#pragma weak name1 = name2

In the form #pragma weak name1 = name2, the symbol name1 becomes a weak reference to name2. If name1 is not defined elsewhere, name1 will have the value name2. If name1 is defined elsewhere, the linker uses that definition and ignores the weak reference to name2. The following directive instructs the linker to resolve any references to bar if it is defined anywhere in the program, and to foo otherwise.

#pragma weak bar = foo

In the identifier form, name2 must be declared and defined within the current compilation unit. For example:

extern void bar(int) {...}
extern void _bar(int);
#pragma weak _bar=bar

When you use the string form, the symbol does not need to be previously declared. If both _bar and bar in the following example are extern "C", the functions do not need to be declared. However, bar must be defined in the same object.

extern "C" void bar(int) {...}
#pragma weak "_bar" = "bar"