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From: Tim Rentsch <tr.17687@z991.linuxsc.com>
Newsgroups: comp.lang.c
Subject: Re: gcc and 'include'
Date: Thu, 26 Mar 2026 19:06:28 -0700
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Bart <bc@freeuk.com> writes:

> Take this program:
>
>   #include <stdlib.h>
>
>   inline int F(){return rand();}
>
>   int main(void) {
>       return F();
>   }
>
> This compiles fine with gcc using -O1 -O2 -O3.
>
> But compile without optimising, and it fails to link as it can't find
> a function 'F'.
>
> Is it supposed to behave like that?

The C standard allows the observed behavior.

Here is a different formulation:

    /* ??? */
    inline int F();

    int
    main(void){
        return F();
    }

    inline int
    F(){
        return 0;
    }

This program shows (under gcc, in my environment) ths same behavior
as what you describe.  A conforming implementation may choose to
compile and run the program without problem, or it may choose to
fail the program because there is no definition of F() with
external linkage (and that statement is true regardless of
optimization setting).

If the comment line (with question marks) is replaced by either

    extern

or

    static

then the program links and runs without problem.