Program to implement DEQUEUE (to insert elements from rear/front and also delete elements from rear/front) concept using linked list

linked_dequeue.c

#include <stdio.h>
#include<stdlib.h>
struct node
{
    int val;
    struct node *link;
};

struct node* getnode( )
{
    return ( struct node *)malloc( sizeof( struct node ));
};

struct node* inf( struct node *); struct node* def( struct node *);
struct node* der( struct node *); struct node* inr( struct node *);
void display( struct node *);

int main()
{
  struct node *F;
  int ch;
  F = NULL;
  while( 1 )
  {
      printf("1. To insert at front\n");
      printf("2. To delete from front\n");
      printf("3. To insert at rear\n");
      printf("4. To delete from rear\n");
      printf("5. Display \n");
      printf("6. Exit\n");
      scanf("%d", &ch );
      if( ch == 1 )
        F = inf (F);
      else if( ch == 2 )
       F = def (F);
      else if( ch == 3 )
      F = inr (F);
      else if( ch == 4 )
       F = der (F);
      else if( ch == 5 )
        display(F);
      else
        break;
  }
}

struct node* inf( struct node *FIRST)
{
    struct node *T; struct node *P;
    if( FIRST == NULL )
    {
        FIRST = getnode();
        T = FIRST;
        T->link=NULL;
    }
    else
    {
        P=getnode();
        P->link=FIRST;
        FIRST=P;
        T=FIRST;
    }
    printf("Enter the element to push\n");
    scanf("%d", &T->val);
    return FIRST;
}

struct node* inr( struct node *FIRST)
{
    struct node *T;
    if( FIRST == NULL )
    {
        FIRST = getnode();
        T = FIRST;
    }
    else
    {
        T = FIRST;
        while( T->link != NULL ) T = T->link;

        T->link = getnode( );
        T = T->link;
    }
    printf("Enter the element to push\n");
    scanf("%d", &T->val);
    T->link = NULL;
    return FIRST;
}

struct node* def( struct node *FIRST)
{ struct node* T; T=FIRST;
   if(FIRST==NULL)
 {
     printf("NO Elements\n"); return(FIRST);
 }
    printf("The deleted value is: %d\n",T->val);
    T=T->link;
    FIRST=T;
    return FIRST;
}

struct node* der( struct node *FIRST)
{ struct node* T; T=FIRST;
 if(FIRST==NULL)
 {
     printf("NO Elements\n"); return(FIRST);
 }
  struct node* P=T;
   while(T->link!=NULL)
    { P=T;
      T=T->link;
    }
    printf("The deleted value is: %d\n",T->val);
     if(P==T)
     FIRST=NULL;
     else
     P->link=NULL;
    return FIRST;
}

void display( struct node *T)
{  if(T==NULL)
   {
       printf("Empty \n"); return;
   }
   printf("The Elements present are: \n");
    while( T!= NULL )
    {
        printf("%d  ",T->val);
        T = T->link;
    }
    printf("\n");
}