Macro define_dlsym_reloc 

macro_rules! define_dlsym_reloc {
    (
        $(
            $(#[$meta:meta])*
            pub $(($where:tt))? fn $sym:ident ( $( $args:ident: $types:ty ),* $(,)? ) $( -> $ret:ty )?;
        )+
    ) => { ... };
}

Define an external symbol but don’t mark it for linking by system linker. Instead the symbol is going to be looked up dynamically at first call using dlsym.

If the symbol defined that way is available then it’s address is saved in a static variable and subsequent calls to the function will use that variable.

However if the symbol is not defined then the program panics.

To avoid the panic the code should first check if the symbol is actually available for example using [has_dyn_symbol] or via a helper function like crate::ffi::has_fiber_id.

The benefit of this approach is that it allows for code which doesn’t need to be recompiled for use in different versions of tarantool or picodata.

More specifically the problem we’re solving is that when your module defines an extern "C" symbol the dynamic linker will attempt to link it while the shared object is being loaded into tarantool (or picodata) and if the symbol is not found the load is aborted completely.