Re: binutils-doc 2.15-5: glitches in ld.info

[Nick Clifton <nickc at redhat dot com>]

Hi Edward,

Thanks for the suggested documentation update.  I have taken the liberty 
of rewriting it slightly, err, well actually rather a lot, but I think 
that I have managed to retain the feel and intent of your original version.

What do you think of this version then ?  If you were coming to this 
subject for the first time, do you think that this new section would 
explain the linker's behaviour ?

Cheers
  Nick

Index: ld/ld.texinfo
===================================================================
RCS file: /cvs/src/src/ld/ld.texinfo,v
retrieving revision 1.139
diff -c -3 -p -r1.139 ld.texinfo
*** ld/ld.texinfo	1 Feb 2005 01:11:27 -0000	1.139
--- ld/ld.texinfo	1 Feb 2005 12:31:35 -0000
*************** the @samp{-f} option.
*** 2741,2751 ****
  @cindex symbol definition, scripts
  @cindex variables, defining
  You may assign a value to a symbol in a linker script.  This will define
! the symbol as a global symbol.
  
  @menu
  * Simple Assignments::		Simple Assignments
  * PROVIDE::			PROVIDE
  @end menu
  
  @node Simple Assignments
--- 2741,2752 ----
  @cindex symbol definition, scripts
  @cindex variables, defining
  You may assign a value to a symbol in a linker script.  This will define
! the symbol and place it into the symbol table with a global scope.
  
  @menu
  * Simple Assignments::		Simple Assignments
  * PROVIDE::			PROVIDE
+ * Source Code Reference::	How to use a linker script defined symbol in source code
  @end menu
  
  @node Simple Assignments
*************** underscore), the linker will silently us
*** 2838,2843 ****
--- 2839,2951 ----
  If the program references @samp{etext} but does not define it, the
  linker will use the definition in the linker script.
  
+ @node Source Code Reference
+ @subsection 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 @samp{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:
+ 
+ @smallexample
+   extern int foo;
+ @end smallexample
+ 
+ But in the linker script it might be defined as:
+ 
+ @smallexample
+   _foo = 1000;
+ @end smallexample
+ 
+ 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 @emph{value} of the symbol.  The
+ second is that the compiler creates an entry in the program's symbol
+ table which holds the symbol's @emph{address}.  ie the symbol table
+ contains the address of the block of memory holding the symbol's
+ value.  So for example the C declaration:
+ 
+ @smallexample
+   int foo = 1000;
+ @end smallexample
+ 
+ Creates a entry called @samp{foo} in the symbol table.  This entry
+ holds the address of an @samp{int} sized block of memory where the
+ number 1000 is currently 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:
+ 
+ @smallexample
+   foo = 1;
+ @end smallexample
+ 
+ Looks up the symbol @samp{foo} in the symbol table, gets the address
+ associated with this symbol and then writes the value 1 into that
+ address.  Whereas:
+ 
+ @smallexample
+   int * a = & foo;
+ @end smallexample
+ 
+ Looks up the symbol @samp{foo} in the symbol table, gets it address
+ and then copies this address into the block of memory associated with
+ the variable @samp{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:
+ 
+ @smallexample
+   foo = 1000;
+ @end smallexample
+ 
+ Creates an entry in the symbol table called @samp{foo} which contains
+ the address of memory location 1000, but nothing special is stored at
+ address 1000.  This means that you cannot access the @emph{value} of a
+ linker script defined symbol - it has no value - all you can do is
+ access the @emph{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:
+ 
+ @smallexample
+ @group
+   start_of_ROM   = .ROM;
+   end_of_ROM     = .ROM + sizeof (.ROM) - 1;
+   start_of_FLASH = .FLASH;
+ @end group
+ @end smallexample
+ 
+ Then the C source code to perform the copy would be:
+ 
+ @smallexample
+ @group
+   extern char start_of_ROM, end_of_ROM, start_of_FLASH;
+   
+   memcpy (& start_of_FLASH, & start_of_ROM, & end_of_ROM - & start_of_ROM);
+ @end group
+ @end smallexample
+ 
+ Note the use of the @samp{&} operators.  These are correct.
+ 
  @node SECTIONS
  @section SECTIONS Command
  @kindex SECTIONS