Why is it that when you don't define a function you get a linker error and not a compiler?

Question

For Example

#include <iostream>

int add(int x, int y);

int main()
 {
    cout << add(5, 5) << endl;
 }

This would compile but not link. I understand the problem, I just don't understand why it compiles fine but doesn't link.

You don't compile different translation units all together. Therefore, this is impossible to do consistently. — chris, Nov 17 '13 at 23:36
Since the **linker** wants to **resolve** the declaration finally, as far it's used anywhere. — πάντα ῥεῖ, Nov 17 '13 at 23:37

score 5 · Answer 1 · answered Nov 17 '13 at 23:38

5

Because the compiler doesn't know whether that function is provided by a library (or other translation unit). There's nothing in your prototype that tells the compiler the function is defined locally (you could use static for that).

answered Nov 17 '13 at 23:38

Ben Voigt

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score 3 · Accepted Answer · answered Nov 17 '13 at 23:41

The input to a C or C++ compiler is one translation unit - more or less one source code file. Once the compiler is finished with that one source code, it has done its job.

If you call/use a symbol, such as a function, which is not part of that translation unit, the compiler assumes it's defined somewhere else.

Later on, you link together all the object files and possibly the libraries you want to use, all references are tied together - it's only at this point, when pulling together everything that's supposed to create an executable, one can know that something is missing.

More, definitely more. Unless you used no `#include` at all. — Ben Voigt, Nov 17 '13 at 23:42

score 1 · Answer 3 · answered Nov 18 '13 at 00:08

When a compiler compiles, it generates the output (object file) with the table of defined symbols (T) and undefined symbols (U) (see man page of nm). Hence there is no requirement that all the references are defined in every translation unit. When all the object files are linked (with any libraries etc), the final binary should have all the symbols defined (unless the target in itself is a library). This is the job of the linker. Hence based on the requested target type (library or not), the linker might not or might give an error for undefined functions. Even if the target is a non-library, if it is not statically linked, it still might refer to shared libraries (.so or .dll), hence if on the target machine while the binary is run, if the shared libraries are missing or if any symbols missing, you might even get a linker error. Hence between compiler, linker and loader, every one is trying to best provide you with the definition of every symbol needed. Here by giving declaring add, you are pacifying the compiler, which hopes that the linker or loader would do the required job. Since you didnt pacify the linker (by say providing it with a shared library reference), it stops and cribs. If you have even pacified the linker, you would have got the error in the loader.

Why is it that when you don't define a function you get a linker error and not a compiler?

3 Answers3

Linked