This keyword instructs the compiler to export symbols weakly.

The __weak keyword can be applied to function and variable declarations, and to function definitions.
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 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 generates an error message, unless the linker option --muldefweak is used. In this case, the linker chooses one for use by all calls.
Functions declared with __weak and then defined without __weak behave as nonweak 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 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.
Function definitions
Weakly defined functions cannot be inlined.
Examples
__weak const int c;                // assume 'c' is not present in final link
const int *f1() { return &c; } // '&c' returns non-NULL if
                                   // compiled and linked /ropi
__weak int i;                      // assume 'i' is not present in final link
int *f2() { return &i; }       // '&i' returns non-NULL if
                                   // compiled and linked /rwpi
__weak void f(void);               // assume 'f' is not present in final link
typedef void (*FP)(void);
FP g() { return f; }               // 'g' returns non-NULL if
                                   // compiled and linked /ropi

英文不好，麻烦大侠通俗解释下，先谢谢了