main.c
------------
#include <stdio.h>

int main(void){
    do();
    return 0;
};
-------------
do.c
-------------
#include <stdio.h>
void do(){
    printf("hello world");
}
-------------
gcc main.c do.c
-->
a.c:1:6: error: expected identifier or ‘(’ before ‘do’
 void do(void){
      ^~
segment.c: In function ‘main’:
segment.c:4:5: error: expected expression before ‘)’ token
  do();
     ^