#include <stdio.h>
#define max 20
int stk[max];
int top=-1;
void push(int);
int pop();
void peep();
void display();
int isFull();
int isEmpty();

int main()
{
    int ch,item;
    do
    {
        printf("....Stack Operations....\n");
        printf("Press 1 for Push\n");
        printf("Press 2 for pop\n");
        printf("Press 3 for peep\n");
        printf("Press 4 for display\n");
        
        printf("Enter your choice \n");
        scanf("%d",&ch);
        switch(ch)
        {
            case 1:
                    printf("Enter the item :- \n");
                    scanf("%d",&item);
                    push(item);
                    break;
            case 2:
                    int k=pop();
                    printf("The poped element is %d\n",k);
                    break;
            case 3:
                    peep();
                    break;
            case 4:
                    display();
                    break;
            default:
                    printf("Invalid Choice\n");
        }
    }while(ch>=1 && ch<=5);
    
    void push(int item)
    {
        if(isFull())
        printf("Stack Overflow\n");
        else
        {
            top=top+1;
            stk[top]=item;
        }
    }
    int pop()
    {
        int s;
        if(isEmpty())
        printf("Stack Underflow\n");
        else
        {
            s=stk[top];
            top=top-1;
        }
        return s;
    }
    void peep()
    {
        if(isEmpty())
        printf("Stack Underflow\n");
        else
        {
            printf("Topmost Element of the stack is %d",stk[top]);
        }
    }
    void display()
    {
        if(isEmpty())
        printf("Stack Underflow\n");
        else
        {
            for(int i=top;i>=0;i--)
            printf("%d ",stk[i]);
        }
    }
    int isFull()
    {
        if(top==max-1)
        return 1;
        else
        return 0;
    }
    int isEmpty()
    {
        if(top==-1)
        return 1;
        else
        return 0;
    }
}

我希望程序运行所有堆栈操作，但我得到这种类型的特定错误：-

/usr/bin/ld: /tmp/ccHJ1ZkB.o: in function `main':
main.c:(.text+0xf7): undefined reference to `push'
/usr/bin/ld: main.c:(.text+0x103): undefined reference to `pop'
/usr/bin/ld: main.c:(.text+0x12b): undefined reference to `peep'
/usr/bin/ld: main.c:(.text+0x137): undefined reference to `display'
collect2: error: ld returned 1 exit status