RealView Compilation Tools Compiler Reference Guide - ARM ...

Compiler-specific Features
Usage
Functions and variable declarations
For declarations, this storage class specifies an extern object declaration
that, even if not present, does not cause the linker to fault an unresolved
reference.
For example:
__weak void f(void);
...
f(); // call f weakly
If the reference to a missing weak function is made from code that
compiles to a branch or branch link instruction, then either:
• The reference is resolved as branching to the next instruction. This
effectively makes the branch a NOP.
• The branch is replaced by a NOP instruction.
Function definitions
Functions defined with __weak export their symbols weakly. A weakly
defined function behaves like a normally defined function unless a non
weakly defined function of the same name is linked into the same image.
If both a non weakly defined function and a weakly defined function exist
in the same image then all calls to the function resolve to call the non
weak function. If multiple weak definitions are available, the linker
chooses one for use by all calls.
Functions declared with __weak and then defined without __weak behave
as non weak functions.
Restrictions
There are restrictions when you qualify function and variable declarations, and function
definitions, with __weak.
Functions and variable declarations
A function or variable cannot be used both weakly and non weakly in the
same compilation. For example the following code uses f() weakly from
g() and h():
void f(void);
void g()
{
f();
}
ARM DUI 0348A Copyright © 2007, 2010 ARM Limited. All rights reserved. 4-21
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