Re: Upvalues vs C static variable

[Sean Conner <sean@...>]

It was thus said that the Great Albert Chan once stated:
> When to use lua upvalue mechanism instead of simpler C static variable ?
> 
> Example, lua 5.4 work-1 math.random :
> 
> It uses upvalue to store the 128-bits random state.
> Why not use static uint64_t state[2] ?

  To prevent unintentional sharing of data.

  Example, a random number generator named spc_rng() (my own special random
number generator) that when seeded with value X will produce the sequence A
-> B -> C -> D ... (why?  Perhaps testing purposes or repeatability or what
have you).

  Case 1---the state is stored in a static C variable.  Your program
instantiates two separate Lua VMs (say, you are using then in different
threads of the program).  As each thread runs, there will be calls
interspaced to spc_rnd() between the two threads:

	thread-1	A
	thread-2	B
	thread-1	C
	thread-1	D
	thread-2	E
	thread-1	F
	thread-2	G

so thread1 will "see" the sequence "A C D F" and thread2 will "see" the
sequence "B E G".

  Case 2---the state is stored as an upvalue.  Your program instantiates two
separate Lua VMs.  As each thread runs, there will be calls interspaced to
spc_rnd() between the two threads:

	thread-1	A
	thread-2	A
	thread-1	B
	thread-1	C
	thread-2	B
	thread-1	D
	thread-2	C

  Here, thread1 will "see" the sequence "A B C D" and thread2 will "see" the
sequence "A B C".  Which case is important depends upon what you are trying
to do, but in general, case 2 is a safer approach to take since it leads to
less surprises.

  -spc