iPhone assembly GAS directive declare a extern function

In a assembly code for iPhone GCC4.2, how to delare a extern function? Example:

_main:
    bl myfunc
    bx lr 

We know if a common compiler, we can use "import myfunc" or "extern 开发者_开发百科myfunc" to declare this function, but GCC44.2 in iOS developer doesn't recognize them.

---

Apple's GAS is a very old fork. I think it wants ".globl". Check this doc.