gcc - C program acting weird -

i've started implementing circular queue in c, , have following lines of code:

#include <stdio.h> #include <stdlib.h> #include "cirq.h"  //allocate circular queue cirq cq_alloc(void){   cirq cq = null;   element *head;   element *tail;    if((head = malloc(sizeof(struct element*))) &&       (tail = malloc(sizeof(struct element *)))){     head->content = 0; // head node keeps track of size.     tail->content = null;     head->next = tail;     tail->next = head;     cq = &head;   } else {     printf("error: no space more cqueues.\n");   }   return cq; }  int cq_size(cirq q){   return (int)(*q)->content; }  int main(){   cirq q = cq_alloc();   printf("size of element ptr %lu\n", sizeof(struct element *));   printf("%d\n", cq_size(q));   return 0; } 

now when compile , run program, having commented out line in main prints out sizeof(struct element *)), program runs fine , right size of queue, 0. when leave line in, size of struct printed out, after segmentation fault: 11. also, make things clear, struct element has void *data , struct element *next fields. how can adding in line prints stuff change behavior of program much?

edit: cirq.h

#ifndef cirq_h #define cirq_h typedef struct element **cirq; // cirq handle  typedef struct element {   void *content;   struct element *next; } element;  extern cirq cq_alloc(void);// allocate queue extern int cq_size(cirq q);// return size of queue extern void cq_enq(cirq q, void *value);// add value queue extern void *cq_deq(cirq q);// dequeue , return queue value extern void *cq_peek(cirq q);// return value @ queue head extern void cq_rot(cirq q);// requeue head element @ tail extern void cq_free(cirq q);// return space allocated queue #endif 

it doesn't matter cirq is, fact return address of local object problem.

this here

cq = &head; 

is causing undefined behavior, because that's address of pointer head stored locally in function only, when function returns it's deallocated , invalid. using elsewhere (outside function) undefined behavior.

also, not typedef pointer. never that, let code reader know it pointer.