!NULL: A variable of type Undefined.

APP_USER_DIR: Provides access to the application user directory.

APP_USER_DIR_QUERY: Locates existing application user directories.

ARG_PRESENT: Returns TRUE if the value of the specified variable can be passed back to the caller.

BREAKPOINT: Sets and clears breakpoints for debugging.

BYTEORDER: Converts between host and network byte ordering.

CALL_FUNCTION: Calls a function given the function's name as a string.

CALL_METHOD: Calls an IDL object method.

CALL_PROCEDURE: Calls a procedure given the procedure's name as a string.

CATCH: Declares and clears exception handlers.

CODE_COVERAGE: Return line numbers of executed and un-executed code within a routine.

COMMAND_LINE_ARGS: Returns string values supplied when the user starts IDL with the -arg or -args command line options.

CPU: Changes the values stored in the read-only !CPU system variable.

CREATE_STRUCT: Creates and concatenates structures.

COMPILE_OPT: Change default rules for compiling routines.

DEFINE_KEY: Programs keyboard function keys.

DEFINE_MSGBLK: Defines and loads a new message block into the current IDL session.

DEFINE_MSGBLK_FROM_FILE: Reads the definition of a message block from a file, and loads it into the current IDL session.

DEFSYSV: Creates a new system variable.

EXECUTE: Compiles and executes IDL statements contained in a string.

EXIT: Quits IDL and exits back to the operating system.

EXPAND_PATH: Expands path-definition string into full path name for use with the !PATH system variable.

HEAP_FREE: Recursively frees all heap variables referenced by its input argument.

HEAP_GC: Performs garbage collection on heap variables.

HEAP_NOSAVE: Used to clear the save attribute of pointer or object heap variables.

HEAP_REFCOUNT: Returns the current reference count for a pointer or object reference.

HEAP_SAVE: Used to query whether a pointer or object heap variable is savable. It can also be used to change the heap variable save attribute.

IDL_VALIDNAME: Determines whether a string may be used as a valid IDL variable name or structure tag name.

KEYWORD_SET: Returns True if given expression is defined and nonzero or an array.

LAMBDA: Create inline Lambda functions.

LAMBDAP: Create inline Lambda procedures.

LMGR: Determines the type of license used by the current IDL session.

LOGICAL_AND: Performs a logical AND operation on its arguments.

LOGICAL_OR: Performs a logical OR operation on its arguments.

LOGICAL_TRUE: Determines whether its arguments are non-zero (or non-NULL).

MEMORY: Provides information about the amount of dynamic memory currently in use by the IDL session.

MESSAGE: Issues error and informational messages.

N_ELEMENTS: Returns the number of elements contained in an expression or variable.

N_PARAMS: Returns the number of non-keyword parameters used in calling a procedure or function.

N_TAGS: Returns the number of tags in a structure.

OBJ_CLASS: Determines the class name of an object.

OBJ_DESTROY: Destroys an object reference.

OBJ_HASMETHOD: Determines whether an object class implements or inherits a specified method or methods.

OBJ_ISA: Determines inheritance relationship of an object.

OBJ_NEW: Creates an object reference.

OBJ_VALID: Verifies validity of object references.

ON_ERROR: Designates the error recovery method.

ON_IOERROR: Declares I/O error exception handler.

PATH_CACHE: Controls IDL’s path caching mechanism.

PREF_COMMIT: Commits IDL preferences in the pending state.

PREF_GET: Returns information about IDL preferences.

PREF_SET: Sets new values for IDL preferences.

PROFILER: Analyze the performance of IDL code applications.

PTR_FREE: Destroys a pointer.

PTR_NEW: Creates a pointer.

PTR_VALID: Verifies the validity of pointers.

PTRARR: Creates an array of pointers.

RECALL_COMMANDS: Returns entries in IDL’s command recall buffer.

RESOLVE_ALL: Compiles any uncompiled routines.

RESOLVE_ROUTINE: Compiles a routine.

RETALL: Returns control to the main program level.

RETURN: Returns control to the next-higher program level.

ROUTINE_INFO: Provides information about compiled procedures and functions.

STOP: Stops the execution of a running program or batch file.

STRMESSAGE: Returns the text of a given error number.

STRUCT_ASSIGN: Uses relaxed-structure assignment to copy structures.

STRUCT_HIDE: Prevents the IDL HELP procedure from displaying information about structures or objects.

SWAP_ENDIAN: Reverses the byte ordering of scalars, arrays or structures.

SWAP_ENDIAN_INPLACE: Reverses the byte ordering of scalars, arrays or structures, without making a copy.

TAG_NAMES: Returns the names of tags in a structure.

TEMPORARY: Returns a temporary copy of a variable, and sets the original variable to Undefined.

WAIT: Suspends execution of the program for a specified period.