Question

I am sending a sturct dataBlock to my threads which contains a pointer to same listRoot(for all threads), an int indexInForLoop, and an int listSize; The Node from the list has a int value which ash to be processed and a int worker which is nrOfListElements % nrOfWorkers(Threads). But somehow I thing the struct I send to each thread is the same, but it shouldn’t each should have 1 different int variable with the indexFromForLoop. What am I doing wrong?

terminal output:

node #1 worker: 1  value: 86
node #2 worker: 0  value: 77
node #3 worker: 1  value: 15
node #4 worker: 0  value: 93
node #5 worker: 1  value: 35
? List got populated
workersInput: 2
in for, ret= 0
in for, ret= 0
* Thread start:             id: 3067579200^
forID_ 1 
->val: 86   
valid for sqrt 
* Thread start:             id: 3075971904^
forID_ 1 
->val: 86   
valid for sqrt 

Code:

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

#define workTime 5
#define workersLimitNr 3

struct dataBlock{
    struct node *root;
    int listSize;
    int forIndex;
};

struct node { // std linked list node
    int value;
    int worker;
    struct node *next;
};

int slots = 0; // only 3 threads are allowed to access the list
int availableCheck(){   // check if thread can acces the list
    if(slots < 3) return 0;
    else return -1;
}

pthread_mutex_t mutp = PTHREAD_MUTEX_INITIALIZER;   //condvar mutex
pthread_cond_t  condvar = PTHREAD_COND_INITIALIZER;   //condvar

void * worker( void *data ){
    struct dataBlock *inData = (struct dataBlock *) data;
    struct node *root = inData->root;
    int listSize =  inData->listSize;
    int forIndex = inData ->forIndex;

    printf( "* Thread start:            ^\n"); 

    pthread_mutex_lock( &mutp );
    if(availableCheck() < 0){
        printf( " ^^^ List not available yet... \n" ); 
        pthread_cond_wait( &condvar, &mutp );
    } 
    struct node *it = root;
    printf("forID_ %d \n", forIndex);
    while(it->next != NULL){
        if(forIndex == it->worker){
            printf("valid for sqrt \n");
            if(it->value > 2){
                sqrt(it->value);
                break;
            }
        }
        it = it->next;
        printf("->val: %d   \n", it->value);
    }

    pthread_cond_signal( &condvar ); // 
    pthread_mutex_unlock( &mutp ); 
    return NULL;
}

int main( int argc, char *argv[] ){
    if ( argc != 3 ){
        printf( "Programm must be called with \n NR of elements and NR of workers! \n " );
        exit( 1 );
    }

    int i;
    struct node *root;
    struct node *iterator;  

//prepare list for task
    int listSize = atoi(argv[1]);
    int nrWorkers = atoi(argv[2]);
    root = malloc(sizeof( struct node) );

    root->value = rand() % 100;
    root->worker = 0;
    iterator = root;

    for( i=1; i<listSize; i++ ){
        iterator->next = malloc(sizeof(struct node));
        iterator = iterator->next;
        iterator->value = rand() % 100;
        iterator->worker = i % nrWorkers;
        printf("node #%d worker: %d  value: %d\n", i, iterator->worker,iterator->value);
    }
    printf("? List got populated\n");

// Create all threads to parse the link list
    int ret;    
    printf("workersInput: %d\n",nrWorkers);

    pthread_t w_thread;
    pthread_t* w_threads = malloc(nrWorkers * sizeof(w_thread));

    struct dataBlock *data = malloc(sizeof(struct dataBlock));
    data->root = root;
    data->listSize = listSize;

    for( i=0; i < nrWorkers; i++ ){ 
        data->forIndex = i;
        ret = pthread_create ( &w_threads[i], NULL, worker, (void *) data );
        if( ret ) {
            perror("Thread creation fail");
            exit(2);    
        }   
        printf("in for, ret= %d\n",ret);
    } 

    for ( i = 0; i < nrWorkers; i++){
        pthread_join(w_threads[i],NULL);
    }

    free(root);
    free(iterator);
    return 0;
}

EDIT: I used a pthread_self() and I confirm there are different threats

Answer 1

Using

data->forIndex = i;

you overwrite the previous value(s). And since you have only one dataBlock each thread will look to the same place. And since creating a thread does not mean “start it before the next statement in the loop is executed” it is pure luck what value each thread sees.

You have to allocate one dataBlock for each thread and give each dataBLock only to one thread.