6.8.8. 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, and EXPORT or GLOBAL).


EXTERN symbol {[WEAK,attr]}



is a symbol name defined in a separately assembled source file, object file, or library. The symbol name is case-sensitive.


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.


is symbol visibility when linked into a dynamic component. By default, the symbol binding defines visibility, that is, global and weak symbols are visible to other external objects and local symbols are not hidden.

Valid attributes are:


symbol is visible to other components, and can be redefined by other components.


symbol cannot be referenced outside the component where it is defined, either directly or indirectly.

The linker also accepts INTERNAL and currently treats it as HIDDEN. If you specify both, for example:


the assembler chooses the most restrictive (INTERNAL).


symbol is visible to other components, and cannot be redefined by other components.


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.


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.
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