C: Typedef structure and pointers anomaly -

i on data structures classes , confused me.

it's related pointers properties guess, on research didn't find real explanation, idea why c allows this?

run-able code: http://ideone.com/kgh3lf

#include <stdio.h> #include <stdlib.h>  /* declaring typedef struct  */ typedef struct{     int a;     char b[10]; }struct_one;  /* declaring structure, intentional wrong calling of first structure */ struct struct_two{     int p;     char q[10];      /* doesn't work expected... should be: struct_one var; */     // struct struct_one var;      /* 1 work!!, , i'm not sure why */     struct struct_one *ptr; };  int main(void) {     /* code */     return 0; } 

you incorrectly assumed second structure "calls" first one. doesn't.

in reality first struct declaration declares untagged struct type typedef alias struct_one. way refer type struct_one. struct_one, not struct struct_one.

the second struct declaration declares struct struct_two, refers type struct struct_one. latter type has absolutely nothing declared type struct_one: struct struct_one , struct_one 2 different, unrelated types. reference struct struct_one inside second struct seen introduction, declaration of new type struct struct_one. compiler assumes define type later (if necessary).

since not doing else in code, not run problems caused such mistake. additional illustration of happen, see following example

int main(void) {     struct_one *p = 0;     struct struct_two s2;     s2.ptr = p;            return 0; } 

this code produce diagnostic message compiler, because s2.ptr = p; assignment illegal: pointer types unrelated. pointer of left hand side struct struct_one * , pointer on right-hand side struct_one *.

once again, in c language when use struct <something> syntax in contexts not require complete type, can write virtually in place of <something>: complete gibberish (as long lexically valid identifier). if type not yet known, compiler assume introducing new type, e.g.

int main() {   struct klshcjkzdcdsamcbsj78q43698 *p = 0; } 

the above valid c program.