PLT MzScheme: Language Manual

System Utilities

15.1  Time

15.1.1  Real Time and Date

(current-seconds) returns the current time in seconds. This time is always an exact integer based on a platform-specific starting date (with a platform-specific minimum and maximum value).

The value of (current-seconds) increases as time passes (increasing by 1 for each second that passes). The current time in seconds can be compared with a time returned by file-or-directory-modify-seconds (see section 11.3.2).

(seconds->date secs-n) takes secs-n, a platform-specific time in seconds (an exact integer) returned by current-seconds or file-or-directory-modify-seconds, and returns an instance of the date structure type, which has the following fields:

  • second : 0 to 61 (60 and 61 are for unusual leap-seconds)

  • minute : 0 to 59

  • hour : 0 to 23

  • day : 1 to 31

  • month : 1 to 12

  • year : e.g., 1996

  • week-day : 0 (Sunday) to 6 (Saturday)

  • year-day : 0 to 365 (364 in non-leap years)

  • dst? : #t (daylight savings time) or #f

  • time-zone-offset : the number of seconds east of GMT for this time zone (e.g., Pacific Standard Time is -28800), an exact integer 36

All fields of the date structure type are accessible by all inspectors (see section 4.6).

The value returned by current-seconds or file-or-directory-modify-seconds is not portable among platforms. Convert a time in seconds using seconds->date when portability is needed.

See also Chapter 11 in PLT MzLib: Libraries Manual for additional date utilities.

15.1.2  Machine Time

(current-milliseconds) returns the current ``time'' in fixnum milliseconds. This time is based on a platform-specific starting date or on the machine's startup time. Since the result is a fixnum, the value is only strictly increasing for a limited (though reasonably long) time.

(current-process-milliseconds) returns the amount of processor time in fixnum milliseconds that has been consumed by the MzScheme process on the underlying operating system. (Under Unix and Mac OS X, this includes both user and system time.) The precision of the result is platform-specific, and since the result is a fixnum, the value is only strictly increasing for a limited (though reasonably long) time.

(current-gc-milliseconds) returns the amount of processor time in fixnum milliseconds that has been consumed by MzScheme's garbage collection so far. This time is a portion of the time reported by (current-process-milliseconds).

15.1.3  Timing Execution

The time-apply procedure collects timing information for a procedure application:

  • (time-apply proc arg-list) invokes the procedure proc with the arguments in arg-list. Four values are returned: a list containing the result(s) of applying proc, the number of milliseconds of CPU time required to obtain this result, the number of ``real'' milliseconds required for the result, and the number of milliseconds of CPU time (included in the second result) spent on garbage collection.

The reliability of the timing numbers depends on the platform; see section 15.1.2 for more information on time accounting. If multiple MzScheme threads are running, then the reported time may include work performed by other threads.

The time syntactic form reports timing information directly to the current output port:

  • (time expr) times the evaluation of expr, printing timing information to the current output port. The result of the time expression is the result of expr.

15.2  Operating System Processes

(subprocess stdout-output-port stdin-input-port stderr-output-port command-path arg-string ···) creates a new process in the underlying operating system to execute command-path asynchronously. The command-path argument is a path to a program executable, and the arg-strings are command-line arguments for the program.

Under Mac OS Classic, arguments are not supported. However, a single arg-string argument triggers a hack specific to Mac OS Classic; in that case, command-path must be "by-id" and the arg-string argument is a four-character string specifying an application ID, which is used to find the application instead of an executable path.

Under Windows, the first arg-string can be 'exact, which triggers a Windows-specific hack: the second arg-string is used exactly as the command-line for the subprocess, and no additional arg-strings can be supplied. Otherwise, a command-line string is constructed from command-path and arg-string so that a typical Windows console application can parse it back to an array of arguments.37 If 'exact is provided on a non-Windows platform, the exn:application:mismatch exception is raised.

Unless it is #f, stdout-output-port is used for the launched process's standard output, stdin-input-port is used for the process's standard input, and stderr-output-port is used for the process's standard error. All provided ports must be file-stream ports. Any of the ports can be #f, in which case a system pipe is created and returned by subprocess. For each port that is provided, no pipe is created and the corresponding returned value is #f.

The subprocess procedure returns four values:

  • a subprocess value representing the create process;

  • an input port piped from the process's standard output, or #f if stdout-output-port was a port;

  • an output port piped to the process standard input, or #f if stdin-input-port was a port;

  • an input port piped from the process's standard error, or #f if stderr-output-port was a port.

Important: All ports returned from subprocess must be explicitly closed with close-input-port and close-output-port.

The returned ports are placed into the management of the current custodian (see section 9.2). The exn:misc exception is raised when a low-level error prevents the spawning of a process or the creation of operating system pipes for process communication.

A subprocess value can be used to obtain further information about the process:

  • (subprocess-wait subprocess) blocks until the process terminates, then returns void.

  • (subprocess-status subprocess) returns 'running if the process is still running, or its exit code otherwise. The exit code is an exact integer, and 0 typically indicates success. If the process terminated due to a fault or signal, the exit code is non-zero.

  • (subprocess-kill subprocess force?) terminates the subprocess if force? is true and if the process still running, then returns void. If an error occurs during termination, the exn:misc exception is raised.

    If force? is #f under Unix and Mac OS X, the subprocess is sent an interrupt signal instead of a kill signal (and the subprocess might handle the signal without terminating). Under Windows, no action is taken when force? is #f.

  • (subprocess-pid subprocess) returns the operating system's numerical ID for the process (if any), valid only as long as the process is running. The ID is an exact integer. Under Mac OS Classic, the reported ID is always 0.

  • (subprocess? v) returns #t if v is a subprocess value, #f otherwise.

MzLib provides procedures for executing shell commands (as opposed to directly executing a program); see Chapter 24 in PLT MzLib: Libraries Manual for details.

15.3  Windows Actions

(shell-execute verb-string target-string parameters-string dir-path show-mode-symbol) performs the action specified by verb-string on target-string in Windows. For example,

(shell-execute #f "" "" (current-directory) 'SW_SHOWNORMAL

opens the PLT Scheme home page in a browser window. For platforms other than Windows, the exn:misc:unsupported exception is raised.

The verb-string can be #f, in which case the operating system will use a default verb. Common verbs include "open", "edit", "find", "explore", and "print".

The target-string is the target for the action, usually a filename path. The file could be executable, or it could be a file with a recognized extension that can be handled by an installed application.

The parameters-string argument is passed on to the system to perform the action. For example, in the case of opening an executable, the parameters-string is used as the command line (after the executable name).

The dir-path is used as the current directory when performing the action.

The show-mode-symbol sets the display mode for an Window affected by the action. It must be one of the following symbols; the description of each symbol's meaning is taken from the Windows API documentation.

  • 'sw_hide -- Hides the window and activates another window.

  • 'sw_maximize -- Maximizes the window.

  • 'sw_minimize -- Minimizes the window and activates the next top-level window in the z-order.

  • 'sw_restore -- Activates and displays the window. If the window is minimized or maximized, Windows restores it to its original size and position.

  • 'sw_show -- Activates the window and displays it in its current size and position.

  • 'sw_showdefault -- Uses a default.

  • 'sw_showmaximized -- Activates the window and displays it as a maximized window.

  • 'sw_showminimized -- Activates the window and displays it as a minimized window.

  • 'sw_showminnoactive -- Displays the window as a minimized window. The active window remains active.

  • 'sw_showna -- Displays the window in its current state. The active window remains active.

  • 'sw_shownoactivate -- Displays a window in its most recent size and position. The active window remains active.

  • 'sw_shownormal -- Activates and displays a window. If the window is minimized or maximized, Windows restores it to its original size and position.

If the action fails, the exn:misc exception is raised. If the action succeeds, the result is #f. In future versions of MzScheme, the result may be a subprocess value (see section 15.2) if the operating system did returns a process handle (but if a subprocess value is returned, its process ID will be 0 instead of the real process ID).

15.4  Operating System Environment Variables

(getenv name-string) gets the value of an operating system environment variable. The name-string argument cannot contain a null character; if an environment variable named by name-string exists, its value is returned (as a string); otherwise, #f is returned.

(putenv name-string value-string) sets the value of an operating system environment variable. The name-string and value-string arguments are strings that cannot contain a null character; the environment variable named by name-string is set to value-string. The return value is #t if the assignment succeeds, #f otherwise.

Although Mac OS Classic does not have operating system environment variables, getenv returns values installed with putenv (which always succeeds) in the same MzScheme session. When MzScheme is started, an initial environment is read from an Environment file in the current directory if it exists. An Environment file must contain a sequence of two-item lists where the name string is the first item in the list and the value string is the second. For example, an Environment file might contain the following:

("PLTCOLLECTS" ";My Disk:Extra Collections:") 
("USER" "joeuser"

15.5  Runtime Information

(system-type [details?]) returns a symbol indicating the type of the operating system for a running MzScheme if details? is #f or not provided. The possible values are:

  • 'unix

  • 'windows

  • 'macos

  • 'macosx

  • 'oskit

Future ports of MzScheme will expand this list of system types. If details? is not #f, then the result is a string, which contains further details about the operating system and the current machine in a platform-specific format.

(system-library-subpath) returns a relative directory pathname string. This string can be used to build pathnames to system-specific files. For example, when MzScheme is running under Solaris on a Sparc architecture, the subpath is "sparc-solaris", while the subpath for Windows on an Intel architecture is "win32\\i386".

(version) returns an immutable string indicating the currently executing version of MzScheme.

(banner) returns an immutable string for MzScheme's start-up banner text (or the banner text for an embedding program, such as MrEd). The banner string ends with a newline.

36 The value produced for the time-zone-offset field tends to be sensitive to the value of the "TZ" environment variable, especially on Unix platforms. Consult the system documentation (usually under tzset) for details.

37 For information on the Windows command-line conventions, search for ``command line parsing'' at