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(nasm.info)Section 8.2.4

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8.2.4. Exporting Symbols to the Library User
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   If you want to export symbols to the user of the library, you have to
declare whether they are functions or data, and if they are data, you
have to give the size of the data item. This is because the dynamic
linker has to build procedure linkage table entries for any exported
functions, and also moves exported data items away from the library's
data section in which they were declared.

   So to export a function to users of the library, you must use

     global  func:function           ; declare it as a function
     
     func:   push    ebp
     
             ; etc.

   And to export a data item such as an array, you would have to code

     global  array:data array.end-array      ; give the size too
     
     array:  resd    128
     .end:

   Be careful: If you export a variable to the library user, by
declaring it as `GLOBAL' and supplying a size, the variable will end up
living in the data section of the main program, rather than in your
library's data section, where you declared it. So you will have to
access your own global variable with the `..got' mechanism rather than
`..gotoff', as if it were external (which, effectively, it has become).

   Equally, if you need to store the address of an exported global in
one of your data sections, you can't do it by means of the standard
sort of code:

     dataptr:        dd      global_data_item        ; WRONG

   NASM will interpret this code as an ordinary relocation, in which
`global_data_item' is merely an offset from the beginning of the
`.data' section (or whatever); so this reference will end up pointing
at your data section instead of at the exported global which resides
elsewhere.

   Instead of the above code, then, you must write

     dataptr:        dd      global_data_item wrt ..sym

   which makes use of the special `WRT' type `..sym' to instruct NASM
to search the symbol table for a particular symbol at that address,
rather than just relocating by section base.

   Either method will work for functions: referring to one of your
functions by means of

     funcptr:        dd      my_function

   will give the user the address of the code you wrote, whereas

     funcptr:        dd      my_function wrt .sym

   will give the address of the procedure linkage table for the
function, which is where the calling program will _believe_ the
function lives.  Either address is a valid way to call the function.