Re: Globals vs. Locals
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- Subject: Re: Globals vs. Locals
- From: Rici Lake <lua@...>
- Date: Sat, 6 Aug 2005 01:34:03 -0500
On 5-Aug-05, at 7:24 PM, Glenn Maynard wrote:
"declaring" is entirely different in Lua and C (the word hardly applies
to globals in Lua). That's why the comparison seems mostly
meaningless.
I just can't resist a challenge.
rlake@freeb:~/src/cdecl$ cat a.c
a = 0;
seta(i) {
a = i;
}
geta() {
return a;
}
rlake@freeb:~/src/cdecl$ cat main.c
a = 1;
main() {
printf("The value of a is %d\n", geta());
a = 2;
printf("The value of a is %d\n", geta());
seta(3);
printf("The value of a is %d\n", geta());
return 0;
}
rlake@freeb:~/src/cdecl$ gcc -o main -z muldefs main.c a.c
rlake@freeb:~/src/cdecl$ ./main
The value of a is 1
The value of a is 2
The value of a is 3
-----------------------------
OK, it's different than Lua. But completely different? I think not.
Leaving aside Lua's ability to define functions inside functions, which
standard C lacks altogether, a local declaration inside a Lua function
differs from a variable declaration inside a C function only in that
the C variable is statically-typed. The scoping is identical, aside
from the fact that Lua, unlike C, implicitly declares the variables in
a for statement.
Similarly, a local declaration in Lua outside of a function is
essentially the same as a static declaration in a C file. Both declare
file-local variables.
The similarity between Lua local declarations and C local declarations
is often missed by Lua programmers wondering how to find out the "name
of a variable". Like C, Lua (local) variables do not have names; they
are simply mapped to storage (on the stack) and two named variables may
well share the same storage location if the compiler can demonstrates
that they are not co-extent. (In Lua's case, this means that their
scopes are disjoint.)
Global variables in Lua are not that similar to global variables in C,
mostly because you cannot create new global variables in C at runtime.
However, as the above example demonstrates, they work pretty well in
the same way: the compiled C file contains a string which names the
variable, and it is looked up in a hash table (at least in ELF format).
In Lua, the global variable is looked up at runtime in a hash table.
I did cheat a bit in the above example, of course. First, I told the
linker not to complain about the fact that 'a' is defined by both
main.c and a.c. Second, I deleted several useful warnings produced by
gcc. In fact, C90 does forbid the use of 'a = 1;' in main.c (and the
similar initialization in a.c), but it has no problems with the
function definitions.