Why signal instead of broadcast works if we need to print the results of multiple thread calculations in specific order?

Question

In my code below, I create a pool of threads which do some computation on a common array. I want the computation to be done in parallel however, I want to print the results of the computation such that a thread id x doesn't print its results before thread id y, where x > y. So first I want to print the results of id 0, then 1, 2, etc.

I'm using pthreads to do this. Originally I used pthread_cond_broadcast to wake up blocked threads (everything worked) but then I tried pthread_cond_signal just out of curiosity. Interestingly enough, the program still works correctly.

But I don't understand why. That is:

1. I spawn, say 5 threads. All of them complete their calculations.
2. 4 of them get blocked, while thread id 0 prints its result and signals.
3. According to the spec pthread_cond_signal "shall unblock at least one of the threads that are blocked". So maybe it unblocks thread id 3.
4. thread id 3 cannot proceed because it's not its turn yet to print the result so it waits.
5. deadlock ensues.

So why does pthread_cond_signal still work? Is it because of repeated sheer luck or because my OS creates a queue of blocked threads and thread id 1 just happens to be in the head of this queue?

This is my code (the wait/signal logic is in function ComputeThread):

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

#define DEFAULT_VAL 5
#define ARR_LEN 5
#define THREADS_NUM 5

typedef struct helper_t {
    int * currId;
    int threadId;
    int * computeArr;
    pthread_mutex_t * mutex;
    pthread_cond_t * cond;
} helper_t;

int * initComputeArr(int len) {
    int i;
    int * computeArr = (int *) malloc(sizeof(int) * len);
    for(i = 0; i < len; i++) {
        computeArr[i] = DEFAULT_VAL;
    }
    return computeArr;
}

void mallocError() {
    printf("malloc error\n");
    exit(EXIT_FAILURE);
}

helper_t * initHelpers(pthread_mutex_t * mutex, pthread_cond_t * cond, int * computeArr) {
    int i;
    helper_t * helpers = (helper_t *) malloc(sizeof(helper_t) * THREADS_NUM);
    int * currId = (int *) malloc(sizeof(int));
    if(!helpers || !currId) {
        mallocError();
    } else {
        *currId = 0;
        for(i = 0; i < THREADS_NUM; i++) {
            helpers[i].mutex = mutex;
            helpers[i].cond = cond;
            helpers[i].computeArr = computeArr;
            helpers[i].currId = currId;
            helpers[i].threadId = i;
        }
    }
    return helpers;
}

void printHelper(helper_t * h) {
    printf("threadId %d, currId %d\n", h->threadId, *h->currId);
}

void printHelpers(helper_t * h, int len) {
    int i;
    for(i = 0; i < len; i++) {
        printHelper(&h[i]);
    }
}

int calc(int * arr, int uptoIndex) {
    int i, sum = 0;
    for(i = 0; i <= uptoIndex; i++) {
        sum += arr[i];
    }
    return sum;
}

void * ComputeThread(void * arg) {
    int calcResult;
    helper_t * h = (helper_t *) arg;
    pthread_mutex_t * mutex = h->mutex;
    pthread_cond_t * cond = h->cond;
    calcResult = calc(h->computeArr, h->threadId);
    sleep(1);
    pthread_mutex_lock(mutex);
    while(*h->currId != h->threadId) {
        printf("id %d waiting...\n", h->threadId);
        pthread_cond_wait(cond, mutex);
    }
    printf("curr %d, threadId %d, result %d\n", *h->currId, h->threadId, calcResult);
    (*h->currId)++;
    pthread_cond_signal(cond);
    pthread_mutex_unlock(mutex);
    pthread_exit((void *) calcResult);
}

int main() {
    int i;
    pthread_mutex_t mutex;
    pthread_cond_t cond;
    int * computeArr;
    int * calcResutls;
    helper_t * helpers;
    pthread_t threads[THREADS_NUM];

    computeArr = initComputeArr(ARR_LEN);
    calcResutls = initComputeArr(ARR_LEN);
    helpers = initHelpers(&mutex, &cond, computeArr);
    printHelpers(helpers, THREADS_NUM);

    for(i = 0; i < THREADS_NUM; i++) {
        pthread_create(&threads[i], NULL, ComputeThread, (void *) &helpers[i]);
    }

    for(i = 0; i < THREADS_NUM; i++) {
        pthread_join(threads[i], (void **) &calcResutls[i]);
    }

    for(i = 0; i < ARR_LEN; i++) {
        printf("%d, ", calcResutls[i]);
    }
    printf("\n");
    printf("end of calc\n");

    return 0;
}

Answer 1

I see undefined behaviour getting invoked as the code misses to initialise the mutex and the condition.