Longest Palindromic Subsequence Multithread in C - c

Question

I'm trying to learn how to multithread with c, and thought that the longest palindromic subsequence problem would be a good place to start.
The idea is that we run two threads and compare their results to find the answer. One thread deals with "odd" subsequences, the other with "even" ones.
Although the code below seems to work, my question is: where in the program should I check for multi-threading errors? It is very new to me so I just need to know what parts may be prone to the issues that multi-threading brings.
#include <stdio.h>
#include <stdlib.h>
#include <pthread.h>
struct str{
char* seq;
int len;
};
void *odd(void* arg){
struct str index = *(struct str*)arg;
int maxAns = 1;
for(int i = 1; i < index.len; i++){
int low = i - 1;
int high = i + 1;
int currMax = 1;
while(low >= 0 && high < index.len && index.seq[low] == index.seq[high]){
low--;
high++;
currMax=currMax+2;
}
if(currMax > maxAns){
maxAns = currMax;
}
}
int* res = malloc(sizeof(int));
*res = maxAns;
free(arg);
return (void*)res;
}
void *even(void* arg){
struct str index = *(struct str*)arg;
int maxAns = 0;
for(int i = 0; i < index.len; i++){
int low = i;
int high = i + 1;
int currMax = 0;
while(low >= 0 && high < index.len && index.seq[low] == index.seq[high]){
low--;
high++;
currMax=currMax+2;
}
if(currMax > maxAns){
maxAns = currMax;
}
}
int* res = malloc(sizeof(int));
*res = maxAns;
free(arg);
return (void*)res;
}
int main(void){
char seq0[] = "aaasaaasadaadsdafa";
int len = sizeof(seq0)/sizeof(seq0[0])-1;
struct str* s0 = malloc(sizeof(struct str));
struct str* s1 = malloc(sizeof(struct str));
s0->seq = (char*)seq0;
s1->seq = (char*)seq0;
s0->len = len;
s1->len = len;
pthread_t t0;
pthread_t t1;
int* res0;
int* res1;
if (pthread_create(&t0, NULL, &odd, s0)!=0){
return 0;
}
if (pthread_create(&t1, NULL, &even, s1)!=0){
return 00;
}
if(pthread_join(t0, (void**)&res0)!=0){
return 1;
}
if(pthread_join(t1, (void**)&res1)!=0){
return 11;
}
if(*res0 > *res1){
printf("%d\n", *res0);
}else{
printf("%d\n", *res1);
}
free(s0);
free(s1);
return 0;
}