extern "C" void _main(multiboot_data* mbd, unsigned int magic);

void _main(void* mbd, unsigned int magic)
{
}

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g++ -o main.o -c main.cpp -Wall -Werror -nostdlib -nostartfiles -nodefaultlibs

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cc1plus: warnings being treated as errors
main.cpp:1: warning: ‘_main’ initialized and declared ‘extern’
main.cpp:1: error: variable or field ‘_main’ declared void
main.cpp:1: error: ‘multiboot_data’ was not declared in this scope
main.cpp:1: error: ‘mbd’ was not declared in this scope
main.cpp:1: error: expected primary-expression before ‘unsigned’
main.cpp:1: error: initializer expression list treated as compound expression
main.cpp: In function ‘void _main(void*, unsigned int)’:
main.cpp:3: error: ‘void _main(void*, unsigned int)’ redeclared as different kind of symbol
main.cpp:1: error: previous declaration of ‘int _main’