HOW TO CREATE USER DEFINED FUNCTION IN C

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

void addition(void);
void subtraction(void);
void multiply(void);
void divition(void);
void main()
{
    char name;
    clrscr();
    printf("\tpress A for Addition.\n\n");
    printf("\tpress S for Subtraction.\n\n");
    printf("\tpress M for Multiply.\n\n");
    printf("\tpress D for Divition.\n\n\t");
    name=getchar();
    switch(name)
    {
        case 'A':
        addition();
        break;
        case 'a':
        addition();
        break;
        case 'S':
        subtraction();
        break;
        case 's':
        subtraction();
        break;
        case 'M':
        multiply();
        break;
        case 'm':
        multiply();
        break;
        case 'D':
        divition();
        break;
        case 'd':
        divition();
        break;
        default:
        printf("\tplease re-enter your choise.\n\n");
        printf("\tThank you for chosing  u & i softwere.\n\n\tCreated By Dixit Panchal.\n\n\tpress any key to continues\n");
        break;
     }
getch();
}
    void addition(void)

    {
    int i,u,ans;
    printf("\tplease enter your two value which you want to Addtion.\n\n\t");
    scanf("%d\t%d",&i,&u);
    ans=i+u;
    printf("\tYour answer is = %d",ans);
    }

    void subtraction(void)

    {
    int u,i,ans;
    printf("\tplease enter your two value which you want to Subtraction.\n\n\t");
    scanf("%d\t%d",&u,&i);
    ans=u-i;
    printf("\tYour answer is = %d",ans);
    }

    void multiply(void)

    {
    int u,i,ans;
    printf("\tplease enter your value which you want to Multiply.\n\n\t");
    scanf("%d\t%d",&u,&i);
    ans=u*i;
    printf("\tYour ans is = %d",ans);
    }

    void divition(void)

    {
    int u,i,ans;
    printf("\tplease enter your value which you want to Divition.\n\n\t");
    scanf("%d\t%d",&u,&i);
    ans=u/i;
    printf("\tYour answer is = %d",ans);
    }