__COUNTER__ macro in gcc/clang

Both gcc and clang define __COUNTER__ marco:

Defined to an integer value that starts at zero and is incremented each time the COUNTER macro is expanded.

Check following code:

# cat foo.c
#include <stdio.h>

void foo1(void)
{
    printf("%s:%d\n", __func__, __COUNTER__);
}

void foo2(void)
{
    printf("%s:%d\n", __func__, __COUNTER__);
}
# cat bar.c
#include <stdio.h>

void bar1(void)
{
    printf("%s:%d\n", __func__, __COUNTER__);
}

void bar2(void)
{
    printf("%s:%d\n", __func__, __COUNTER__);
}
# cat main.c
#include "foo.h"
#include "bar.h"

int main(void)
{
    foo1();
    foo2();
    bar1();
    bar2();
    return 0;
}

Run the program:

# ./main
foo_1:0
foo_2:1
bar_1:0
bar_2:1

You can see for every translate unit (.c) file, the __COUNTER__ begins at 0.

P.S., the code can be referenced here.