Traverse a vector by dividing the search into threads

Question

Hello, I have a problem, I have a college assignment where I have a very large vector, where I have to get each processor thread through a part of that vector increasing the speed of the search, but when I run my code, appears 2 times the printf of the value in vector 0, I do not know pq, I believe must be occurring some race condition yet, but I do not know where.

#include <pthread.h>
#include <semaphore.h>
#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <math.h>
#define TAM 100000000

long *vetor;
long cont=0;
int vet[3];
sem_t semaforo;

void preencher(){
    long i;
    srand( (unsigned)time(NULL) );
    for(i=0;i<TAM;i++) vetor[i]=rand()%TAM;
}

void contar(void *argp) {
    int i, temp;
    int alvo;
    alvo = (long) argp;
    sem_wait(&semaforo);
    vet[0] = (TAM/4);
    printf("VALOR NO VETOR[0]=%d\n", vet[0]);
    for (i = 0; i < vet[0]; i++) {
    if(vetor[i]==alvo){

        temp = cont;
        temp++;
        cont = temp;

     }
    }
    sem_post(&semaforo);
}

void contar2(void *argp) {
        int i, temp;
        int alvo;
        alvo = (long) argp;
        sem_wait(&semaforo);
        vet[1] = (TAM/2);
        printf("VALOR NO VETOR[1]=%d\n", vet[1]);
        for (i = vet[0]; i < vet[1]; i++) {
        if(vetor[i]==alvo){

            temp = cont;
            temp++;
            cont = temp;

         }
        }
        sem_post(&semaforo);
}

void contar3(void *argp) {
        int i, temp;
        int alvo;
        alvo = (long) argp;
        sem_wait(&semaforo);
        vet[2] = (TAM/4)*3;
        printf("VALOR NO VETOR[2]=%d\n", vet[2]);
        for (i = vet[1]; i < vet[2]; i++) {
        if(vetor[i]==alvo){

            temp = cont;
            temp++;
            cont = temp;

         }
        }
        sem_post(&semaforo);
}

void contar4(void *argp) {
        int i, temp;
        int alvo;
        alvo = (long) argp;
        sem_wait(&semaforo);
        for (i = vet[2]; i < TAM; i++) {
        if(vetor[i]==alvo){

            temp = cont;
            temp++;
            cont = temp;

         }
        }
        sem_post(&semaforo);
}

int main(int argc, char * argv[])
{
    pthread_t t1, t2, t3, t4;
    sem_init(&semaforo,0,1);
    long chave;
    vetor = (long *)malloc(TAM*sizeof(long));
    preencher();
    printf("\nDigite a chave a ser buscada no vetor:\n");
    scanf("%ld",&chave);
    contar((void *)chave);
    int rc;
    rc = pthread_create(&t1, NULL, (void *)contar, NULL);
    rc = pthread_create(&t2, NULL, (void *)contar2, NULL);
    rc = pthread_create(&t3, NULL, (void *)contar3, NULL);
    rc = pthread_create(&t4, NULL, (void *)contar4, NULL);
    rc = pthread_join(t1, NULL);
    rc = pthread_join(t2, NULL);
    rc = pthread_join(t3, NULL);
    rc = pthread_join(t4, NULL);
    printf("Ocorrencias: %ld",cont);
    return(0);
}

Answer 1

The problem is not race condition but algorithm itself. You call the function contar() twice: once in the thread main and other in the first thread. Behold:

contar((void *)chave);
int rc;
rc = pthread_create(&t1, NULL, (void *)contar, NULL);