There is a serious problem with my code. What should I do? Can you help me modify it?

#include<stdio.h>
#include<stdlib.h>
#include<limits.h>

typedef struct Llist *List;

struct Llist{
    int data;
    struct Llist *next;
};

List readL(int num);
void attach(int num, List* rear);
List crt_pare(List a, List b);
int compare(int a, int b);
void print_result(List P);
int main()
{
    int n, m;
    List PtrL1, PtrL2, PtrL3;
    
    while(scanf("%d%d", &n, &m), n && m){
        PtrL1 = readL(n);
        PtrL2 = readL(m);
        PtrL3 = crt_pare(PtrL1, PtrL2);
        print_result(PtrL3);
    }
    return 0;
}
List readL(int num)
{
    List ptrL, rearL, temp;
    int r_n;
    
    ptrL = (List)malloc(sizeof(struct Llist));
    ptrL ->next = NULL;
    rearL = ptrL;
    while(num--){
        scanf("%d", &r_n);
        attach(r_n, &rearL);
    }
    temp = ptrL;
    ptrL = ptrL->next;
    free(temp);
    return ptrL;
}
List crt_pare(List a, List b)
{
    List c_1, c_2, c_3, c_temp;
    int a_flag = 0, b_flag = 0;
    List PtrL;
    c_1 = a; c_2 = b;
    c_3 = (List)malloc(sizeof(struct Llist));
    c_3 ->next = NULL;
    PtrL = c_3;
    while(c_1 || c_2){
        if(c_1 ->next)
            a_flag = 1;
        if(c_2 ->next)
            b_flag = 1;
        if(a_flag >0 && b_flag >0)
            a_flag = 0; b_flag = 0;
        switch(compare(c_1 ->data, c_2 ->data)){
            case 1: attach(c_2 ->data, &PtrL);
                    if(b_flag == 0)
                        c_2 = c_2 ->next;
                    else
                        c_2 ->data = INT_MAX;
                    break;
            case 0: attach(c_1 ->data, &PtrL);
                    if(a_flag == 0)
                        c_1 = c_1 ->next;
                    else
                        c_1 ->data = INT_MAX;
                    break;
        }
        if(a_flag >0 && b_flag >0)
            break;
    }
    c_temp = c_3;
    c_3 = c_3 ->next;
    free(c_3);
    return c_3;
}
void attach(int num, List* prear){
    List PtrL;
    PtrL = (List)malloc(sizeof(struct Llist));
    PtrL ->next = NULL;
    PtrL ->data = num;
    (*prear) ->next = PtrL;
    *prear = PtrL;
}
int compare(int a, int b)
{
    if(a >= b)
        return 1;
    else
        return 0;
}
void print_result(List P)
{
    if (!P)
        printf("^v^!!!, error, <..>");
    while(P){
        printf("%d ", P->data);
        P = P ->next;
    }
    printf("n");
}

Input
Multiple sets of data, each with three rows; the first row has n and m lengths for sequences A and B, the second row has n elements for A, and the third row has m elements for B (elements separated by Spaces). The input ends when n=0 and m=0.

Output
For each set of data output a row, for the merged sequence, each data separated by a space.
For example, enter Copy
5 5
1, 3, 5, 7, 9
2, 4, 6, 8, 10
5 6
1, 2, 2, 3, 5
2, 4, 6, 8, 10, 12
0 0
The example outputs Copy
1, 2, 3, 4, 5, 6, 7, 8, 9, 10
1, 2, 2, 3, 4, 5, 6, 8, 10, 12