duplicate symbol test

a.c

#include <stdio.h>

int a = 1;

int main() {
    printf("%d\n", a);
}

b.c

int a = 2;

log.txt

$ gcc a.c b.c -o test
duplicate symbol _a in:
    /var/folders/sp/s96yk_gj46jf2nwkgtmylmnw0000gn/T/a-28e783.o
    /var/folders/sp/s96yk_gj46jf2nwkgtmylmnw0000gn/T/b-e8927c.o
ld: 1 duplicate symbol for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)