3.5.5 Source Code Reference

Accessing a linker script defined variable from source code is not intuitive. In particular a linker script symbol is not equivalent to a variable declaration in a high level language, it is instead a symbol that does not have a value.

Before going further, it is important to note that compilers often transform names in the source code into different names when they are stored in the symbol table. For example, Fortran compilers commonly prepend or append an underscore, and C++ performs extensive ‘name mangling’. Therefore there might be a discrepancy between the name of a variable as it is used in source code and the name of the same variable as it is defined in a linker script. For example in C a linker script variable might be referred to as:

  extern int foo;

But in the linker script it might be defined as:

  _foo = 1000;

In the remaining examples however it is assumed that no name transformation has taken place.

When a symbol is declared in a high level language such as C, two things happen. The first is that the compiler reserves enough space in the program’s memory to hold the value of the symbol. The second is that the compiler creates an entry in the program’s symbol table which holds the symbol’s address. ie the symbol table contains the address of the block of memory holding the symbol’s value. So for example the following C declaration, at file scope:

  int foo = 1000;

creates an entry called ‘foo’ in the symbol table. This entry holds the address of an ‘int’ sized block of memory where the number 1000 is initially stored.

When a program references a symbol the compiler generates code that first accesses the symbol table to find the address of the symbol’s memory block and then code to read the value from that memory block. So:

  foo = 1;

looks up the symbol ‘foo’ in the symbol table, gets the address associated with this symbol and then writes the value 1 into that address. Whereas:

  int * a = & foo;

looks up the symbol ‘foo’ in the symbol table, gets its address and then copies this address into the block of memory associated with the variable ‘a’.

Linker scripts symbol declarations, by contrast, create an entry in the symbol table but do not assign any memory to them. Thus they are an address without a value. So for example the linker script definition:

  foo = 1000;

creates an entry in the symbol table called ‘foo’ which holds the address of memory location 1000, but nothing special is stored at address 1000. This means that you cannot access the value of a linker script defined symbol - it has no value - all you can do is access the address of a linker script defined symbol.

Hence when you are using a linker script defined symbol in source code you should always take the address of the symbol, and never attempt to use its value. For example suppose you want to copy the contents of a section of memory called .ROM into a section called .FLASH and the linker script contains these declarations:

  start_of_ROM   = .ROM;
  end_of_ROM     = .ROM + sizeof (.ROM);
  start_of_FLASH = .FLASH;

Then the C source code to perform the copy would be:

  extern char start_of_ROM, end_of_ROM, start_of_FLASH;
  memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM);

Note the use of the ‘&’ operators. These are correct. Alternatively the symbols can be treated as the names of vectors or arrays and then the code will again work as expected:

  extern char start_of_ROM[], end_of_ROM[], start_of_FLASH[];
  memcpy (start_of_FLASH, start_of_ROM, end_of_ROM - start_of_ROM);

Note how using this method does not require the use of ‘&’ operators.