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cmacroslustre

What does ({;}) and ({0;}) in a macro definition signify?

I was going through the lustre source code and was stuck at the macro definition: 

#define ldlm_namespace_proc_unregister(ns)      ({;})  
#define ldlm_namespace_proc_register(ns)        ({0;})

defined in the file lustre/ldlm/ldlm_resource.c.
What does this macro definition signify?

Solution

  • Macros are a plain text replacement. This macro means that a piece of code ldlm_namespace_proc_register(x) will be transformed to ({0;}). Nothing more and nothing less.

    If you are also wondering about the meaning of the code ({;}) and ({0;}) then these are GCC expression statements.

    According to that documentation, ({0;}) should be exactly the same as 0, and ({;}) is an expression of type void.

    Speculation follows: the purpose of these macros might be to support the user of the library writing code like this:

    int result = ldlm_namespace_proc_register(x);
// ...
ldlm_namespace_proc_unregister(x);

    but also that depending on compiler switches or other configuration, this code may either actually call a function, or in the case where your lines are enabled, actually do nothing.