In the GNU Compiler Collection (GCC), a symbol can be annotated as weak using the __attribute__((weak)) syntax. This attribute allows you to define a symbol that can be overridden by another definition of the same symbol in a different translation unit. Here’s how it works:

Weak Symbols in GCC

1. Definition: A weak symbol is a symbol that has a lower linkage than a strong symbol. If both a weak and a strong symbol with the same name are linked together, the strong symbol will take precedence.

2. Usage: You can declare a weak symbol in your code like this:

   void my_function() __attribute__((weak));
   

   This declaration tells the compiler that my_function is a weak symbol.

3. Linking Behavior: When you link your program, if there is another definition of my_function that is not marked as weak (i.e., a strong symbol), the linker will use that strong definition. If no strong definition exists, the weak definition will be used.

4. Practical Example: This feature is often used in libraries where you might want to provide default implementations of functions that can be overridden by user-defined implementations. For example:

   void my_function() {
       // Default implementation
   }
   
   void my_function() __attribute__((weak));
   

   In this case, if a user provides their own implementation of my_function, that implementation will be used instead of the default.

Summary

Using the __attribute__((weak)) in GCC allows developers to create flexible APIs and libraries where default behaviors can be overridden by user-defined functions. This is particularly useful in scenarios like plugin systems or when providing default handlers that can be customized.

For more detailed information, you can refer to the GCC documentation on attributes.