Address of a variable

Address of a variable
You are encouraged to solve this task according to the task description, using any language you may know.

Basic Data Operation
This is a basic data operation. It represents a fundamental action on a basic data type.

You may see other such operations in the Basic Data Operations category, or:

Integer Operations
Arithmetic | Comparison

Boolean Operations
Bitwise | Logical

String Operations
Concatenation | Interpolation | Comparison | Matching

Memory Operations
Pointers & references | Addresses

Demonstrate how to get the address of a variable and how to set the address of a variable.

Ada

Get The Address

The_Address : System.Address;
I : Integer;
The_Address := I'Address;

Set The Address

Set the address of a variable to address A100 in hexidecimal

I : Integer;
for I'Address use 16#A100#;

Set the address of one varible to the address of another variable, creating an overlay.

I : Integer;
J : Integer;
For I'Address use J'Address;

C/C++

Compiler: GCC

Get the address

Note that void* is a "pure" address which doesn't carry the type information anymore. If you need the type information (e.g. to recover the variable itself in a type safe manner), use a pointer to the appropriate type instead; in this case int*.

int i;
void* address_of_i = &i;

Set the address

While C++ doesn't directly support putting a variable at a given address, the same effect can be achieved by creating a reference to that address:

int& i = *(int*)0xA100;

Overlaying of variables is done with anonymous unions; however at global/namespace scope such variables have to be static (i.e. local to the current file):

static union
{
  int i;
  int j;
};

C++ only: An alternative (and cleaner) solution is to use references:

int i;
int& j = i;

Note that in this case, the variables can be non-static.

Delphi

Pascal supports the @ ( address of ) operator and the var : [type] absolute declaration.

To get the address of any variable, structure, procedure or function use the @ operator.

 var
   Int : integer ;
   p   : pointer ;
 begin
   P := @int ;
   writeln(integer(p^));
 end;

A variable can be declared as absolute ie: to reside at a specific address.

 Var
   CrtMode : integer absolute $0040 ;
   Str     : string[100] ;
   StrLen  : byte absolute Str ;

Forth

Variables and created memory blocks return their address when refrerenced. The "fetch" operator @ could also be pronounced "dereference".

variable foo
foo .  \ some large number, an address
8 foo !
foo @ .  \ 8

You can define a constant or value with an address, which then acts like a variable. This can be used to refer to fixed addresses (such as I/O ports), graphics buffers, or allocated memory.

$3F8 constant LPT1:
8 LPT1: !

100 cells allocate throw value buffer
42 buffer 20 cells + !

Perl

To get the address, stringify the reference to a variable. Also see Devel::Peek.

print \my $v;  # SCALAR(0x8167cf4)

Use Devel::Pointer::PP if you want to dereference a certain address in memory.

Changing the address of a variable is not easily possible, but see perlapi. Wanting to go against the automatic memory management is a sign that this is only used to hack around the deficiencies of dafter languages. I can imagine address munging is commonly used to make variable aliasing possible, but Perl already has a higher level syntax for that.

Toka

Get the Address

The default behaviour of a data element in Toka is to return its address. This makes obtaining the address trivial:

variable foo
foo .

Set the Address

You can manually assign a name to any memory address (or other number), but you should make sure it's part of allocated memory first.

 hex abcdef is-data foo
 foo .