Recursion 2 Explanation

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define sg struct gammal
struct gammal{
-char name[20];
-int payment;
-struct gammal *next, *prev;
};
sg* add (sg *g, sg *head){
-if(head==NULL || g->next == head){
--sg *t = (sg*)malloc(sizeof(sg));
--printf("Enter the name: ");
--scanf("%s",t->name);
--printf("Enter payment: ");
--scanf("%d",&t->payment);
--if(head == NULL){
---t->next = t;
---t->prev = t;
---return t;
--}
--t->next = head;
--head->prev = t;
--g->next = t;
--t->prev = g;
--return g;
-}
-g->next = add(g->next,head);
-return g;
}
void show (sg *g,sg *head){
-do{
--printf("------ ");
--printf("%s ",g->name);
--printf("%d ",g->payment);
--g = g->prev;
-}while(g != head);
-printf("------ ");
}
int main()
{
-int a;
-sg *head=NULL;
-do{
--printf("1) Add "
----"2) Show "
----"Please, enter a number: ");
--scanf("%d",&a);
--if(a==1)
---head = add(head,head);
--else if(a==2)
---show(head,head);
-}while( a );
}