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global-varslanguage

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This system is a compatibility layer for defining global variables. Global variables cannot be dynamically bound and sometimes is faster than variables defined with defvar or defparameter.

Also, when you are using globals-vars to define a variable, you tell the user of the library that this variable is not intended to by dynamically bound.

Here is a test for speed, comparing access to standard variable and global variable:

POFTHEDAY> (declaim (optimize (debug 1) (speed 3)))

POFTHEDAY> (defvar *global* 0)
*GLOBAL*

POFTHEDAY> (global-vars:define-global-var -global- 0)
-GLOBAL-

POFTHEDAY> (time (loop repeat 1000000000
                       do (incf *global*)))
Evaluation took:
  2.339 seconds of real time
  2.339325 seconds of total run time (2.336514 user, 0.002811 system)
  100.00% CPU
  5,164,301,132 processor cycles
  0 bytes consed

POFTHEDAY> (time (loop repeat 1000000000
                       do (incf -global-)))
Evaluation took:
  1.560 seconds of real time
  1.560328 seconds of total run time (1.558862 user, 0.001466 system)
  100.00% CPU
  3,444,078,626 processor cycles
  0 bytes consed

As you can see, accessing of variable, defined as global is almost twice faster on SBCL.

There is also macros define-global-var* and define-global-parameter*. They will define variables which will not be available in compile-time. Why does somebody might want this?