Expression(s)

The arguments are interpreted differently depending on the keyword selected. If no keyword is selected, HELP displays basic information for its parameters. For example, to see the type and structure of the variable A, enter:

HELP, A

ALL_KEYS

Set this keyword to show current function-key definitions as set by DEFINE_KEY. If no arguments are supplied, information on all function keys is displayed. If arguments are provided, they must be scalar strings containing the names of function keys, and information on the specified keys is given. Under Unix, this keyword is different from KEYS because every key is displayed, no matter what its current programming. Under VMS and Windows, the two keywords mean the same thing. On the Macintosh, keys cannot be defined via DEFINE_KEY.

BREAKPOINTS

Set this keyword to display the breakpoint table which shows the program module and location for each breakpoint.

CALLS

Set this keyword to a named variable in which to store the procedure call stack. Each string element contains the name of the program module, source file name, and line number. Array element zero contains the information about the caller of HELP, element one contains information about its caller, etc. This keyword is useful for programs that require traceback information.

DEVICE

Set this keyword to show information about the currently selected graphics device. This information is dependent on the abilities of the current device, but the name of the device is always given. Arguments to HELP are ignored when DEVICE is specified.

DLM

Set this keyword to display all known dynamically loadable modules and their state (loaded or not loaded).

FILES

Set this keyword to display information about file units. If no arguments are supplied in the call to HELP, information on all open file units (except the special units 0, -1, and -2) is displayed. If arguments are provided, they are taken to be integer file unit numbers, and information on the specified file units is given.

For example, the command:

HELP, /FILES, -2, -1, 0

gives information below about the default file units:

Unit Attributes Name
-2   Write, Truncate, Tty, Reserved   <stderr>
-1   Write, Truncate, Tty, Reserved   <stdout>
0   Read, Tty, Reserved              <stdin>

The attributes column tells about the characteristics of the file. For instance, the file connected to logical file unit 2 is called "stderr" and is the standard error file. It is opened for write access (Write), is a new file (Truncate), is a terminal (Tty), and cannot be closed by the CLOSE command (Reserved).

HEAP_VARIABLES

Set this keyword to display help information for all the current heap variables.

KEYS

Set this keyword to show current function key definitions as set by DEFINE_ KEY, for those function keys that are currently programmed to perform a function. Under Unix, this keyword is different from ALL_KEYS because that keyword displays every key, no matter what its current programming. Under VMS and Windows, the two keywords mean the same thing. On the Macintosh, keys cannot be defined via DEFINE_KEY. If no arguments are supplied, information on all function keys is displayed. If arguments are provided, they must be scalar strings containing the names of function keys, and information on the specified keys is given.

LAST_MESSAGE

Set this keyword to display the last error message issued by IDL.

MEMORY

Set this keyword to see a report on the amount of dynamic memory (in bytes) currently in use by the IDL session, and the number of times dynamic memory has been allocated and deallocated. Arguments to HELP are ignored when MEMORY is specified.

MESSAGES

Set this keyword to display all known message blocks and the error space range into which they are loaded.

NAMES

A string of the names of variables whose values are to be printed. This string, which must be enclosed in quotes, can contain the standard wildcard matching characters, "*" and "?". For example, to print only the values of variables beginning with "A", use the command HELP, NAME='a*' . Similarly, HELP, NAME='?' , prints the values of all variables with a single-character name.

OBJECTS

Set this keyword to display information on defined object classes. If no arguments are provided, all currently-defined object classes are shown. If arguments are provided, the definition of the object class for the heap variables referred to is displayed.

Information is provided on inherited superclasses and all known methods. A method is known to IDL only if it has been compiled in the current IDL session and called by its own class or a subclass. Methods that have not been compiled yet will not be shown. Thus, the list of methods displayed by HELP is not necessarily a complete list of all possible method for the object class.

If called within a class' method, the OBJECTS keyword also displays the instance data of the object on which it was called.

OUTPUT

Set this keyword equal to a named variable that will contain a string array containing the formatted output of the HELP command. Each line of formatted output becomes a single element in the string array.

CAUTION: The OUTPUT keyword is primarily for use in capturing HELP output in order to display it someplace else, such as in a text widget. This keyword is not intended to be used in obtaining programmatic information about the IDL session, and is formatted to be human readable. Research Systems reserves the right to change the format and content of this text at any time, without warning . If you find yourself using OUTPUT for a non-display purpose, you should consider submitting an enhancement request for a function that will provide the information you require in a safe form.

RECALL_COMMANDS

Set this keyword to display the saved commands in the command input buffer. By default, IDL saves the last 20 lines of input in a buffer from which they can be recalled for command line editing. Arguments to HELP are ignored when RECALL is specified.

The number of lines saved can be changed by assigning the desired number of lines to the environment variable !EDIT_INPUT in the IDL startup file. See for details.

ROUTINES

Set this keyword to show a list of all compiled procedures and functions with their parameter names. Keyword parameters accepted by each module are shown to the right of the routine name.

SOURCE_FILES

Set this keyword to display information on procedures and functions written in the IDL language that have been compiled during the current IDL session. Full path names (relative to the current directory) of compiled .pro files are displayed.

STRUCTURES

Set this keyword to display information on structure-type variables. If no arguments are provided, all currently-defined structures are shown. If arguments are provided, the structure definition for those expressions is displayed. It is often more convenient to use HELP, /STRUCTURES instead of PRINT to look at the contents of a structure variable because it shows the names of the fields as well as the data.

SYSTEM_VARIABLES

Set this keyword to display information on all system variables. Arguments are ignored.

TRACEBACK

Set this keyword to display the current nesting of procedures and functions.