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| <h4 class="subsection">3.5.5 Source Code Reference</h4> |
| |
| <p>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. |
| </p> |
| <p>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 ‘<samp>name |
| mangling</samp>’. 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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> extern int foo; |
| </pre></div> |
| |
| <p>But in the linker script it might be defined as: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> _foo = 1000; |
| </pre></div> |
| |
| <p>In the remaining examples however it is assumed that no name |
| transformation has taken place. |
| </p> |
| <p>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 <em>value</em> of the symbol. The |
| second is that the compiler creates an entry in the program’s symbol |
| table which holds the symbol’s <em>address</em>. 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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> int foo = 1000; |
| </pre></div> |
| |
| <p>creates an entry called ‘<samp>foo</samp>’ in the symbol table. This entry |
| holds the address of an ‘<samp>int</samp>’ sized block of memory where the |
| number 1000 is initially stored. |
| </p> |
| <p>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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> foo = 1; |
| </pre></div> |
| |
| <p>looks up the symbol ‘<samp>foo</samp>’ in the symbol table, gets the address |
| associated with this symbol and then writes the value 1 into that |
| address. Whereas: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> int * a = & foo; |
| </pre></div> |
| |
| <p>looks up the symbol ‘<samp>foo</samp>’ in the symbol table, gets its address |
| and then copies this address into the block of memory associated with |
| the variable ‘<samp>a</samp>’. |
| </p> |
| <p>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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> foo = 1000; |
| </pre></div> |
| |
| <p>creates an entry in the symbol table called ‘<samp>foo</samp>’ which holds |
| the address of memory location 1000, but nothing special is stored at |
| address 1000. This means that you cannot access the <em>value</em> of a |
| linker script defined symbol - it has no value - all you can do is |
| access the <em>address</em> of a linker script defined symbol. |
| </p> |
| <p>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: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> start_of_ROM = .ROM; |
| end_of_ROM = .ROM + sizeof (.ROM) - 1; |
| start_of_FLASH = .FLASH; |
| </pre></div> |
| |
| <p>Then the C source code to perform the copy would be: |
| </p> |
| <div class="smallexample"> |
| <pre class="smallexample"> extern char start_of_ROM, end_of_ROM, start_of_FLASH; |
| |
| memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM); |
| </pre></div> |
| |
| <p>Note the use of the ‘<samp>&</samp>’ operators. These are correct. |
| </p> |
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