Multiple definition of inline function

cinline()

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 inline.h and two translation units main.cpp and tran.cpp.

Details of code are as below

inline.h

#ifndef __HEADER__
#include <stdio.h>
extern inline int func1(void)
{ return 5; }

static inline int func2(void)
{ return 6; }

inline int func3(void)
{ return 7; }
#endif

main.c

#define <stdio.h>
#include <inline.h>
int main(int argc, char *argv[])
{
    printf("%d\n",func1());
    printf("%d\n",func2());
    printf("%d\n",func3());
    return 0;
}

tran.cpp

//(note that the functions are not inline here)
#include <stdio.h>
int func1(void)
{ return 500; }

int func2(void)
{ return 600; }

int func3(void)
{ return 700; }

The above code compiles in g++, but does not compile in gcc (even if you make changes related to gcc like changing the code to .c, not using any C++ header files, etc.). The error displayed is "duplicate definition of inline function – func3".

Can you clarify why this difference is present across compilers?

Also, when you run the program (g++ compiled) by creating two separate compilation units (main.o and tran.o) and create an executable a.out, the output obtained is:

500
6
700

Why does the compiler pick up the definition of the function which is not inline. Actually, since #include is used to "add" the inline definition I had expected 5,6,7 as the output. My understanding was during compilation since the inline definition is found, the function call would be "replaced" by inline function definition.

Can you please tell me in detailed steps the process of compilation and linking which would lead us to 500,6,700 output. I can only understand the output 6.

Best Answer

This answer is divided into the following sections:

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

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

The problem can be successfully reproduced by

Rename tran.cpp to tran.c
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.

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.

Function pointers in C

Let's start with a basic function which we will be pointing to:

int addInt(int n, int m) {
    return n+m;
}

First thing, let's define a pointer to a function which receives 2 ints and returns an int:

int (*functionPtr)(int,int);

Now we can safely point to our function:

functionPtr = &addInt;

Now that we have a pointer to the function, let's use it:

int sum = (*functionPtr)(2, 3); // sum == 5

Passing the pointer to another function is basically the same:

int add2to3(int (*functionPtr)(int, int)) {
    return (*functionPtr)(2, 3);
}

We can use function pointers in return values as well (try to keep up, it gets messy):

// this is a function called functionFactory which receives parameter n
// and returns a pointer to another function which receives two ints
// and it returns another int
int (*functionFactory(int n))(int, int) {
    printf("Got parameter %d", n);
    int (*functionPtr)(int,int) = &addInt;
    return functionPtr;
}

But it's much nicer to use a typedef:

typedef int (*myFuncDef)(int, int);
// note that the typedef name is indeed myFuncDef

myFuncDef functionFactory(int n) {
    printf("Got parameter %d", n);
    myFuncDef functionPtr = &addInt;
    return functionPtr;
}

The difference between a definition and a declaration

A declaration introduces an identifier and describes its type, be it a type, object, or function. A declaration is what the compiler needs to accept references to that identifier. These are declarations:

extern int bar;
extern int g(int, int);
double f(int, double); // extern can be omitted for function declarations
class foo; // no extern allowed for type declarations

A definition actually instantiates/implements this identifier. It's what the linker needs in order to link references to those entities. These are definitions corresponding to the above declarations:

int bar;
int g(int lhs, int rhs) {return lhs*rhs;}
double f(int i, double d) {return i+d;}
class foo {};

A definition can be used in the place of a declaration.

An identifier can be declared as often as you want. Thus, the following is legal in C and C++:

double f(int, double);
double f(int, double);
extern double f(int, double); // the same as the two above
extern double f(int, double);

However, it must be defined exactly once. If you forget to define something that's been declared and referenced somewhere, then the linker doesn't know what to link references to and complains about a missing symbols. If you define something more than once, then the linker doesn't know which of the definitions to link references to and complains about duplicated symbols.

Since the debate what is a class declaration vs. a class definition in C++ keeps coming up (in answers and comments to other questions) , I'll paste a quote from the C++ standard here.
At 3.1/2, C++03 says:

A declaration is a definition unless it [...] is a class name declaration [...].

3.1/3 then gives a few examples. Amongst them:

[Example: [...]
struct S { int a; int b; }; // defines S, S::a, and S::b [...]
struct S; // declares S
—end example

To sum it up: The C++ standard considers struct x; to be a declaration and struct x {}; a definition. (In other words, "forward declaration" a misnomer, since there are no other forms of class declarations in C++.)

Thanks to litb (Johannes Schaub) who dug out the actual chapter and verse in one of his answers.