链接错误与编译错误

Question

为什么结构的双重声明会导致编译错误 ，而函数的双重定义会导致链接错误 ？

Answer 1

Because functions definitions are included in executable at link time but decalration or syntax check all are done at compile time

Consider one thing also when you are calling any function and compiler is not able to find the declaration of function then it will generate warning as implicit declaration of func() .

To remove this warning message we provide forward declaration of func, int func(); and it compiled without any warning message.

Do you think why does this happen ? It happens because compiler did not find that func() symbol. It's all upto the compiler to make code error free according to the Grammar of the Language.

But building of final executable is done at link time and then linker started looking for function defition of func() , If found then fine and if not .. then Linker error

could not have resolved external symbol _func()

Note: any external symbols are resolved at link time

On gcc for only compilation use this : (this may vary according to compiler)

gcc -Werror -c test.c --> It will generate test.o file

then try to link it and make executable

gcc -Werror -o test test.o --> test is executable

Answer 2

The standard doesn't say when an error should be reported - it's up to the compiler, but basically it's because that's when the errors are caught .

First, the compiler parses the files. It's easy to see if a struct or class was defined multiple times in the same translation unit ( only this is an error, between translation units you can have multiple class-type definition ), because it deals with that translation unit.

Second, it links the object files together (linking). Only now can it tell that the same symbol was exported multiple times, because that's when the error occurs per-say.

Answer 3

When compiling a program the compiler needs to know which exact definition of a structure to use. But we only need to know which exact function to use only when we are trying to link the program.

So if a structure is defined twice compiler is confused during compilation hence complains at compile time.

For a function during compile time you can have multiple definitions but confusion arises only during linking so it complains during linking.

Answer 4

What you are saying is not necessarily true.

If you "inline" the function definition in a header and then subsequently write its definition in the compilation unit you will get an error

...already has a definition.

The case you are referring to is when two different compilation units define (or see a definition) of the same function, so there is no compilation error in any individual compilation unit, it is the combination of them that causes the link error.

Note incidentally that this is where the keyword inline actually makes a difference. For a non-templated function defined in a header, if you use the inline keyword, it will mean multiple compilation units can exist with this function defined. It does not actually guarantee the compiler will inline it.