GCC:Multiple definitions of a global variable doesn't give linker error

Question

I stumbled upon this weird behaviour when compiling C programs using gcc.

Let's say we have these two simple source files:

fun.c

#include <stdio.h>

// int var = 10;   Results in Gcc compiler error if global variable is initialized
int var;

void fun(void) {
    printf("Fun: %d\n", var);
}

main.c

#include <stdio.h>
int var = 10;
int main(void) {
    fun();
    printf("Main: %d\n", var);
}

Surprisingly, when compiled as gcc main.c fun.c -o main.out this doesn't produce multiple definition linker error.

One would expect multiple definition linker error to occur just the same, regardless of global variable initialization. Does it mean that compiler makes uninitialized global variables extern by default?

Possible duplicate of Two variables with same name and type, in two different .c files, compile with gcc — LPs
– LPs, Commented Mar 16, 2016 at 16:09
stackoverflow.com/q/21275992/3049655 , stackoverflow.com/questions/1490693/… — Spikatrix
– Spikatrix, Commented Mar 16, 2016 at 16:22
Related note, when accessing within a shared library, you can find both with dlsym(). This is what the RTLD_NEXT argument is for. I’m not aware of how the order is determined, however.Probably a good practice to avoid it, or share it by defining it once and using the extern keyword in other files. — mzpq
– mzpq, Commented Dec 25, 2018 at 22:39

Lee Daniel Crocker · Accepted Answer · 2016-03-16 16:07:56Z

1

A global variable can have any number of declarations, but only one definition. An initializer makes it a definition, so it will complain about having two of those (even if they agree).

answered Mar 16, 2016 at 16:07

Lee Daniel Crocker

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3 Comments

Marin Veršić Over a year ago

I do not believe you are correct because int var; is a variable declaration and definition. The fact that you can use it right away proves it is also a definition, does it not?

Lee Daniel Crocker Over a year ago

The C99 standard calls that a "tentative definition"; The "multiple definition" error message (and I) are still using the old K&R terminology.

Marin Veršić Over a year ago

Yeah, that is it, as I just found out from @Cool Guy's comment

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