Implementing longest palindromic substring algorithm using pthreads as practice

longest_palindromic_substring.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <pthread.h>
int sum;
struct input {
  char* str;
  int i;
};
int NUM_THREADS;


void* add(void* ptr){ 

  struct input* input = (struct input*) ptr;
  int strLen = strlen(input->str)+1;
  if(input->i-1 < 0 || input->i+1 >= strLen){

    //printf("Boundary %d %d %d, %s\n", input->i-i-1, input->i, input->i+i+1, input->str);
    char* ret = malloc(sizeof(char)*2);

    *ret = input->str[input->i];
    ret[1] = '\0';
    //printf("Palindromic substring: %s\n", ret);
    return ret;
  }

  //  printf("%d %d %d\n", input->i-i-1, input->i, input->i+i+1);
  int i = 0;
  while(input->i-i-1 >= 0 && input->i+i+1 < strLen && input->str[input->i-i-1] == input->str[input->i+i+1]){
        i++;
  }
  int n = 0;
  while(input->i-n-1 >= 0 && input->i+n < strLen && input->str[input->i-n-1] == input->str[input->i+n]){
        n++;
  }
  if(n > i){
    char* ret_val = malloc((n*2+1)*sizeof(char));
    strncpy(ret_val, input->str+input->i-n, n*2);
    ret_val[n*2] = '\0';

    //printf("Palindromic substring: %s\n", ret_val);
    return ret_val;
  }
  else{
    char* ret_val = malloc((i*2+2)*sizeof(char));
    strncpy(ret_val, input->str+input->i-i, i*2+1);
    ret_val[i*2+1] = '\0';
    //printf("Palindromic substring: %s\n", ret_val);
    return ret_val;
  }
  if(i == 0){
    char* ret = malloc(sizeof(char)*2);

    *ret = input->str[input->i];
    ret[1] = '\0';
    //printf("Palindromic substring: %s\n", ret);
    return ret;
  }
  return "";
  

}
int main(){
  char* s = "civilwartestingwhetherthatnaptionoranynartionsoconceivedandsodedicatedcanlongendureWeareqmetonagreatbattlefiemldoftzhatwarWehavecometodedicpateaportionofthatfieldasafinalrestingplaceforthosewhoheregavetheirlivesthatthatnationmightliveItisaltogetherfangandproperthatweshoulddothisButinalargersensewecannotdedicatewecannotconsecratewecannothallowthisgroundThebravelmenlivinganddeadwhostruggledherehaveconsecrateditfaraboveourpoorponwertoaddordetractTgheworldadswfilllittlenotlenorlongrememberwhatwesayherebutitcanneverforgetwhattheydidhereItisforusthelivingrathertobededicatedheretotheulnfinishedworkwhichtheywhofoughtherehavethusfarsonoblyadvancedItisratherforustobeherededicatedtothegreattdafskremainingbeforeusthatfromthesehonoreddeadwetakeincreaseddevotiontothatcauseforwhichtheygavethelastpfullmeasureofdevotionthatweherehighlyresolvethatthesedeadshallnothavediedinvainthatthisnationunsderGodshallhaveanewbirthoffreedomandthatgovernmentofthepeoplebythepeopleforthepeopleshallnotperishfromtheearth";
  int strLen = strlen(s)+1;
  NUM_THREADS = 32;
  pthread_t* threads = malloc((strLen)*sizeof(pthread_t));
  struct input* args = malloc(sizeof(struct input));
  int numThreadsSoFar = 0;
  void* max_val;
  int max_len = 0;
  void* cur_val;
  while(numThreadsSoFar < strLen-strLen%NUM_THREADS){
    for(int i=numThreadsSoFar; i<numThreadsSoFar+NUM_THREADS; i++){
      args = malloc(sizeof(struct input));
      args->str = s;
      args->i = i;
      pthread_create(&threads[i], NULL, add, args);
    }
    for(int i=numThreadsSoFar; i<numThreadsSoFar+NUM_THREADS; i++){
      pthread_join(threads[i], &cur_val);
      if(strlen(cur_val) > max_len){
        max_len = strlen(cur_val);
        max_val = cur_val;
      }

    }
    numThreadsSoFar += NUM_THREADS;
  }
  for(int i=numThreadsSoFar; i<strLen; i++){
    args = malloc(sizeof(struct input));
    args->str = s;
    args->i = i;
    pthread_create(&threads[i], NULL, add, args);
  }
  for(int i=numThreadsSoFar; i<strLen; i++){
    pthread_join(threads[i], &cur_val);
    if(strlen(cur_val) > max_len){
        max_len = strlen(cur_val);
        max_val = cur_val;
    }
  }
  printf("Longest palindromic substring: %s\n", (void*)max_val);
}