - `USERDIR` and `USERNAME` are defined and ``%USERDIR%\%USERNAME%`' is an existing directory.

  - `USERDIR` and `USER` are defined and ``%USERDIR%\%USER%`' is an existing directory.

  - `USERNAME` is defined and ``%USERNAME%`' is an existing directory on the Windows system drive.

  - `USER` is defined and ``%USER%`' is an existing directory on the Windows system drive.

  - Finally, if all else fails, the Windows system drive is used as the home directory.

procedure: system-library-pathname pathname
  Locates pathname in MIT/GNU Scheme's system library directory. An error of type `condition-type:file-operation-error` is signalled if pathname cannot be located on the library search path.

      (system-library-pathname "compiler.com")
        => #[pathname 45 "/usr/local/lib/mit-scheme/compiler.com"]

procedure: system-library-directory-pathname pathname
  Locates the pathname of an MIT/GNU Scheme system library directory. An error of type `condition-type:file-operation-error` is signalled if pathname cannot be located on the library search path.

      (system-library-directory-pathname "options")
           => #[pathname 44 "/usr/local/lib/mit-scheme/options/"]

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## 15.2 Working Directory

When MIT/GNU Scheme is started, the current working directory (or simply, working directory) is initialized in an operating-system dependent manner; usually, it is the directory in which Scheme was invoked. The working directory can be determined from within Scheme by calling the `pwd` procedure, and changed by calling the `cd` procedure. Each REP loop has its own working directory, and inferior REP loops initialize their working directory from the value in effect in their superior at the time they are created.

procedure: working-directory-pathname
procedure: pwd
  Returns the current working directory as a pathname that has no name, type, or version components, just host, device, and directory components. `pwd` is an alias for `working-directory-pathname`; the long name is intended for programs and the short name for interactive use.

procedure: set-working-directory-pathname! filename
procedure: cd filename
  Makes filename the current working directory and returns the new current working directory as a pathname. Filename is coerced to a pathname using `pathname-as-directory`. `cd` is an alias for `set-working-directory-pathname!`; the long name is intended for programs and the short name for interactive use.

  Additionally, `set-working-directory-pathname!` modifies the value of
  `*default-pathname-defaults*` by merging the new working directory into it.

  When this procedure is executed in the top-level REP loop, it changes the working directory of the running Scheme executable.

      (set-working-directory-pathname! "/usr/morris/blisp")
           =>  #[pathname "/usr/morris/blisp/"]
      (set-working-directory-pathname! "~")
           =>  #[pathname "/usr/morris/"]

  This procedure signals an error if filename does not refer to an existing directory.

  If filename describes a relative rather than absolute pathname, this procedure interprets it as relative to the current working directory, before changing the working directory.

      (working-directory-pathname)
           =>  #[pathname "/usr/morris/"]
      (set-working-directory-pathname! "foo")
           =>  #[pathname "/usr/morris/foo/"]

procedure: with-working-directory-pathname filename thunk
  This procedure temporarily rebinds the current working directory to filename, invokes thunk (a procedure of no arguments), then restores the previous working directory and returns the value yielded by thunk. Filename is coerced to a pathname using `pathname-as-directory`. In addition to binding the working directory, `with-working-directory-pathname` also binds the variable `*default-pathname-defaults*`, merging the old value of that variable with the new working directory pathname. Both bindings are performed in exactly the same way as dynamic binding of a variable (see section 2.3 Dynamic Binding).

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## 15.3 File Manipulation

This section describes procedures that manipulate files and directories. Any of these procedures can signal a number of errors for many reasons. The specifics of these errors are much too operating-system dependent to document here. However, if such an error is signalled by one of these procedures, it will be of type `condition-type:file-operation-error`.

procedure: file-exists? filename
procedure: file-exists-direct? filename
procedure: file-exists-indirect? filename
  These procedures return `#t` if filename is an existing file or directory; otherwise they return `#f`. In operating systems that support symbolic links, if the file is a symbolic link, `file-exists-direct?` tests for the existence of the link, while `file-exists-indirect?` and `file-exists?` test for the existence of the file pointed to by the link.

procedure: copy-file source-filename target-filename
  Makes a copy of the file named by source-filename. The copy is performed by creating a new file called target-filename, and filling it with the same data as source-filename.

procedure: rename-file source-filename target-filename
  Changes the name of source-filename to be target-filename. In the unix implementation, this will not rename across file systems.

procedure: delete-file filename
  Deletes the file named filename.

procedure: delete-file-no-errors filename
  Like `delete-file`, but returns a boolean value indicating whether an error occurred during the deletion. If no errors occurred, `#t` is returned. If an error of type `condition-type:file-error` or `condition-type:port-error` is signalled, `#f` is returned.

procedure: hard-link-file source-filename target-filename
  Makes a hard link from source-filename to target-filename. This operation gives the file specified by source-filename a new name, in addition to the old name.

  This currently works only on unix systems. It is further restricted to work only when source-filename and target-filename refer to names in the same file system.

procedure: soft-link-file source-filename target-filename
  Creates a new soft link called target-filename that points at the file source-filename. (Soft links are also sometimes called symbolic links.) Note that source-filename will be interpreted as a string (although you may specify it as a pathname object, if you wish). The contents of this string will be stored in the file system as the soft link. When a file operation attempts to open the link, the contents of the link are interpreted relative to the link's location at that time.

  This currently works only on unix systems.

procedure: make-directory filename
  Creates a new directory named filename. Signals an error if filename already exists, or if the directory cannot be created.

procedure: delete-directory filename
  Deletes the directory named filename. Signals an error if the directory does not exist, is not a directory, or contains any files or subdirectories.

procedure: ->truename filename
  This procedure attempts to discover and return the "true name" of the file associated with filename within the file system. An error of type `condition-type:file-operation-error` is signalled if the appropriate file cannot be located within the file system.

procedure: call-with-temporary-file-pathname procedure
  Calls `temporary-file-pathname` to create a temporary file, then calls procedure with one argument, the pathname referring to that file. When procedure returns, if the temporary file still exists, it is deleted; then, the value yielded by procedure is returned. If procedure escapes from its continuation, and the file still exists, it is deleted.

procedure: temporary-file-pathname [directory]
  Creates a new empty temporary file and returns a pathname referring to it. The temporary file is created with Scheme's default permissions, so barring unusual circumstances it can be opened for input and/or output without error. The temporary file will remain in existence until explicitly deleted. If the file still exists when the Scheme process terminates, it will be deleted.

  If directory is specified, the temporary file will be stored there. If it is not specified, or if it is `#f`, the temporary file will be stored in the directory returned by `temporary-directory-pathname`.

procedure: temporary-directory-pathname
  Returns the pathname of an existing directory that can be used to store temporary files. These directory names are tried, in order, until a writeable directory is found:

  - The directories specified by the environment variables `TMPDIR`, `TEMP`, or `TMP`.

  - Under unix, the directories ``/var/tmp`', ``/usr/tmp`', or ``/tmp`'.

  - Under OS/2 or Windows, the following directories on the system drive: ``\temp`', ``\tmp`', or ``\`'.

  - Under OS/2 or Windows, the current directory, as specified by `*default-pathname-defaults*`.

procedure: file-directory? filename
  Returns `#t` if the file named filename exists and is a directory. Otherwise returns `#f`. In operating systems that support symbolic links, if filename names a symbolic link, this examines the file linked to, not the link itself.

  This is equivalent to

      (eq? 'directory (file-type-indirect filename))

procedure: file-regular? filename
  Returns `#t` if the file named filename exists and is a regular file (i.e. not a directory, symbolic link, device file, etc.). Otherwise returns `#f`. In operating systems that support symbolic links, if filename names a symbolic link, this examines the file linked to, not the link itself.

  This is equivalent to

      (eq? 'regular (file-type-indirect filename))

procedure: file-symbolic-link? filename
  In operating systems that support symbolic links, if the file named filename exists and is a symbolic link, this procedure returns the contents of the symbolic link as a newly allocated string. The returned value is the name of the file that the symbolic link points to and must be interpreted relative to the directory of filename. If filename either does not exist or is not a symbolic link, or if the operating system does not support symbolic links, this procedure returns `#f`.

procedure: file-type-direct filename
procedure: file-type-indirect filename
  If the file named filename exists, `file-type-direct` returns a symbol specifying what type of file it is. For example, if filename refers to a directory, the symbol `directory` is returned. If filename doesn't refer to an existing file, `#f` is returned.

  If filename refers to a symbolic link, `file-type-direct` returns the type of the link itself, while `file-type-indirect` returns the type of the file linked to.

  At this time, the symbols that can be returned are the following. The names are intended to be self-explanatory. Most of these names can only be returned on particular operating systems, and so the operating-system name is prefixed to the name.

      regular
      directory
      unix-symbolic-link
      unix-character-device
      unix-block-device
      unix-named-pipe
      unix-socket
      os2-named-pipe
      win32-named-pipe

procedure: file-readable? filename
  Returns `#t` if filename names a file that can be opened for input; i.e. a readable file. Otherwise returns `#f`.

procedure: file-writeable? filename
  Returns `#t` if filename names a file that can be opened for output; i.e. a writeable file. Otherwise returns `#f`.

procedure: file-executable? filename
  Returns `#t` if filename names a file that can be executed. Otherwise returns `#f`. Under unix, an executable file is identified by its mode bits. Under OS/2, an executable file has one of the file extensions ``.exe`', ``.com`', ``.cmd`', or ``.bat`'. Under Windows, an executable file has one of the file extensions ``.exe`', ``.com`', or ``.bat`'.

procedure: file-access filename mode
  Mode must be an exact integer between `0` and `7` inclusive; it is a bitwise-encoded predicate selector with `1` meaning "executable", `2` meaning "writeable", and `4` meaning "readable". `file-access` returns `#t` if filename exists and satisfies the predicates selected by mode. For example, if mode is `5`, then filename must be both readable and executable. If filename doesn't exist, or if it does not satisfy the selected predicates, `#f` is returned.

procedure: file-eq? filename1 filename2
  Determines whether filename1 and filename2 refer to the same file. Under unix, this is done by comparing the inodes and devices of the two files. Under OS/2 and Windows, this is done by comparing the filename strings.

procedure: file-modes filename
  If filename names an existing file, `file-modes` returns an exact non-negative integer encoding the file's permissions. The encoding of this integer is operating-system dependent. Under unix, it is the least-significant 12 bits of the `st_mode` element of the `struct stat` structure. Under OS/2 and Windows, it is the file attribute bits, which are described below. If filename does not name an existing file, `#f` is returned.

procedure: set-file-modes! filename modes
  Filename must name an existing file. Modes must be an exact non-negative integer that could have been returned by a call to `file-modes`. `set-file-modes!` modifies the file's permissions to be those encoded by modes.

variable: os2-file-mode/read-only
variable: os2-file-mode/hidden
variable: os2-file-mode/system
variable: os2-file-mode/directory
variable: os2-file-mode/archived
  The values of these variables are the "mode bits" that comprise the value returned by `file-modes` under OS/2. These bits are small integers that are combined by adding to form a complete set of modes. The integer zero represents a set of modes in which none of these bits are set.

variable: nt-file-mode/read-only
variable: nt-file-mode/hidden
variable: nt-file-mode/system
variable: nt-file-mode/directory
variable: nt-file-mode/archive
variable: nt-file-mode/normal
variable: nt-file-mode/temporary
variable: nt-file-mode/compressed
  The values of these variables are the "mode bits" that comprise the value returned by `file-modes` under Windows. These bits are small integers that are combined by adding to form a complete set of modes. The integer zero represents a set of modes in which none of these bits are set.

procedure: file-modification-time filename
  Returns the modification time of filename as an exact non-negative integer. The result may be compared to other file times using ordinary integer arithmetic. If filename names a file that does not exist, `file-modification-time` returns `#f`.

  In operating systems that support symbolic links, if filename names a symbolic link, `file-modification-time` returns the modification time of the file linked to. An alternate procedure, `file-modification-time-direct`, returns the modification time of the link itself; in all other respects it is identical to `file-modification-time`. For symmetry, `file-modification-time-indirect` is a synonym of `file-modification-time`.

procedure: file-access-time filename
  Returns the access time of filename as an exact non-negative integer. The result may be compared to other file times using ordinary integer arithmetic. If filename names a file that does not exist, `file-access-time` returns `#f`.

  In operating systems that support symbolic links, if filename names a symbolic link, `file-access-time` returns the access time of the file linked to. An alternate procedure, `file-access-time-direct`, returns the access time of the link itself; in all other respects it is identical to `file-access-time`. For symmetry, `file-access-time-indirect` is a synonym of `file-access-time`.

procedure: set-file-times! filename access-time modification-time
  Filename must name an existing file, while access-time and modification-time must be valid file times that might have been returned by `file-access-time` and `file-modification-time`, respectively. `set-file-times!` alters the access and modification times of the file specified by filename to the values given by access-time and modification-time, respectively. For convenience, either of the time arguments may be specified as `#f`; in this case the corresponding time is not changed. `set-file-times!` returns an unspecified value.

procedure: current-file-time
  Returns the current time as an exact non-negative integer, in the same format used by the above file-time procedures. This number can be compared to other file times using ordinary arithmetic operations.

procedure: file-touch filename
  Touches the file named filename. If the file already exists, its modification time is set to the current file time and `#f` is returned. Otherwise, the file is created and `#t` is returned. This is an atomic test-and-set operation, so it is useful as a synchronization mechanism.

procedure: file-length filename
  Returns the length, in bytes, of the file named filename as an exact non-negative integer.

procedure: file-attributes filename
  This procedure determines if the file named filename exists, and returns information about it if so; if the file does not exist, it returns `#f`.

  In operating systems that support symbolic links, if filename names a symbolic link, `file-attributes` returns the attributes of the link itself. An alternate procedure, `file-attributes-indirect`, returns the attributes of the file linked to; in all other respects it is identical to `file-attributes`. For symmetry, `file-attributes-direct` is a synonym of `file-attributes`.

The information returned by `file-attributes` is decoded by accessor procedures. The following accessors are defined in all operating systems:

procedure: file-attributes/type attributes
  The file type: `#t` if the file is a directory, a character string (the name linked to) if a symbolic link, or `#f` for all other types of file.

procedure: file-attributes/access-time attributes
  The last access time of the file, an exact non-negative integer.

procedure: file-attributes/modification-time attributes
  The last modification time of the file, an exact non-negative integer.

procedure: file-attributes/change-time attributes
  The last change time of the file, an exact non-negative integer.

procedure: file-attributes/length attributes
  The length of the file in bytes.

procedure: file-attributes/mode-string attributes
  The mode string of the file, a newly allocated string showing the file's mode bits. Under unix, this string is in unix format. Under OS/2 and Windows, this string shows the standard "DOS" attributes in their usual format.

procedure: file-attributes/n-links attributes
  The number of links to the file, an exact positive integer. Under Windows and OS/2, this is always `1`.

The following additional accessors are defined under unix:

procedure: file-attributes/uid attributes
  The user id of the file's owner, an exact non-negative integer.

procedure: file-attributes/gid attributes
  The group id of the file's group, an exact non-negative integer.

procedure: file-attributes/inode-number attributes
  The inode number of the file, an exact non-negative integer.

The following additional accessor is defined under OS/2 and Windows:

procedure: file-attributes/modes attributes
  The attribute bits of the file. This is an exact non-negative integer containing the file's attribute bits, exactly as specified by the operating system's API.

The following additional accessor is defined under OS/2:

procedure: file-attributes/allocated-length attributes
  The allocated length of the file, which can be larger than the length of the file due to fixed-length allocation units.

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## 15.4 Directory Reader

procedure: directory-read directory [sort?]
  Directory must be an object that can be converted into a pathname by
  `->pathname`. The directory specified by directory is read, and the contents of the directory is returned as a newly allocated list of absolute pathnames. The result is sorted according to the usual sorting conventions for directories, unless sort? is specified as `#f`. If directory has name, type, or version components, the returned list contains only those pathnames whose name, type, and version components match those of directory; `wild` or `#f` as one of these components means "match anything".

  The OS/2 and Windows implementations support "globbing", in which the characters `*` and `?` are interpreted to mean "match anything" and "match any character", respectively. This "globbing" is supported only in the file part of directory.

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## 15.5 Date and Time

MIT/GNU Scheme provides a simple set of procedures for manipulating date and time information. There are four time representations, each of which serves a different purpose. Each representation may be converted to any of the others.

The primary time representation, universal time, is an exact non-negative integer counting the number of seconds that have elapsed since midnight January 1, 1900 UTC. (UTC stands for Coordinated Universal Time, and is the modern name for Greenwich Mean Time.) This format is produced by `get-universal-time` and `decoded-time->universal-time`.

The second representation, decoded time, is a record structure in which the time is broken down into components, such as month, minute, etc. Decoded time is always relative to a particular time zone, which is a component of the structure. This format is produced by `global-decoded-time` and `local-decoded-time`.

The third representation, file time, is an exact non-negative integer that is larger for increasing time. Unlike universal time, this representation is operating-system dependent. This format is produced by all of the file-attribute procedures, for example `file-modification-time` and `file-attributes`.

The fourth representation, the time string, is an external representation for time. This format is defined by RFC-822, Standard for the format of ARPA Internet text messages, with the modification that years are represented as four-digit numbers rather than two-digit numbers. This format is the standard format for Internet email and numerous other network protocols.

Within this section, argument variables named universal-time, decoded-time, file-time, and time-string are respectively required to be of the corresponding format.

> 15.5.1 Universal Time |  |
> 15.5.2 Decoded Time |  |
> 15.5.3 File Time |  |
> 15.5.4 Time-Format Conversion |  |
> 15.5.5 External Representation of Time |  |

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### 15.5.1 Universal Time

procedure: get-universal-time
  Return the current time in universal format.

      (get-universal-time) => 3131453078

variable: epoch
  `epoch` is the representation of midnight January 1, 1970 UTC in universal-time format.

      epoch => 2208988800

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### 15.5.2 Decoded Time

Objects representing standard time components, such as seconds and minutes, are required to be exact non-negative integers. Seconds and minutes must be inclusively between `0` and `59`; hours between `0` and `23`; days between `1` and `31`; months between `1` and `12`; years are represented in "four-digit" form, in which 1999 is represented as `1999` --- not `99`.

procedure: local-decoded-time
  Return the current time in decoded format. The decoded time is represented in the local time zone.

      (pp (local-decoded-time))
      -| #[decoded-time 76]
      -| (second 2)
      -| (minute 12)
      -| (hour 11)
      -| (day 27)
      -| (month 4)
      -| (year 1999)
      -| (day-of-week 1)
      -| (daylight-savings-time 1)
      -| (zone 5)

procedure: global-decoded-time
  Return the current time in decoded format. The decoded time is represented in UTC.

      (pp (global-decoded-time))
      -| #[decoded-time 77]
      -| (second 8)
      -| (minute 12)
      -| (hour 15)
      -| (day 27)
      -| (month 4)
      -| (year 1999)
      -| (day-of-week 1)
      -| (daylight-savings-time 0)
      -| (zone 0)

procedure: make-decoded-time second minute hour day month year [zone]
  Return a new decoded-time object representing the given time. The arguments must be valid components according to the above rules, and must form a valid date.

  If zone is not supplied or is `#f`, the resulting decoded time will be represented in the local time zone. Otherwise, zone must be a valid time zone, and the result will be represented in that zone.

  Warning: because this procedure depends on the operating system's runtime library, it is not capable of representing all dates. In particular, on most unix systems, it is not possible to encode dates that occur prior to midnight, January 1, 1970 UTC. Attempting to do this will signal an error.

      (pp (make-decoded-time 0 9 11 26 3 1999))
      -| #[decoded-time 19]
      -| (second 0)
      -| (minute 9)
      -| (hour 11)
      -| (day 26)
      -| (month 3)
      -| (year 1999)
      -| (day-of-week 4)
      -| (daylight-savings-time 0)
      -| (zone 5)

      (pp (make-decoded-time 0 9 11 26 3 1999 3))
      -| #[decoded-time 80]
      -| (second 0)
      -| (minute 9)
      -| (hour 11)
      -| (day 26)
      -| (month 3)
      -| (year 1999)
      -| (day-of-week 4)
      -| (daylight-savings-time 0)
      -| (zone 3)

procedure: decoded-time/second decoded-time
procedure: decoded-time/minute decoded-time
procedure: decoded-time/hour decoded-time
procedure: decoded-time/day decoded-time
procedure: decoded-time/month decoded-time
procedure: decoded-time/year decoded-time
  Return the corresponding component of decoded-time.

      (decoded-time/second (local-decoded-time)) => 17
      (decoded-time/year (local-decoded-time)) => 1999
      (decoded-time/day (local-decoded-time)) => 26

procedure: decoded-time/day-of-week decoded-time
  Return the day of the week on which decoded-time falls, encoded as an exact integer between `0` (Monday) and `6` (Sunday), inclusive.

      (decoded-time/day-of-week (local-decoded-time)) => 4

procedure: decoded-time/daylight-savings-time? decoded-time
  Return `#t` if decoded-time is represented using daylight savings time. Otherwise return `#f`.

      (decoded-time/daylight-savings-time? (local-decoded-time))
                        => #f

procedure: decoded-time/zone decoded-time
  Return the time zone in which decoded-time is represented. This is an exact rational number between `-24` and `+24` inclusive, that when multiplied by `3600` is an integer. The value is the number of hours west of UTC.

      (decoded-time/zone (local-decoded-time)) => 5

procedure: time-zone? object
  Returns `#t` if object is an exact number between `-24` and `+24` inclusive, that when multiplied by `3600` is an integer.

      (time-zone? -5)   => #t
      (time-zone? 11/2) => #t
      (time-zone? 11/7) => #f

procedure: month/max-days month
  Returns the maximum number of days possible in month. Month must be an exact integer between `1` and `12` inclusive.

      (month/max-days 2) => 29
      (month/max-days 3) => 31
      (month/max-days 4) => 30

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### 15.5.3 File Time

As stated above, file time is operating-system dependent. As of this writing, two formats are used. For unix and Windows systems, file time is the number of seconds since midnight January 1, 1970 UTC (the standard unix time convention).

OS/2 represents file time as a 32-bit unsigned integer, in which the time components are broken down into unsigned bit fields. The components are always stated in local time. The fields, from MSB to LSB, are:

- 7 bits representing the year, relative to 1900.

- 4 bits representing the month, numbered 1 to 12.

- 5 bits representing the day of the month, numbered 1 to 31.

- 5 bits representing the hour of the day, numbered 0 to 23.

- 6 bits representing the minute, numbered 0 to 59.

- 5 bits representing the second. This field is unusual in that it counts units of two seconds, so it is a number between 0 and 29, representing second counts corresponding to 0 through 58.

The following procedures generate their results in file-time format:

    file-access-time
    file-access-time-direct
    file-access-time-indirect
    file-modification-time
    file-modification-time-direct
    file-modification-time-indirect
    file-attributes/access-time
    file-attributes/modification-time
    file-attributes/change-time

Additionally, `set-file-times!` accepts its time arguments in file-time format.

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### 15.5.4 Time-Format Conversion

The procedures described in this section convert times from one format to another.

procedure: universal-time->local-decoded-time universal-time
procedure: universal-time->global-decoded-time universal-time
  Converts an argument in universal-time format to decoded-time format. The result is in the local time zone or UTC, respectively.

      (pp (universal-time->local-decoded-time (get-universal-time)))
      -| #[decoded-time 21]
      -| (second 23)
      -| (minute 57)
      -| (hour 17)
      -| (day 29)
      -| (month 4)
      -| (year 1999)
      -| (day-of-week 3)
      -| (daylight-savings-time 1)
      -| (zone 5)

      (pp (universal-time->global-decoded-time
           (get-universal-time)))
      -| #[decoded-time 22]
      -| (second 27)
      -| (minute 57)
      -| (hour 21)
      -| (day 29)
      -| (month 4)
      -| (year 1999)
      -| (day-of-week 3)
      -| (daylight-savings-time 0)
      -| (zone 0)

procedure: universal-time->file-time universal-time
  Converts an argument in universal-time format to file-time format.

      (universal-time->file-time (get-universal-time))
          => 925422988

procedure: universal-time->local-time-string universal-time
procedure: universal-time->global-time-string universal-time
  Converts an argument in universal-time format to a time string. The result is in the local time zone or UTC, respectively.

      (universal-time->local-time-string (get-universal-time))
          => "Thu, 29 Apr 1999 17:55:31 -0400"
      (universal-time->global-time-string (get-universal-time))
          => "Thu, 29 Apr 1999 21:55:51 +0000"

procedure: decoded-time->universal-time decoded-time
  Converts an argument in decoded-time format to universal-time format.

      (decoded-time->universal-time (local-decoded-time))
          => 3134411942
      (decoded-time->universal-time (global-decoded-time))
          => 3134411947

procedure: decoded-time->file-time decoded-time
  Converts an argument in decoded-time format to file-time format.

      (decoded-time->file-time (local-decoded-time))
          => 925423191
      (decoded-time->file-time (global-decoded-time))
          => 925423195

procedure: decoded-time->string decoded-time
  Convert an argument in decoded-time format to a time string.

      (decoded-time->string (local-decoded-time))
          => "Thu, 29 Apr 1999 18:00:43 -0400"
      (decoded-time->string (global-decoded-time))
          => "Thu, 29 Apr 1999 22:00:46 +0000"

procedure: file-time->universal-time file-time
  Converts an argument in universal-time format to file-time format.

      (file-time->universal-time (file-modification-time "/"))
          => 3133891907

procedure: file-time->local-decoded-time file-time
procedure: file-time->global-decoded-time file-time
  Converts an argument in file-time format to decoded-time format. The result is in the local time zone or UTC, respectively.

      (pp (file-time->local-decoded-time
           (file-modification-time "/")))
      -| #[decoded-time 26]
      -| (second 47)
      -| (minute 31)
      -| (hour 17)
      -| (day 23)
      -| (month 4)
      -| (year 1999)
      -| (day-of-week 4)
      -| (daylight-savings-time 1)
      -| (zone 5)

      (pp (file-time->global-decoded-time
           (file-modification-time "/")))
      -| #[decoded-time 27]
      -| (second 47)
      -| (minute 31)
      -| (hour 21)
      -| (day 23)
      -| (month 4)
      -| (year 1999)
      -| (day-of-week 4)
      -| (daylight-savings-time 0)
      -| (zone 0)

procedure: file-time->local-time-string file-time
procedure: file-time->global-time-string file-time
  Converts an argument in file-time format to a time string. The result is in the local time zone or UTC, respectively.

      (file-time->local-time-string (file-modification-time "/"))
          => "Fri, 23 Apr 1999 17:31:47 -0400"
      (file-time->global-time-string (file-modification-time "/"))
          => "Fri, 23 Apr 1999 21:31:47 +0000"

procedure: string->universal-time time-string
  Converts a time-string argument to universal-time format.

      (string->universal-time "Fri, 23 Apr 1999 21:31:47 +0000")
          => 3133888307
      (string->universal-time "Fri, 23 Apr 1999 17:31:47 -0400")
          => 3133888307

procedure: string->decoded-time time-string
  Converts a time-string argument to decoded-time format.

      (pp (string->decoded-time "Fri, 23 Apr 1999 17:31:47 -0400"))
      -| #[decoded-time 30]
      -| (second 47)
      -| (minute 31)
      -| (hour 17)
      -| (day 23)
      -| (month 4)
      -| (year 1999)
      -| (day-of-week 4)
      -| (daylight-savings-time 0)
      -| (zone 4)

procedure: string->file-time time-string
  Converts a time-string argument to file-time format.

      (string->file-time "Fri, 23 Apr 1999 17:31:47 -0400")
          => 924899507

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### 15.5.5 External Representation of Time

The normal external representation for time is the time string, as described above. The procedures in this section generate alternate external representations of time which are more verbose and may be more suitable for presentation to human readers.

procedure: decoded-time/date-string decoded-time
procedure: decoded-time/time-string decoded-time
  These procedures return strings containing external representations of the date and time, respectively, represented by decoded-time. The results are implicitly in local time.

      (decoded-time/date-string (local-decoded-time))
          => "Tuesday March 30, 1999"
      (decoded-time/time-string (local-decoded-time))
          => "11:22:38 AM"

procedure: day-of-week/long-string day-of-week
procedure: day-of-week/short-string day-of-week
  Returns a string representing the given day-of-week. The argument must be an exact non-negative integer between `0` and `6` inclusive. `day-of-week/long-string` returns a long string that fully spells out the name of the day. `day-of-week/short-string` returns a shortened string that abbreviates the day to three letters.

      (day-of-week/long-string 0)  => "Monday"
      (day-of-week/short-string 0) => "Mon"
      (day-of-week/short-string 3) => "Thu"

procedure: month/long-string month
procedure: month/short-string month
  Returns a string representing the given month. The argument must be an exact non-negative integer between `1` and `12` inclusive. `month/long-string` returns a long string that fully spells out the name of the month. `month/short-string` returns a shortened string that abbreviates the month to three letters.

      (month/long-string 1)   => "January"
      (month/short-string 1)  => "Jan"
      (month/short-string 10) => "Oct"

procedure: time-zone->string
  Returns a string corresponding to the given time zone. This string is the same string that is used to generate RFC-822 time strings.

      (time-zone->string 5)    => "-0500"
      (time-zone->string -4)   => "+0400"
      (time-zone->string 11/2) => "-0530"

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## 15.6 Machine Time

The previous section dealt with procedures that manipulate clock time. This section describes procedures that deal with computer time: elapsed CPU time, elapsed real time, and so forth. These procedures are useful for measuring the amount of time it takes to execute code.

Some of the procedures in this section manipulate a time representation called ticks. A tick is a unit of time that is unspecified here but can be converted to and from seconds by supplied procedures. A count in ticks is represented as an exact integer. At present each tick is one millisecond, but this may change in the future.

procedure: process-time-clock
  Returns the amount of process time, in ticks, that has elapsed since Scheme was started. Process time is measured by the operating system and is time during which the Scheme process is computing. It does not include time in system calls, but depending on the operating system it may include time used by subprocesses.

      (process-time-clock) => 21290

procedure: real-time-clock
  Returns the amount of real time, in ticks, that has elapsed since Scheme was started. Real time is the time measured by an ordinary clock.

      (real-time-clock) => 33474836

procedure: internal-time/ticks->seconds ticks
  Returns the number of seconds corresponding to ticks. The result is always a real number.

      (internal-time/ticks->seconds 21290) => 21.29
      (internal-time/ticks->seconds 33474836) => 33474.836

procedure: internal-time/seconds->ticks seconds
  Returns the number of ticks corresponding to seconds. Seconds must be a real number.

      (internal-time/seconds->ticks 20.88) => 20880
      (internal-time/seconds->ticks 20.83) => 20830

procedure: system-clock
  Returns the amount of process time, in seconds, that has elapsed since Scheme was started. Roughly equivalent to:

      (internal-time/ticks->seconds (process-time-clock))

  Example:

      (system-clock) => 20.88

procedure: runtime
  Returns the amount of process time, in seconds, that has elapsed since Scheme was started. However, it does not include time spent in garbage collection.

      (runtime) => 20.83

procedure: with-timings thunk receiver
  Calls thunk with no arguments. After thunk returns, receiver is called with three arguments describing the time spent while computing thunk: the elapsed run time, the amount of time spent in the garbage collector, and the elapsed real time. All three times are in ticks.

  This procedure is most useful for doing performance measurements, and is designed to have relatively low overhead.

      (with-timings
       (lambda () ... hairy computation ...)
       (lambda (run-time gc-time real-time)
         (write (internal-time/ticks->seconds run-time))
         (write-char #\space)
         (write (internal-time/ticks->seconds gc-time))
         (write-char #\space)
         (write (internal-time/ticks->seconds real-time))
         (newline)))

procedure: measure-interval runtime? procedure
  Calls procedure, passing it the current process time, in seconds, as an argument. The result of this call must be another procedure. When procedure returns, the resulting procedure is tail-recursively called with the ending time, in seconds, as an argument.

  If runtime? is `#f`, the elapsed time is deducted from the elapsed system time returned by `runtime`.

  While this procedure can be used for time measurement, its interface is somewhat clumsy for that purpose. We recommend that you use `with-timings` instead, because it is more convenient and has lower overhead.

      (measure-interval #t
                        (lambda (start-time)
                          (let ((v ... hairy computation ...))
                            (lambda (end-time)
                              (write (- end-time start-time))
                              (newline)
                              v))))

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## 15.7 Subprocesses

MIT/GNU Scheme provides the ability to run and control subprocesses. This support is divided into two parts: a low-level set of primitives that maps onto the underlying operating system's process-control primitives, and a high-level set of procedures for starting a subprocess and running it to completion in a single call. Subprocesses that are run in the latter fashion are referred to as synchronous, because they are started and stopped in synchrony with a Scheme procedure call.

This chapter documents Scheme's high-level synchronous-subprocess support. The low-level support is not documented but is available for those who are willing to read the source code.

Synchronous-subprocess support is a run-time-loadable option. To use it, execute

    (load-option 'synchronous-subprocess)

once before calling it.

> 15.7.1 Subprocess Procedures |  |
> 15.7.2 Subprocess Conditions |  |
> 15.7.3 Subprocess Options |  |

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### 15.7.1 Subprocess Procedures

There are two commands for running synchronous subprocesses under Scheme. `run-shell-command` is very simple to use, provides access to all shell features, and is to be preferred in most situations. `run-synchronous-subprocess` allows direct execution of a program and precise control of the command-line arguments passed to the program, but does not provide file globbing, I/O redirection, or other shell features.

procedure: run-shell-command command option ...
  Runs command, which must be a string. Command is passed to a command shell for interpretation; how the shell is chosen is detailed below.

  The options are a sequence of keyword/value pairs that specify optional behavior. See below for more information about options.

  `run-shell-command` waits until the subprocess completes its execution and returns the exit code from the subprocess. If the subprocess is killed or stopped, an error is signalled and the procedure does not return.

procedure: run-synchronous-subprocess program arguments option ...
  Runs program, passing it the given command-line arguments. Program must be either the name of a program on the path, or else a pathname to a specific program. Arguments must be a list of strings; each string is a single command-line argument to the program.

  The options are a sequence of keyword/value pairs that specify optional behavior. See below for more information about options.

  `run-synchronous-subprocess` waits until the subprocess completes its execution and returns the exit code from the subprocess. If the subprocess is killed or stopped, an error is signalled and the procedure does not return.

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### 15.7.2 Subprocess Conditions

If a subprocess spawned by one of the above procedures is killed or suspended, then one of the following errors will be signalled.

condition type: condition-type:subprocess-signalled subprocess reason
  This condition type is a subtype of `condition-type:subprocess-abnormal-termination`. It is signalled when the subprocess is killed.

  Subprocess is an object that represents the subprocess involved. The internals of this object can be accessed but the interface is not documented at this time; see the source code for details.

  Reason is interesting only on unix systems, where it is the signal that killed the process. On other systems it has a fixed value that conveys no useful information.

condition type: condition-type:subprocess-stopped subprocess reason
  This condition type is a subtype of `condition-type:subprocess-abnormal-termination`. It is signalled when the subprocess is stopped or suspended.

  Subprocess is an object that represents the subprocess involved. The internals of this object can be accessed but the interface is not documented at this time; see the source code for details.

  Reason is interesting only on unix systems, where it is the signal that stopped the process. On other systems it has a fixed value that conveys no useful information.

condition type: condition-type:subprocess-abnormal-termination subprocess reason
  This condition type is a subtype of `condition-type:error`. This is an abstract type that is never signalled. It is provided so that condition handlers can be bound to it.

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### 15.7.3 Subprocess Options

The following subprocess options may be passed to `run-shell-command` or `run-synchronous-subprocess`. These options are passed as alternating keyword/value pairs, for example:

    (run-shell-command "ls /"
                       'output my-output-port
                       'output-buffer-size 8192)

The example shows a shell command being run with two options specified: `output` and `output-buffer-size`.

subprocess option: input port
  Specifies the standard input of the subprocess. Port may be an input port, in which case characters are read from port and fed to the subprocess until port reaches end-of-file. Alternatively, port may be `#f`, indicating that the subprocess has no standard input.

  The default value of this option is `#f`.

      (call-with-input-file "foo.in"
        (lambda (port)
          (run-shell-command "cat > /dev/null" 'input port)))

subprocess option: input-line-translation line-ending
  Specifies how line-endings should be translated when writing characters to the subprocess. Ignored if the `input` option is `#f`. Line-ending must be either a string specifying the line ending, or the symbol `default`, meaning to use the operating system's standard line ending. In either case, newline characters to be written to the `input` port are translated to the specified line ending before being written.

  The default value of this option is `default`.

      (call-with-input-file "foo.in"
        (lambda (port)
          (run-shell-command "cat > /dev/null"
                             'input port
                             'input-line-translation "\r\n")))

subprocess option: input-buffer-size n
  Specifies the size of the input buffer for the standard input of the subprocess. (This is the buffer on the Scheme side, and has nothing to do with any buffering done on the subprocess side.) Ignored if the `input` option is `#f`. N must be an exact positive integer specifying the number of characters the buffer can hold.

  The default value of this option is `512`.

      (call-with-input-file "foo.in"
        (lambda (port)
          (run-shell-command "cat > /dev/null"
                             'input port
                             'input-buffer-size 4096)))

subprocess option: output port
  Specifies the standard output and standard error of the subprocess. Port may be an output port, in which case characters are read from the subprocess and fed to port until the subprocess finishes. Alternatively, port may be `#f`, indicating that the subprocess has no standard output or standard error.

  The default value of this option is the value of `(current-output-port)`.

      (call-with-output-file "foo.out"
        (lambda (port)
          (run-shell-command "ls -la /etc" 'output port)))

subprocess option: output-line-translation line-ending
  Specifies how line-endings should be translated when reading characters from the standard output of the subprocess. Ignored if the `output` option is `#f`. Line-ending must be either a string specifying the line ending, or the symbol `default`, meaning to use the operating system's standard line ending. In either case, newline characters read from the subprocess port are translated to the specified line ending.

  The default value of this option is `default`.

      (call-with-output-file "foo.out"
        (lambda (port)
          (run-shell-command "ls -la /etc"
                             'output port
                             'output-line-translation "\r\n")))

subprocess option: output-buffer-size n
  Specifies the size of the output buffer for the standard output of the subprocess. (This is the buffer on the Scheme side, and has nothing to do with any buffering done on the subprocess side.) Ignored if the `output` option is `#f`. N must be an exact positive integer specifying the number of characters the buffer can hold.

  The default value of this option is `512`.

      (call-with-output-file "foo.out"
        (lambda (port)
          (run-shell-command "ls -la /etc"
                             'output port
                             'output-buffer-size 4096)))

subprocess option: redisplay-hook thunk
  Specifies that thunk is to be run periodically when output from the subprocess is available. Thunk must be a procedure of no arguments, or `#f` indicating that no hook is supplied. This option is mostly useful for interactive systems. For example, the Edwin text editor uses this to update output buffers when running some subprocesses.

  The default value of this option is `#f`.

      (run-shell-command "ls -la /etc"
                         'redisplay-hook
                         (lambda ()
                           (update-buffer-contents buffer)))

subprocess option: environment environment
  Specifies the environment variables that are to be used for the subprocess. Environment must be either a vector of strings or `#f` indicating the default environment. If it is a vector of strings, each string must be a name/value pair where the name and value are separated by an equal sign, for example, `"foo=bar"`. To define a variable with no value, just omit the value, as in `"foo="`.

  Note that the variable `scheme-subprocess-environment` is bound to the default subprocess environment.

  The default value of this option is `#f`.

      (run-shell-command "ls -la /etc"
                         'environment
                         (let* ((v scheme-subprocess-environment)
                                (n (vector-length v))
                                (v (vector-grow v (+ n 1))))
                           (vector-set! v n "TERM=none")
                           v))

subprocess option: working-directory pathname
  Specifies the working directory in which the subprocess will run.

  The default value of this option is `(working-directory-pathname)`.

      (run-shell-command "ls -la" 'working-directory "/etc/")

subprocess option: use-pty? boolean
  This option is meaningful only on unix systems; on other systems it is ignored. Specifies whether to communicate with the subprocess using PTY devices; if true, PTYs will be used, otherwise pipes will be used.

  The default value of this option is `#f`.

      (run-shell-command "ls -la /etc" 'use-pty? #t)

subprocess option: shell-file-name pathname
  Specifies the shell program to use for `run-shell-command`.

  The default value of this option is `(os/shell-file-name)`. This is the value of the environment variable `SHELL`, or if `SHELL` is not set, the value is operating-system dependent as follows:

  - On unix systems, ``/bin/sh`' is used.

  - On OS/2 systems, the value of the environment variable `COMSPEC` is used, or if that is not set, ``cmd.exe`' on the current path.

  - On Windows systems, the value of the environment variable `COMSPEC` is used. If that is not set, ``cmd.exe`' is used for Windows NT, or ``command.com`' is used for Windows 9x; in each case the shell is found by searching the path.

      (run-shell-command "ls -la /etc"
                         'shell-file-name "/usr/local/bin/bash")

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## 15.8 TCP Sockets

MIT/GNU Scheme provides access to sockets, which are a mechanism for inter-process communication. TCP stream sockets are supported, which communicate between computers over a TCP/IP network. TCP sockets are supported on all operating systems.

TCP sockets have two distinct interfaces: one interface to implement a client and another to implement a server. The basic protocol is that servers set up a listening port and wait for connections from clients. Implementation of clients is simpler and will be treated first.

The socket procedures accept two special arguments, called host-name and service. Host-name is a string which must be the name of an internet host. It is looked up using the ordinary lookup rules for your computer. For example, if your host is `foo.mit.edu` and host-name is `"bar"`, then it specifies `bar.mit.edu`.

Service specifies the service to which you will connect. A networked computer normally provides several different services, such as telnet or FTP. Each service is associated with a unique port number; for example, the `"www"` service is associated with port `80`. The service argument specifies the port number, either as a string, or directly as an exact non-negative integer. Port strings are decoded by the operating system using a table; for example, on unix the table is in ``/etc/services`'. Usually you will use a port string rather than a number.

procedure: open-tcp-stream-socket host-name service [buffer-size [line-translation]]
  `open-tcp-stream-socket` opens a connection to the host specified by host-name. Host-name is looked up using the ordinary lookup rules for your computer. The connection is established to the service specified by service. The returned value is an I/O port, to which you can read and write characters using ordinary Scheme I/O procedures such as `read-char` and `write-char`.

  Buffer-size specifies the size of the read and write buffers used by the port; if this is unspecified or `#f`, the buffers will hold `4096` bytes.

  Line-translation specifies how end-of-line characters will be translated when reading or writing to the socket. If this is unspecified or `#f`, then lines will be terminated by CR-LF, which is the standard for most internet protocols. Otherwise, it must be a string, which specifies the line-ending character sequence to use.

  When you wish to close the connection, just use `close-port`.

  As an example, here is how you can open a connection to a web server:

      (open-tcp-stream-socket "web.mit.edu" "www")

Next we will treat setting up a TCP server, which is slightly more complicated. Creating a server is a two-part process. First, you must open a server socket, which causes the operating system to listen to the network on a port that you specify. Once the server socket is opened, the operating system will allow clients to connect to your computer on that port.

In the second step of the process, you accept the connection, which completes the connection initiated by the client, and allows you to communicate with the client. Accepting a connection does not affect the server socket; it continues to listen for additional client connections. You can have multiple client connections to the same server socket open simultaneously.

procedure: open-tcp-server-socket service [address]
  This procedure opens a server socket that listens for connections to service; the socket will continue to listen until you close it. The returned value is a server socket object.

  An error is signalled if another process is already listening on the service. Additionally, ports whose number is less than `1024` are privileged on many operating systems, and cannot be used by non-privileged processes; if service specifies such a port and you do not have administrative privileges, an error may be signalled.

  The optional argument address specifies the IP address on which the socket will listen. If this argument is not supplied or is given as `#f`, then the socket listens on all IP addresses for this machine. (This is equivalent to passing the result of calling `host-address-any`.)

procedure: tcp-server-connection-accept server-socket block? peer-address [line-translation]
  Checks to see if a client has connected to server-socket. If so, an I/O port is returned. The returned port can be read and written using ordinary Scheme I/O procedures such as `read-char` and `write-char`.

  The argument block? says what to do if no client has connected at the time of the call. If `#f`, it says to return immediately with two values of `#f`. Otherwise, the call waits until a client connects.

  The argument peer-address is either `#f` or an IP address as allocated by `allocate-host-address`. If it is an IP address, the address is modified to be the address of the client making the connection.

  The optional argument line-translation specifies how end-of-line characters will be translated when reading or writing to the returned socket. If this is unspecified or `#f`, then lines will be terminated by CR-LF, which is the standard for most internet protocols. Otherwise, it must be a string, which specifies the line-ending character sequence to use.

  Note that closing the port returned by this procedure does not affect server-socket; it just closes the particular client connection that was opened by the call. To close server-socket, use `close-tcp-server-socket`.

procedure: close-tcp-server-socket server-socket
  Closes the server socket server-socket. The operating system will cease listening for network connections to that service. Client connections to server-socket that have already been accepted will not be affected.

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## 15.9 Miscellaneous OS Facilities

This section contains assorted operating-system facilities that don't fit into other categories.

variable: microcode-id/operating-system
variable: microcode-id/operating-system-name
  `microcode-id/operating-system` is bound to a symbol that specifies the type of operating system that Scheme is running under. There are three possible values: `unix`, `os/2`, or `nt`.

  `microcode-id/operating-system-name` is a string containing the same name as `microcode-id/operating-system`; the latter is created by interning the former as a symbol.

variable: microcode-id/operating-system-variant
  This variable is a string that identifies the particular variant of the operating system that Scheme is running under. Here are some of the possible values:

      "GNU/Linux"
      "FreeBSD"
      "HP-UX"
      "SunOS"
      "OS/2 2.1"
      "OS/2 4.0"
      "Microsoft Windows NT 4.0 (Build 1381; Service Pack 3)"
      "Microsoft Windows 98 (Build 410)"

  For Windows systems, it is recommended that you match on the prefix of this string and ignore the `"Build"` suffix. This is because the suffix may contain information about service packs or fixes, while the prefix will be constant for a particular version of Windows.

The next few procedures provide access to the domain name service (DNS), which maintains associations between internet host names such as `"www.swiss.ai.mit.edu"` and IP addresses, such as `18.23.0.16`. In MIT/GNU Scheme, we represent an internet host name as a string, and an IP address as a byte vector of length 4 (byte vectors are just character strings that are accessed using `vector-8b-ref` rather than `string-ref`). The bytes in an IP address read in the same order as they do when written out:

    (get-host-by-name "www.swiss") => #("\022\027\000\020")

procedure: get-host-by-name host-name
  Looks up the internet host name host-name using the DNS, returning a vector of IP addresses for the corresponding host, or `#f` if there is no such host. Usually the returned vector has only one element, but if a host has more than one network interface, the vector might have more than one element.

      (get-host-by-name "www.swiss") => #("\022\027\000\020")

procedure: get-host-by-address ip-address
  Does a reverse DNS lookup on ip-address, returning the internet host name corresponding to that address, or `#f` if there is no such host.

      (get-host-by-address "\022\027\000\020") => "swissnet.ai.mit.edu"

procedure: canonical-host-name host-name
  Finds the "canonical" internet host name for host-name. For example:

      (canonical-host-name "zurich")    => "zurich.ai.mit.edu"
      (canonical-host-name "www.swiss") => "swissnet.ai.mit.edu"

  In both examples, the default internet domain ``ai.mit.edu`' is added to host-name. In the second example, `"www.swiss"` is an alias for another computer named `"swissnet"`.

procedure: get-host-name
  Returns the string that identifies the computer that MIT/GNU Scheme is running on. Usually this is an unqualified internet host name, i.e. the host name without the domain suffix:

      (get-host-name) => "aarau"

procedure: os/hostname
  Returns the canonical internet host name of the computer that MIT/GNU Scheme is running on. So, in contrast to the example for `get-host-name`:

      (os/hostname) => "aarau.ai.mit.edu"

procedure: allocate-host-address
  Allocates and returns an IP address object. This is just a string of a fixed length (current 4 bytes) into which an IP address may be stored. This procedure is used to generate an appropriate argument to be passed to `tcp-server-connection-accept`.

      (allocate-host-address) => "Xe\034\241"

procedure: host-address-any
  Return an IP address object that specifies "any host". This object is useful only when passed as the address argument to `open-tcp-server-socket`.

      (host-address-any) => "\000\000\000\000"

procedure: host-address-loopback
  Return an IP address object that specifies the local loopback network interface. The loopback interface is a software network interface that can be used only for communicating between processes on the same computer. This address object is useful only when passed as the address argument to `open-tcp-server-socket`.

      (host-address-loopback) => "\177\000\000\001"

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