C has a keyword called inline. It's a source of considerable grief among people who don't understand what it does—which, as I've discovered recently, includes people who otherwise understand C very well.

It's common to see people make assertions like “you should only ever use inline with static,” which turns out to be reasonable advice. I don't think advice without explanation is very useful, though, so what follows is my attempt to explain inline's strange behaviour.

Let's say you have a program, and, like all good programs, it looks like this:

#include <stdio.h>

inline void foo(void) {
    puts("hi");
}

int main(void) {
    foo();
    return 0;
}

It looks good! Let's compile it:

$ gcc -otest -std=c99 test.c
/tmp/ccBRwXBz.o: In function `main':
test.c:(.text+0x5): undefined reference to `foo'
collect2: ld returned 1 exit status

Of those I've spoken to, most people's reaction is that this is a compiler bug. I'm no gcc evangelist, but in this instance it is actually behaving correctly; the standard doesn't require this program to work.

The definition of foo in my example has the inline keyword and no extern keyword—this is true for every declaration of foo, since it's the only one. N1570 §6.7.4p7 stipulates that when this is the case, the identifier foo is declared with external linkage, but that no external definition is provided.

In other words, foo refers to an external function that we haven't defined. An inline definition of foo has been provided, and is available to the compiler as a substitute for the external definition, but the compiler isn't required to use it.

The upshot of this is that when a function is declared as inline without also being declared static or extern, programs commonly get into a state where they depend on an unspecified compiler decision in order to link. This isn't always the case—the behaviour could be used to provide an inline alternative to an external function defiend in another translation unit, which is presumably why it's still a part of C.

If I were a writer, this is where I would finish with some clever and final-sounding remark.