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Assignment: Defining Symbols
----------------------------

   You may create global symbols, and assign values (addresses) to
global symbols, using any of the C assignment operators:

`SYMBOL  = EXPRESSION ;'
`SYMBOL &= EXPRESSION ;'
`SYMBOL += EXPRESSION ;'
`SYMBOL -= EXPRESSION ;'
`SYMBOL *= EXPRESSION ;'
`SYMBOL /= EXPRESSION ;'
   Two things distinguish assignment from other operators in `ld'
expressions.
   * Assignment may only be used at the root of an expression; `a=b+3;'
     is allowed, but `a+b=3;' is an error.

   * You must place a trailing semicolon (";") at the end of an
     assignment statement.

   Assignment statements may appear:
   * as commands in their own right in an `ld' script; or

   * as independent statements within a `SECTIONS' command; or

   * as part of the contents of a section definition in a `SECTIONS'
     command.

   The first two cases are equivalent in effect--both define a symbol
with an absolute address.  The last case defines a symbol whose address
is relative to a particular section (SECTIONS:.).   

   When a linker expression is evaluated and assigned to a variable, it
is given either an absolute or a relocatable type. An absolute
expression type is one in which the symbol contains the value that it
will have in the output file; a relocatable expression type is one in
which the value is expressed as a fixed offset from the base of a
section.

   The type of the expression is controlled by its position in the
script file.  A symbol assigned within a section definition is created
relative to the base of the section; a symbol assigned in any other
place is created as an absolute symbol. Since a symbol created within a
section definition is relative to the base of the section, it will
remain relocatable if relocatable output is requested.  A symbol may be
created with an absolute value even when assigned to within a section
definition by using the absolute assignment function `ABSOLUTE'.  For
example, to create an absolute symbol whose address is the last byte of
an output section named `.data':
     SECTIONS{ ...
       .data :
         {
           *(.data)
           _edata = ABSOLUTE(.) ;
         }
     ... }

   The linker tries to put off the evaluation of an assignment until all
the terms in the source expression are known (Evaluation:.).  For   
instance, the sizes of sections cannot be known until after allocation,
so assignments dependent upon these are not performed until after
allocation. Some expressions, such as those depending upon the location
counter "dot", `.' must be evaluated during allocation. If the result
of an expression is required, but the value is not available, then an
error results.  For example, a script like the following
     SECTIONS { ...
       text 9+this_isnt_constant :
         { ...
         }
     ... }

will cause the error message "`Non constant expression for initial
address'".

   In some cases, it is desirable for a linker script to define a symbol
only if it is referenced, and only if it is not defined by any object
included in the link.  For example, traditional linkers defined the
symbol `etext'.  However, ANSI C requires that the user be able to use
`etext' as a function name without encountering an error.  The
`PROVIDE' keyword may be used to define a symbol, such as `etext', only
if it is referenced but not defined.  The syntax is `PROVIDE(SYMBOL =
EXPRESSION)'.