How to have inline functions in multiple files

Question

My project has the following 4 files: main.c, rcm.h, rcm.c and queue.c.

In rcm.h I'm declaring all functions implemented in rcm.c and queue.c.

rcm.c looks like:

#include "rcm.h"

void rcm (void) {

  Queue *Q = init(10);

  /* Some other stuff */
}

queue.c` looks like:

#include "rcm.h"

extern inline Queue* init(int n) {
  return malloc(sizeof(Queue*);
}

and rcm.h:

#ifndef __RCM__
#define __RCM__

typedef struct queue { /*...*/ } Queue;

void rcm( void );

inline Queue* init( int n );
#endif

When compiling I get the the warnings

 gcc-7 -O0 -c rcm.c -o rcm.o
 In file included from rcm.c:15:0:
 rcm.h:58:15: warning: inline function 'init' declared but never defined
 inline Queue* init(int size);
               ^~~~
 gcc-7    -c queue.c -o queue.o
 gcc-7 main.c lib/rcm.o queue.o -o main
In file included from main.c:4:0:
inc/rcm.h:58:15: warning: inline function 'init' declared but never defined
 inline Queue* init(int size);
           ^~~~

But, when I am not declaring init() as inline compiles normally.

The question is: Why do you declare the inline function init() in translation units, that do not define it? A function can only be inlined, if the definition is present in the source, it is not possible to inline it from other translation units. — Ctx
– Ctx, Commented Feb 3, 2020 at 12:23
Declare your inline functions (i.e. including the code) in the .h file. — Paul Ogilvie
– Paul Ogilvie, Commented Feb 3, 2020 at 13:08

emacs drives me nuts · Accepted Answer · 2020-02-05 21:17:51Z

1

inline Queue* init( int n );

In order for a compiler to be able to inline a function, it must know he code of a function. Without that knowledge, the compiler must emit a call to that function¹. Hence the warning. In order to use an inline function in several modules, you can define it in the header as:

static inline Queue* init (int n)
{
    /* code here */
}

Cf. for example the GCC documentation for inline.

The reason for the warning is that you want the function to be inline, but you are hiding the code from the compiler: main.c includes the header that declares an inline function but in that compilation unit, init is defined (implemented) nowhere. ¹ Except for functions built-in the compiler. In that case, you don't have to provide the code yourself, it compiler has build-in knowledge about it.

edited Feb 5, 2020 at 21:17

answered Feb 3, 2020 at 13:54

emacs drives me nuts

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