Inline function attribute causes undefined symbol linking error

Hello ARM Community,

I am using DS-5 5.18.1. I have a C function which must be compiled inline to decrease latency of an FIQ handler. I referenced following document for syntax:

http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0472k/chr1359124973480.html

Note that the attribute may accompany the function declaration and/or definition.

I added the attribute to the function declaration as below in example.c:

void foo(void) __attribute__((always_inline));

The following linking error was produced:

L6218E: Undefined symbol foo (referred from example.o).     C/C++ Problem

It is important to note that this error was not present before adding the inline attribute. There are no related warnings, nor any other errors. Why would this single change cause the preceding error?

Thank you,

Ryan