when an executable is loaded, it's address will be changed?

[Evan Lavelle]

Ge Jun wrote:
> Ian Lance Taylor wrote:
> 
>> Ge Jun <gejun@iastate.edu> writes:
>>
>>  
>>
>>> An executable's code has already the absolute address(or the relative
>>> address in that section?) in it.  For
>>> example, the "nm" tool can list the address of the labels.
>>>
>>> My question is, when the executable is loaded by linux, will the
>>> address be changed?
>>>   
>>
>>
>> No, the address of an executable will not be changed.  (The address of
>> a shared library may--indeed, probably will--be changed when it is
>> loaded.)
>>
>> In other words, it is not a requirement that the code in an executable
>> be position independent.
>>  
>>
> 
> Thank you very much!!  I still have a question.  How does the loader 
> work if I have several copies of the program running simutaneously? 
> Their address are the same. 

Their *virtual* addresses are the same; not their *physical* addresses

>  They will be loaded in the same address in 
> the memory(but only one is in there at one time)? 

In general, a loader loads a program to a fixed virtual address. You can 
have multiple programs running at the same virtual addresses, because 
the loader and operating system have set up the processor's page tables 
such that the programs occupy different regions of physical memory. As a 
user, you don't know about the 'real' physical memory that your program 
is in; you only see the virtual memory.

> If possible, could 
> you please also give me a link talking about how does the loader work? I 
> checked by myself but failed to find such a webpage.
> 
> Thank you again!!
> 
> Jun

I don't know any links; you could try Bach's 'The design of the Unix 
operating system' for some more details.

Evan