Passing work to a thread in C

One argument struct per thread, so nothing is shared by accident.

#include <pthread.h>

typedef struct {
    const int *values;
    size_t start;
    size_t end;
    long sum;
} Task;

static void *sum_range(void *arg) {
    Task *task = arg;
    task->sum = 0;
    for (size_t i = task->start; i < task->end; i++) {
        task->sum += task->values[i];
    }
    return NULL;
}

long parallel_sum(const int *values, size_t n, Task *tasks, size_t workers) {
    pthread_t threads[8];
    size_t chunk = (n + workers - 1) / workers;

    for (size_t w = 0; w < workers; w++) {
        size_t start = w * chunk;
        tasks[w].values = values;
        tasks[w].start = start;
        tasks[w].end = (start + chunk < n) ? start + chunk : n;
        pthread_create(&threads[w], NULL, sum_range, &tasks[w]);
    }

    long total = 0;
    for (size_t w = 0; w < workers; w++) {
        pthread_join(threads[w], NULL);
        total += tasks[w].sum;
    }
    return total;
}

How it works

1. A pointer into an array of structs gives each thread its own.
2. The struct carries both the input and the place for the result.
3. The memory must outlive the thread, so no locals of a dead frame.

Keywords and builtins used here

• NULL
• const
• for
• int
• long
• parallel_sum
• return
• size_t
• static
• struct
• sum_range
• typedef
• void

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Step 2 of 3 in Starting threads, step 2 of 10 in Threads & concurrency.

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