On Jul 8, 1:34 pm, gopinath sikha <[email protected]> wrote:
> Hi guys,
>
> How gcc or any c compiler maintains the scope of the variable?
> Especially all the static variable are maintained in the data section.
>
> Can anyone please respond.
As a compiler processes the program, it maintains a table that maps
identifier strings to associated attributes like type information,
line number of declaration, etc., etc. This is called the symbol
table. It's often implemented as a hash table. Self-balancing trees
work fine, too.
There are at least a couple of different ways to process scopes. Here
is one.
The symbol table must allow duplicate keys for this technique, so it's
a multi-map rather than map.
The compiler gives every scope a number. It does this by keeping a
counter initialized at 0. Every time it sees a {, it increments the
counter and the new value becomes the scope number. It also pushes
each new scope number on a stack. At any given time, the stack
contains the numbers of open scopes, with the innermost scope at the
top.
When the compiler processes a declaration, the declared name is added
to the symbol table, and the scope number becomes one of its
attributes. If an attempt is made to add a variable twice with the
same scope number, a re-declaration error is printed for the user.
When the compiler processes a variable use (as in x = 42), it looks up
the variable (x) in the symbol table. If there is more than a single
x, it uses the one with the highest scope number because this must be
the "inner" scope where there is an x. If the variable (x) is not
found at all, then an "undefined identifier" error is printed for the
user.
When the compiler processes the end of a scope, a }, it pops the scope
stack. Say the closing scope is number N. Then the compiler removes
all symbol table entries with scope N as attributes and keeps on
processing.
That's about it. There are some details like handling the id's for
fields within structs, but I'm sure you can figure out how that works
on your own. I.e. when you have struct point { float x, y; } x; x.x
= 42; think about what the compiler is doing when it processes the
{ in the strut declaration, when it sees the x and the y, when it sees
the second x and the third.
Cheers.
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