Merge Sort with PThreads in C

Side Note: I'm beginning to learn how to use pthreads and I'm starting to get the concept. I've been using this example script (written in C++) here to manage a Merge Sort with threads: https://www.geeksforgeeks.org/merge-sort-using-multi-threading/

Since I'm writing my own merge sort in C rather than C++, I rewrote this example script to test and noticed an issue. Rather than MAX 20 for an array of 20 elements, I decided to go with 15. I noticed that the sorting/merging is invalid (yet somewhat close) and I cannot figure out why... In addition, I've altered the code to use a different amount of threads instead of THREAD_MAX 4 so I may change it to 5 or 10 threads.

Could it be the Merge that's yielding invalid results? I've commented it below within main().

My C++ to C convert below:

#include <pthread.h>
#include <time.h>
#include <stdlib.h>

// number of elements in array
#define MAX 15

// number of threads
#define THREAD_MAX 4

//using namespace std;

// array of size MAX
int a[MAX];
int part = 0;

// merge function for merging two parts
void merge(int low, int mid, int high)
{   
    int* left = (int*) malloc( (mid - low + 1) * sizeof(int));
    int* right = (int*) malloc( (high - mid) * sizeof(int));


    // n1 is size of left part and n2 is size
    // of right part
    int n1 = mid - low + 1,
    n2 = high - mid,
    i, j;

    // storing values in left part
    for (i = 0; i < n1; i++)
        left[i] = a[i + low];

    // storing values in right part
    for (i = 0; i < n2; i++)
        right[i] = a[i + mid + 1];

    int k = low;
    i = j = 0;

    // merge left and right in ascending order
    while (i < n1 && j < n2) {
        if (left[i] <= right[j])
            a[k++] = left[i++];
        else
            a[k++] = right[j++];
    }

    // insert remaining values from left
    while (i < n1) {
        a[k++] = left[i++];
    }

    // insert remaining values from right
    while (j < n2) {
        a[k++] = right[j++];
    }

    free(left);
    free(right);
}

// merge sort function
void merge_sort(int low, int high)
{
    // calculating mid point of array
    int mid = low + (high - low) / 2;
    if (low < high) {

        // calling first half
        merge_sort(low, mid);

        // calling second half
        merge_sort(mid + 1, high);

        // merging the two halves
        merge(low, mid, high);
    }
}

// thread function for multi-threading
void* merge_sort123(void* arg)
{
    // which part out of 4 parts
    int thread_part = part++;

    // calculating low and high
    int low = thread_part * (MAX / THREAD_MAX);
    int high = (thread_part + 1) * (MAX / THREAD_MAX) - 1;

    // evaluating mid point
    int mid = low + (high - low) / 2;
    if (low < high) {
        merge_sort(low, mid);
        merge_sort(mid + 1, high);
        merge(low, mid, high);
    }
    return 0;
}



// Driver Code
int main()
{
    // generating random values in array
    for (int i = 0; i < MAX; i++){
        a[i] = rand() % 100;
    //        printf("%d ", a[i]);
    }


    pthread_t threads[THREAD_MAX];

    // creating 4 threads
    for (int i = 0; i < THREAD_MAX; i++)
        pthread_create(&threads[i], NULL, merge_sort123,
                       (void*)NULL);

    // joining all 4 threads
    for (int i = 0; i < THREAD_MAX; i++)
        pthread_join(threads[i], NULL);


    ///////////////////////////////////////////////////////////////
    // --- THIS MAY BE THE PART WHERE THE MERGING IS INVALID --- //
    ///////////////////////////////////////////////////////////////
    // merging the final 4 parts
    merge(0, (MAX / 2 - 1) / 2, MAX / 2 - 1);
    merge(MAX / 2, MAX/2 + (MAX-1-MAX/2)/2, MAX - 1);
    merge(0, (MAX - 1)/2, MAX - 1);


    // displaying sorted array
    printf("\n\nSorted array: ");
    for (int i = 0; i < MAX; i++)
        printf ("%d ", a[i]);


    printf("\n");
    return 0;
}

1 answer

  • answered 2018-10-12 03:15 Craig Estey

    As I mentioned in the top comments, there are a few issues with the original code.

    The above race condition.

    The [seemingly] missing delete at the bottom of merge

    The fact that the number of array elements must be a multiple of the number of threads. If not, the range for the last thread will be calculated incorrectly.

    The final merge in the main thread is fixed/hardwired for 4 threads.


    A general solution is possible. However, unless the array size is very large, it isn't as big a time saver, so it's mostly for practice with multithreading [which is what you wanted, I believe]. See: Multithreaded quicksort or mergesort

    It is easier to pass several parameters to a thread using a control struct. This is good technique for multithreading in general.

    The main thread can prepopulate this with the array ranges for each thread. It can later use these control structs to generalize the final merge.

    Here's a a cleaned up version that works for an arbitrary array size and arbitrary number of threads:

    #include <stdio.h>
    #include <pthread.h>
    #include <time.h>
    #include <stdlib.h>
    
    int opt_a;
    int opt_t;
    int opt_r;
    
    // number of elements in array
    //#define MAX 15
    //#define MAX 16
    int MAX;
    
    // number of threads
    //#define THREAD_MAX 4
    int THREAD_MAX;
    
    //using namespace std;
    
    // array of size MAX
    int *a;
    
    // thread control parameters
    struct tsk {
        int tsk_no;
        int tsk_low;
        int tsk_high;
    };
    
    // merge function for merging two parts
    void
    merge(int low, int mid, int high)
    {
    
        // n1 is size of left part and n2 is size of right part
        int n1 = mid - low + 1;
        int n2 = high - mid;
    
        int *left = malloc(n1 * sizeof(int));
        int *right = malloc(n2 * sizeof(int));
    
        int i;
        int j;
    
        // storing values in left part
        for (i = 0; i < n1; i++)
            left[i] = a[i + low];
    
        // storing values in right part
        for (i = 0; i < n2; i++)
            right[i] = a[i + mid + 1];
    
        int k = low;
    
        i = j = 0;
    
        // merge left and right in ascending order
        while (i < n1 && j < n2) {
            if (left[i] <= right[j])
                a[k++] = left[i++];
            else
                a[k++] = right[j++];
        }
    
        // insert remaining values from left
        while (i < n1)
            a[k++] = left[i++];
    
        // insert remaining values from right
        while (j < n2)
            a[k++] = right[j++];
    
        free(left);
        free(right);
    }
    
    // merge sort function
    void
    merge_sort(int low, int high)
    {
    
        // calculating mid point of array
        int mid = low + (high - low) / 2;
    
        if (low < high) {
            // calling first half
            merge_sort(low, mid);
    
            // calling second half
            merge_sort(mid + 1, high);
    
            // merging the two halves
            merge(low, mid, high);
        }
    }
    
    // thread function for multi-threading
    void *
    merge_sort123(void *arg)
    {
        struct tsk *tsk = arg;
        int low;
        int high;
    
        // calculating low and high
        low = tsk->tsk_low;
        high = tsk->tsk_high;
    
        // evaluating mid point
        int mid = low + (high - low) / 2;
    
        if (low < high) {
            merge_sort(low, mid);
            merge_sort(mid + 1, high);
            merge(low, mid, high);
        }
    
        return 0;
    }
    
    // Driver Code
    int
    main(int argc, char **argv)
    {
        char *cp;
        struct tsk *tsk;
    
        --argc;
        ++argv;
    
        MAX = 15;
        THREAD_MAX = 4;
    
        // use new/general algorithm by default
        opt_a = 1;
    
        for (; argc > 0; --argc, ++argv) {
            cp = *argv;
            if (*cp != '-')
                break;
    
            switch (cp[1]) {
            case 'M':  // array count
                MAX = atoi(cp + 2);
                break;
    
            case 'T':  // thread count
                THREAD_MAX = atoi(cp + 2);
                break;
    
            case 'a':  // change algorithm
                opt_a = !opt_a;
                break;
    
            case 'r':  // do _not_ use rand -- use linear increment
                opt_r = !opt_r;
                break;
    
            case 't':  // tracing
                opt_t = !opt_t;
                break;
    
            default:
                break;
            }
        }
    
        // allocate the array
        a = malloc(sizeof(int) * MAX);
    
        // generating random values in array
        if (opt_t)
            printf("ORIG:");
        for (int i = 0; i < MAX; i++) {
            if (opt_r)
                a[i] = MAX - i;
            else
                a[i] = rand() % 100;
            if (opt_t)
                printf(" %d", a[i]);
        }
        if (opt_t)
            printf("\n");
    
        pthread_t threads[THREAD_MAX];
        struct tsk tsklist[THREAD_MAX];
    
        int len = MAX / THREAD_MAX;
    
        if (opt_t)
            printf("THREADS:%d MAX:%d LEN:%d\n", THREAD_MAX, MAX, len);
    
        int low = 0;
    
        for (int i = 0; i < THREAD_MAX; i++, low += len) {
            tsk = &tsklist[i];
            tsk->tsk_no = i;
    
            if (opt_a) {
                tsk->tsk_low = low;
                tsk->tsk_high = low + len - 1;
                if (i == (THREAD_MAX - 1))
                    tsk->tsk_high = MAX - 1;
            }
    
            else {
                tsk->tsk_low = i * (MAX / THREAD_MAX);
                tsk->tsk_high = (i + 1) * (MAX / THREAD_MAX) - 1;
            }
    
            if (opt_t)
                printf("RANGE %d: %d %d\n", i, tsk->tsk_low, tsk->tsk_high);
        }
    
        // creating 4 threads
        for (int i = 0; i < THREAD_MAX; i++) {
            tsk = &tsklist[i];
            pthread_create(&threads[i], NULL, merge_sort123, tsk);
        }
    
        // joining all 4 threads
        for (int i = 0; i < THREAD_MAX; i++)
            pthread_join(threads[i], NULL);
    
        // show the array values for each thread
        if (opt_t) {
            for (int i = 0; i < THREAD_MAX; i++) {
                tsk = &tsklist[i];
                printf("SUB %d:", tsk->tsk_no);
                for (int j = tsk->tsk_low; j <= tsk->tsk_high; ++j)
                    printf(" %d", a[j]);
                printf("\n");
            }
        }
    
        // merging the final 4 parts
        if (opt_a) {
            struct tsk *tskm = &tsklist[0];
            for (int i = 1; i < THREAD_MAX; i++) {
                struct tsk *tsk = &tsklist[i];
                merge(tskm->tsk_low, tsk->tsk_low - 1, tsk->tsk_high);
            }
        }
        else {
            merge(0, (MAX / 2 - 1) / 2, MAX / 2 - 1);
            merge(MAX / 2, MAX / 2 + (MAX - 1 - MAX / 2) / 2, MAX - 1);
            merge(0, (MAX - 1) / 2, MAX - 1);
        }
    
        // displaying sorted array
        printf("\n\nSorted array:");
        for (int i = 0; i < MAX; i++)
            printf(" %d", a[i]);
        printf("\n");
    
        return 0;
    }