"struct" before already-declared struct - c

Question

I am still new in C. I know you can just use the already-declared struct as new data type such as int, double, etc. However, I encounter a struct written like this:
struct AdjListNode
{
int dest;
int weight;
struct AdjListNode* next;
};
In this struct, the data type of "next" pointer is struct AdjListNode*. What does struct have to do with the already-declared AdjListNode*? Thanks!

Answer 1

What does struct have to do with the already-declared AdjListNode*?
The answer is that the c syntax requires it.
You do not get a type AdjListNode by writing struct AdjListNode { ... };
AdjListNode is a struct tag and you always have to use struct AdjListNode when declaring variables.
See this simple example (without pointer inside the struct):
#include <stdio.h>
struct sSomeName
{
int x;
};
int main(void) {
struct sSomeName var; // OK, variable of type struct sSomeName
struct sSomeName* pVar; // OK, pointer to variable of type struct sSomeName
// sSomeName var2; // ERROR: unknown type name 'sSomeName'
var.x = 5;
pVar = &var;
printf("%d\n", pVar->x);
return 0;
}
So if you want to add a pointer inside the struct, you must write struct sSomeName just as you have to do inside main, i.e. like:
struct sSomeName
{
int x;
struct sSomeName* p;
};
Using typedef
If you want a type named AdjListNode you must use typedef.
A typedef example could look like:
#include <stdio.h>
typedef struct sSomeName sSomeName;
struct sSomeName
{
int x;
sSomeName* p;
};
int main(void) {
sSomeName var;
sSomeName* pVar;
var.x = 5;
var.p = NULL;
pVar = &var;
printf("%d\n", pVar->x);
printf("%p\n", (void*)pVar->p);
return 0;
}

Answer 2

Here with that declaration pointer to structure is declared. This is basically used for implementing linked list or for other data structures like tree.
It does'nt mean that struct is re-declared. It is similar to declare a struct variable.

Answer 3

The structure is created as follows: typedef struct AdjListNode. Example:
#include <stdio.h>
#include <stdlib.h>
typedef struct AdjListNode
{
int dest;
int weight;
struct AdjListNode* next;
}AdjListNode;
typedef struct Nodo{
char *nombre;
int *edad;
struct Nodo *siguiente;
}Nodo;
int main(int argc, char **argv) {
AdjListNode *nodo=malloc(sizeof(AdjListNode));
nodo->dest=1;
nodo->weight=2;
nodo->next=NULL;
printf("Nodo-->dest: %d", nodo->dest);
free(nodo);
}