Random Queue in C - c

Question

I've written this code with all correct understandings i have. please check my problems.
#include<stdio.h>
#include<stdlib.h>
// Define a structure for the dequeue elements
This structure is all good, with data, next, previous pointers.
typedef struct RanElmt_ {
void *data;
struct DeqElmt_ *prev;
struct DeqElmt_ *next;
void (*destroy)(void *data);
//Your Code here
} RanElmt;
THis is ok too, acording to what i think is correct.
typedef struct RandQ_{
int size;
struct RanElmt *head;
struct RanElmt *tail;
}RandQ;
RandQ * RandomizedQueue(void (*destroy)(void *data)){
RandQ *relmt = (RandQ*)malloc(sizeof(RandQ));
} // construct an empty randomized queue
int isREmpty(RandQ *rQ){
if ( rQ->size == 0)
return 1;
return 0;
} // is the queue empty?
int rsize(RandQ *rQ){
return rQ->size;
}
// return the number of items on the queue
ACtually this is only one function,(enqueue) I'm going to get the idea and code other functions(dequeue, sample etc..)
int enqueue(RandQ *rQ, const void *data){
RanElmt *relmt = (RanElmt*)malloc(sizeof(RanElmt));
relmt->data = (void*)data;
if (rQ->head == NULL){
relmt = rQ->head;
relmt = rQ->tail;
relmt->prev = NULL;
relmt->next = NULL;
}
else{
rQ->head = relmt;
}
(rQ->head)->prev = relmt;
relmt->prev = rQ->head;
rQ->head = relmt;
} // add the item
main(){
Deque(free);
printf(" okk \n");
}
THis program is giving these errors:
Errors i'm getting

Answer 1

In C struct tags and type names live in different name spaces. That is struct RanElmt and RanElmt are two different types, in addition struct RanElmt is not completely defined.
Your RandQ should be defined something like
typedef struct RandQ_{
int size;
struct RanElmt_ *head; // or RanElmt* head;
struct RanElmt_ *tail; // or RanElmt* tail;
}RandQ;
in addition your RanElmt is probably not what you want, maybe you meant:
typedef struct RanElmt_ {
void *data;
struct RanElmt_ *prev; // pointer to a struct of the same type
struct RanElmt_ *next; // pointer to a struct of the same type
void (*destroy)(void *data);
// You cannot put code here in C (or even a function definition AFAIK).
} RanElmt;

Answer 2

You have confused the struct tag and the typedeffed alias for the queue elements in the definition of the queue:
typedef struct RandQ_{
int size;
struct RanElmt *head;
struct RanElmt *tail;
} RandQ;
Here, the head and tail are of the type struct RanElmt. This struct doesn't exist in your program. You have a struct RanElmt_ (with trailing underscore) that you can also call ´RanElmtwithout thestructkeyword, because you have combined the struct definition with atypedef`.
The compiler still generates the code, because pointers to unknown structs are okay, unless you try to get at their data. Obviously the compiler can't access the struct fields if it doesn't know them.
There's no need for the underscore. The names of structs are in a separate namespace, so you can have both a struct called RandQ and a type (in global namespace) called RanQ. I recommend to use the same name for struct tag and aliassed type.
You can also get rid of the need to use the struct keyword inside the struct defnition if you separate the typedef from the struct definition:
typedef struct RanElmt RanElmt; // use just RanElmt from now on
struct RanElmt {
void *data;
RanElmt *prev;
RanElmt *next;
} RanElmt;
Your code has several other problems, but I think he program is in an early state, so I don't address them here.