prog.c:12:13: warning: declaration does not declare anything
struct A;
^
prog.c: In function ‘main’:
prog.c:21:6: error: ‘struct B’ has no member named ‘value’
b.value = 5;
^
prog.c:19:14: warning: variable ‘b’ set but not used [-Wunused-but-set-variable]
struct B b;
^
prog.c:18:14: warning: unused variable ‘a’ [-Wunused-variable]
struct A a;
^