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most-positive-fixnum, most-negative-fixnum

most-positive-fixnum, most-negative-fixnum Constant Variable

Constant Value:

implementation-dependent.

Description:

most-positive-fixnum is that fixnum closest in value to positive infinity provided by the implementation, and greater than or equal to both 215 - 1 and array-dimension-limit.

most-negative-fixnum is that fixnum closest in value to negative infinity provided by the implementation, and less than or equal to 215.

Expanded Reference: most-positive-fixnum, most-negative-fixnum​

Inspecting the Values​

These constants define the range of the fixnum type, which varies across implementations. The standard guarantees that fixnums span at least the range from -2^15 to 2^15-1.

most-positive-fixnum
=> 4611686018427387903

most-negative-fixnum
=> -4611686018427387904

Boundary Behavior​

Adding 1 to most-positive-fixnum produces a bignum. Subtracting 1 from most-negative-fixnum also produces a bignum.

(typep most-positive-fixnum 'fixnum)
=> T
(typep (1+ most-positive-fixnum) 'fixnum)
=> NIL
(typep (1+ most-positive-fixnum) 'bignum)
=> T

(typep most-negative-fixnum 'fixnum)
=> T
(typep (1- most-negative-fixnum) 'fixnum)
=> NIL
(typep (1- most-negative-fixnum) 'bignum)
=> T

Portability Note​

Code that depends on specific fixnum ranges is inherently non-portable. The fixnum range varies widely: 32-bit implementations may have fixnums as small as 2^29, while 64-bit implementations commonly use 2^62 or larger. Always use these constants rather than hardcoded values.

;; Portable fixnum range check
(defun fits-in-fixnum-p (n)
  (<= most-negative-fixnum n most-positive-fixnum))

(fits-in-fixnum-p 1000000)
=> T
(fits-in-fixnum-p (expt 2 100))
=> NIL

Determining Fixnum Size in Bits​

You can compute how many bits a fixnum occupies on the current implementation.

(integer-length most-positive-fixnum)
=> 62

(+ 1 (integer-length most-positive-fixnum))
=> 63