Symbol Table in C Linked list

#include<stdio.h>
#include<conio.h>

#include<string.h>
#define null 0
int size=0;
void insert();
void del();
int search(char lab[]);
void modify();
void display();

struct symbtab
{
    char label[10];
    int addr;
    struct symbtab *next;
};

struct symbtab *first,*last;

void main()
{
        int op;
        int y;
        char la[10];
        do
        {
            printf("\nSYMBOL TABLE IMPLEMENTATION\n");
            printf("1.INSERT\n");
            printf("2.DISPLAY\n");
            printf("3.DELETE\n");
            printf("4.SEARCH\n");
            printf("5.MODIFY\n");
            printf("6.END\n");
            printf("\nEnter your option: ");
            scanf("%d",&op);
            switch(op)
            {
        case 1:
                insert();
                display();
                break;
        case 2:
                        display();
                break;
        case 3:
                del();
                display();
                        break;
        case 4:
                printf("Enter the label to be searched: ");
                scanf("%s",la);
                y=search(la);
                if(y==1)
                    printf("The label is already in the symbol table\n");
                else
                     printf("The label is not found in the symbol tablel\n");
                break;
        case 5:
                modify();
                display();
                break;
        case 6:
                break;
    }
        }
        while(op<6);
        getch();
}

void insert()
{
    int n;
    char l[10];
    printf("Enter the label: ");
    scanf("%s",l);
    n=search(l);
    if(n==1)
         printf("The label is already in the symbol table. Duplicate cant be inserted\n");
    else
     {
        struct symbtab *p;
        p=malloc(sizeof(struct symbtab));
        strcpy(p->label,l);
        printf("Enter the address: ");
        scanf("%d",&p->addr);
        p->next=null;
        if(size==0)
        {
                        first=p;
                        last=p;
        }
        else
        {
                        last->next=p;
                        last=p;
        }
        size++;
    }
}

void display()
{
    int i;
    struct symbtab *p;
    p=first;
    printf("\nLABEL\tADDRESS\n");
    for(i=0;i<size;i++)
    {
                    printf("%s\t%d\n",p->label,p->addr);
                    p=p->next;
    }
}
int search(char lab[])
{
    int i,flag=0;
    struct symbtab *p;
    p=first;
    for(i=0;i<size;i++)
    {
            if(strcmp(p->label,lab)==0)
                  flag=1;
             p=p->next;
    }
    return flag;
}

void modify()
{
    char l[10],nl[10];
    int add,choice,i,s;
    struct symbtab *p;
    p=first;
    printf("What do you want to modify?\n");
    printf("1.Only the label\n");
    printf("2.Only the address of a particular label\n");
    printf("3.Both the label and address\n");
    printf("Enter your choice: ");
    scanf("%d",&choice);
    switch(choice)
                {
        case 1:
                printf("Enter the old label\n");
                scanf("%s",l);
                printf("Enter the new label: ");
                scanf("%s",nl);
                s=search(l);
                if(s==0)
                                printf("\nNo such label");
                else
                {
                                for(i=0;i<size;i++)
                                {
                                                if(strcmp(p->label,l)==0)
                                                {
                                                                strcpy(p->label,nl);
                                                }
                                                p=p->next;
                                }
                }
                break;
        case 2:
                printf("Enter the label whose address is to be modified: ");
                scanf("%s",l);
                printf("Enter the new address: ");
                scanf("%d",&add);
                s=search(l);
                if(s==0)
                                printf("\nNo such label");
                else
                {
                                for(i=0;i<size;i++)
                                {
                                                if(strcmp(p->label,l)==0)
                                                {
                                                                p->addr=add;
                                                }
                                                p=p->next;
                                }
                }
                break;
        case 3:
                printf("Enter the old label: ");
                scanf("%s",l);
                printf("Enter the new label: ");
                scanf("%s",nl);
                printf("Enter the new address: ");
                scanf("%d",&add);
                s=search(l);
                if(s==0)
                                printf("\nNo such label");
                else
                {
                  for(i=0;i<size;i++)
                    {
                     if(strcmp(p->label,l)==0)
                       {
                     strcpy(p->label,nl);
                     p->addr=add;
                            }
                            p=p->next;
                    }
                }
                break;
         }
}

void del()
{
    int a;
    char l[10];
    struct symbtab *p,*q;
    p=first;
    printf("Enter the label to be deleted: ");
    scanf("%s",l);
    a=search(l);
    if(a==0)
        printf("Label not found\n");
        else
        {
            if(strcmp(first->label,l)==0)
            first=first->next;
                        else if(strcmp(last->label,l)==0)
            {
                     q=p->next;
                     while(strcmp(q->label,l)!=0)
                     {
                       p=p->next;
                        q=q->next;
                    }
                    p->next=null;
                    last=p;
                }
                else
                {
                     q=p->next;
                     while(strcmp(q->label,l)!=0)
                    {
                 p=p->next;
                 q=q->next;
                }
              p->next=q->next;
              }
         size--;
     }
}