multiple definition of inline function

Question

I have gone through some posts related to this topic but was not able to sort out my doubt completely. This might be a very naive question.

I have a header file

Answer 1

Maybe you should post the actual code. The snippets you show don't compile:

• inline.h has extern inline int func1(void) That doesn't make any sense.
• main.h has #define <stdio.h> I think you meant include instead.

Once I fixed those and compiled with gcc, it compiled fine and I got the following output

5
6
7

When I compile with g++, I get this output:

5
6
700

That happens because func3() is not static in inline.h

Answer 2

This answer is divided into the following sections:

1. How to reproduce the duplicate definition of inline function - func3 problem and why.
2. Why defintion of func3 is a duplicate instead of func1.
3. Why it compiles using g++

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How to produce the duplicate definition of inline function - func3 problem

The problem can be successfully reproduced by

1. Rename tran.cpp to tran.c
2. Compile with gcc -o main main.c tran.c

@K71993 is actually compiling using the old gnu89 inline semantics, which is different from C99. The reason for renaming tran.cpp to tran.c is to tell the gcc driver to treat it as C source instead of C++ source.

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Why definition of func3 is a duplicate instead of func1.

GNU 89 inline semantics

The following text is quoted from GCC Document: An Inline Function is As Fast As a Macro explains why func3 is a duplicate definition instead of func1, since func3 (instead of func1) is an externally visible symbol (in GNU89 inline semantics)

When an inline function is not static, then the compiler must assume that there may be calls from other source files; since a global symbol can be defined only once in any program, the function must not be defined in the other source files, so the calls therein cannot be integrated. Therefore, a non-static inline function is always compiled on its own in the usual fashion.

If you specify both inline and extern in the function definition, then the definition is used only for inlining. In no case is the function compiled on its own, not even if you refer to its address explicitly. Such an address becomes an external reference, as if you had only declared the function, and had not defined it.

C99 inline semantics

If compiled with C99 standard, i.e., gcc -o main main.c tran.c -std=c99, the linker will complain that definition of func1 is a duplicate due to the reason that extern inline in C99 is a external definition as mentioned in other posts and comments.

Please also refer to this execellent answer about semantic differents between GNU89 inline and C99 inline.

Why it compiles using g++.

When compiled with g++, the source program are considered as C++ source. Since func1, func2 and func3 are defined in multiple translation units and their definitions are different, the One Defintion Rule of C++ is violated. Since the compiler is not required to generate dignostic message when definitions spans multiple translation units, the behavior is undefined.

Answer 3

The compile error you see is actually a linker error.

gcc and g++ are treating static inline a little differently. inline was first part of C++ and then made into an extension to many C compilers, before being added to standard C. The standard semantics could be different, but it could just be the implementations that are different.

It could also have something to do with some crazy stuff that happens with C++ code that gets rid of duplicate template stuff catching other duplicate stuff as well.

Answer 4

Your code is invalid from the C++ point of view, since it blatantly violates the One Definition Rule. The only reason you managed to compile it by C++ compiler is the loose error checking in your C++ compiler (it happens to be one of those parts of ODR where "no diagnostic is required").

Your code is not valid C, because it provides duplicate external definition of function func1. Note that it is func1, not func3 that is problematic from the C point of view. There's nothing formally wrong with your func3. Your func2 is also OK, as long as the two definitions never "meet" each other in the same translation unit.

One possible reason you might be getting a different diagnostic report from your compiler is that your C compiler might be supporting inline functions in some non-standard compiler-specific way (either a pre-C99 compiler or a modern compiler run in non-standard "legacy" mode).

Frankly, I find it hard to believe you are getting an error report about func3 from any compiler, assuming the code you posted accurately represents what you are trying to compile. Most likely what you posted is not the real code.

Answer 5

The compiling error is because there is a duplicate definition of func1();

Because func1() is defined using extern inline, it will produce a external definition.

However, there is also an external definition in tran.c, which cause multiple definition error.

However, func2() and func3() do not produce an external definition, hence no redefinition error.

You might want to look at here http://www.greenend.org.uk/rjk/2003/03/inline.html.

Also, take a note that c++ and c treats inline functions differently, and even in c, different standards (c89 vs. c99) treats inline functions differently.

Answer 6

basically Inline is a late entry to GCC ( I mean c compiler). "[ . . . ] An inline definition does not provide an external definition for the function, and does not forbid an external definition in another translation unit. An inline definition provides an alternative to an external definition, which a translator may use to implement any call to the function in the same translation unit. It is unspecified whether a call to the function uses the inline definition or the external definition." — ISO 9899:1999(E), the C99 standard, section 6.7.4