C: Var and Function have same name -- a bug of ld? - c

Question

In my project (https://github.com/zzt93/os-lab1), I encounter that a global variable has the same name with a function, but compile it produce on error or warning, which cause a bug.
A simple program which can almost reproduce this problem:
//a.c
struct {
int t;
int *s;
} empty, full;
int main(){
printf("full is at %p", &full);
printf("empty is at %p", &empty);
empty.t = 1;
return 0;
}
//b.c
int empty() {
return 1;
}
Compiling them use gcc -o res.out -Wall -g -Wextra a.c b.c
will just produce some warning like this (notice: in my project, it even produce no error):
/usr/bin/ld: Warning: alignment 1 of symbol empty in /tmp/ccq70SCM.o is smaller than 16 in /tmp/ccVCOeWq.o
/usr/bin/ld: Warning: size of symbol empty changed from 16 in /tmp/ccVCOeWq.o to 11 in /tmp/ccq70SCM.o
/usr/bin/ld: Warning: type of symbol empty changed from 1 to 2 in /tmp/ccq70SCM.o
it seems it take struct empty and function empty as the same one.
Decompile it, you can clearly see that linker link the address of function empty rather than that struct empty.So try running res.out will cause segment fault.
40054e: be 73 05 40 00 mov $0x400573,%esi
400553: bf 12 06 40 00 mov $0x400612,%edi
400558: b8 00 00 00 00 mov $0x0,%eax
40055d: e8 ae fe ff ff callq 400410 <printf#plt>
400562: c7 05 07 00 00 00 01 movl $0x1,0x7(%rip) # 400573 <empty>
400569: 00 00 00
40056c: b8 00 00 00 00 mov $0x0,%eax
400571: 5d pop %rbp
400572: c3 retq
0000000000400573 <empty>:
400573: 55 push %rbp
400574: 48 89 e5 mov %rsp,%rbp
400577: b8 01 00 00 00 mov $0x1,%eax
40057c: 5d pop %rbp
40057d: c3 retq
40057e: 66 90 xchg %ax,%ax
Question:
Why linker choose function rather than that struct? Am i right to think it as a bug?
why add a static for the declaration of struct can prevent this error? -- I understand that static make the variable invisible outside this file, but notice I add static to struct empty not function empty solving the problem.
Edit::
And strange enough, in the symbol table of res.out, there is only one empty
Name Value Class Type Size Line Section
empty |0000000000400573| T | FUNC|000000000000000b| |.text
I am using
gcc version 4.9.2

Answer 1

Adding static prevents the error because static, when applied to functions or global variables, makes the symbol not be exported to the linker - in simple words, it makes it "private" to that file.
If you don't use static, the linker will see both definitions, but the types don't match. However, since compilation is applied file by file, the linker has no way to know the correct type of a variable - it must trust that you did your job and didn't lie.
This is why header files are important - it makes sure that types match in different files.