Re: Overlays

[Al Riddoch <ajr@ecs.soton.ac.uk>]

On Sun, Apr 28, 2002 at 02:56:54PM +0200, Francesco Orsenigo wrote:
> How can I control which parts of my code are loaded in memory?
> In other terms, when the program needs to call a function (or a global 
> variable?) this function is loaded with the all the symbols (functions and 
> gvars) in the entire module (in my case, the .c file?) even if i don't need 
> them?
> 
> I'm splitting my program into many of .c files, so that functions that are 
> used in a particular situations are grouped, kept apart from functions that 
> must be used in other situations and can be unloaded...
> 

When you build your program, the concept of modules as .c or .o files
is essentially lost, and all the code is contained in the binary. Code
in shared libraries you use is handled differently, and for simplicity
I won't go into that here.

On a modern Unix-like system, when you run the binary, its contents are
mapped into memory using the virtual memory subsystem. This means that
the contents of the file are not actually loaded, but a section of
the systems address space is set aside for your programs code. This
space is divided into pages, which were 4096 bytes on intel, but may
be larger on more recent CPUs. When the program needs to call some code,
the page containing that code is loaded into memory first, on a page
by page basis.

I hope this gives you enough information to partly understand what goes
on. If you have any questions, feel free to ask.

Al