how to call function in executable from my library?

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In Linux/ELF you can pass the -export-dynamic option to the linker (-rdynamic on the compiler driver gcc) so symbols from the executable are exported to shared objects.

You’d have a dummy print implementation in your library, which would be shadowed by the implementation on your executable, since the executable is usually searched before shared objects for symbol resolution.

This has the disadvantage that it’s not very fine-grained, you could end up overriding some symbol you didn’t intend to. The finer-grained option would be to create a list of symbols to be exported as:

{
    print;
    <other symbols>
};

and pass that list to the linker, e.g. from gcc: -Wl,--dynamic-list=<file with list of symbols>

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