Including a header file from another directory

When referencing to header files relative to your c file you should use #include "path/to/header.h"

The form #include <someheader.h> is only used for internal headers or for explicitly added directories (in gcc with the -I option).

write

#include "../b/structure.h"

in place of

#include <structures.h>

then go in directory in c & compile your main.c with

gcc main.c