ARM VERSION 1.2 - Manual (Page 342)

Directives Reference
7-60 Copyright © 2000, 2001 ARM Limited. All rights reserved. ARM DUI 0068B
7.7.9 EXTERN
The EXTERN directive provides the assembler with a name that is not defined in the
current assembly.
EXTERN is very similar to IMPORT , except that the name is not imported if no reference to
it is found in the current assembly (see IMPORT on page 7-62, and EXPORT or
GLOBAL on page 7-58).
Syntax
EXTERN symbol{[WEAK]}
where:
symbol is a symbol name defined in a separately assembled source file, object
file, or library. The symbol name is case-sensitive.
[WEAK] prevents the linker generating an error message if the symbol is not
defined elsewhere. It also prevents the linker searching libraries that are
not already included.
Usage
The name is resolved at link time to a symbol defined in a separate object file. The
symbol is treated as a program address. If
[WEAK] is not specified, the linker generates
an error if no corresponding symbol is found at link time.
If [WEAK] is specified and no corresponding symbol is found at link time:
• If the reference is the destination of a B or BL instruction, the value of the symbol
is taken as the address of the following instruction. This makes the B or BL
instruction effectively a NOP .
• Otherwise, the value of the symbol is taken as zero.
Example
This example tests to see if the C++ library has been linked, and branches conditionally
on the result.
AREA Example, CODE, READONLY
EXTERN __CPP_INITIALIZE[WEAK] ; If C++ library linked, gets the address of
; __CPP_INITIALIZE function.
LDR r0,__CPP_INITIALIZE ; If not linked, address is zeroed.
CMP r0,#0 ; Test if zero.
BEQ nocplusplus ; Branch on the result.