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(nasm.info)Section 6.5.3

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6.5.3. `elf' Extensions to the `GLOBAL' Directive
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   `ELF' object files can contain more information about a global symbol
than just its address: they can contain the size of the symbol and its
type as well. These are not merely debugger conveniences, but are
actually necessary when the program being written is a shared library.
NASM therefore supports some extensions to the `GLOBAL' directive,
allowing you to specify these features.

   You can specify whether a global variable is a function or a data
object by suffixing the name with a colon and the word `function' or
`data'. (`object' is a synonym for `data'.) For example:

     global   hashlookup:function, hashtable:data

   exports the global symbol `hashlookup' as a function and `hashtable'
as a data object.

   You can also specify the size of the data associated with the
symbol, as a numeric expression (which may involve labels, and even
forward references) after the type specifier. Like this:

     global  hashtable:data (hashtable.end - hashtable)
     
     hashtable:
             db this,that,theother  ; some data here
     .end:

   This makes NASM automatically calculate the length of the table and
place that information into the `ELF' symbol table.

   Declaring the type and size of global symbols is necessary when
writing shared library code. For more information, see *Note Section
8.2.4::.