Error code repeating values in threads?

Your code has some problems:

• You are currently using “aspas inglesas” instead of "aspas duplas comuns" . That does not compile. However, this is also very easy to fix.

• Your Operacao function does not use the soma callback at all. And this callback is always NULL .

• The call to pthread_exit in the main function is unnecessary because the Operacao function already does this.

• In its operation function, you read the number twice, but store both numbers read in the same memory location n . In this way, the first reading will be discarded and only the result of the second will be used. Since you display n + n after, it will display double the second value read.

• It is not good practice (and the compiler can give you a warning) does not use return in a function whose return type is not void (and remember which void * is not the same as void ), even if there is no possible way in which the function could finish its execution normally. The workaround is to set return NULL; to Operacao . This does not change the function of this function, but it serves to make the compiler satisfied in cases where it claims to do so.

I think what you wanted was this:

#include <pthread.h>
#include <stdio.h>

void *operacao() {
    int a, b;
    printf("Digite o primeiro número: ");
    scanf("%d", &a);
    printf("Digite o segundo número: ");
    scanf("%d", &b);
    printf("A soma é %d.", a + b);
    pthread_exit(NULL);
    return NULL; // Nunca é executado.
}

int main() {
    pthread_t thread;
    printf("Criando uma nova thread\n");
    int flag = pthread_create(&thread, NULL, operacao);
    if (flag != 0) {
        printf("Erro na criação de uma nova thread\n");
        return 1;
    }
    operacao();
    return 0;
}

Or, if you want to tell the thread at the time of requesting the user:

#include <pthread.h>
#include <stdio.h>

void *operacao(const char *nome_thread) {
    int a, b;
    printf("A thread %s pede que você digite o primeiro número: ", nome_thread);
    scanf("%d", &a);
    printf("A thread %s pede que você digite o segundo número: ", nome_thread);
    scanf("%d", &b);
    printf("A soma é %d.", a + b);
    pthread_exit(NULL);
    return NULL; // Nunca é executado.
}

int main() {
    pthread_t thread;
    printf("Criando uma nova thread\n");
    int flag = pthread_create(&thread, NULL, operacao, "auxiliar");
    if (flag != 0) {
        printf("Erro na criação de uma nova thread\n");
        return 1;
    }
    operacao("principal");
    return 0;
}

See here working on ideone.