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:

Functions defined with __weak export their symbols weakly. A weakly defined function behaves like a normally defined function unless a nonweakly defined function of the same name is linked into the same image. If both a nonweakly defined function and a weakly defined function exist in the same image then all calls to the function resolve to call the nonweak 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 nonweak functions.

A function or variable cannot be used both weakly and nonweakly in the same compilation. For example, the following code uses f() weakly from g() and h():

void f(void);
void g()
{
    f();
}
__weak void f(void);
void h()
{
    f();
}

It is not possible to use a function or variable weakly from the same compilation that defines the function or variable. The following code uses f() nonweakly from h():

__weak void f(void);
void h()
{
    f();
}
void f() {}

The linker does not load the function or variable from a library unless another compilation uses the function or variable nonweakly. If the reference remains unresolved, its value is assumed to be NULL. Unresolved references, however, are not NULL if the reference is from code to a position-independent section or to a missing __weak function.

Weakly defined functions cannot be inlined.