從執行緒返回結果

轉換為 void *的具體資料型別的指標可用於將值傳遞給執行緒函式並返回結果。

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

struct thread_args
{
    int a;
    double b;
};

struct thread_result
{
    long x;
    double y;
};

void *thread_func(void *args_void)
{
    struct thread_args *args = args_void;
    /* The thread cannot return a pointer to a local variable */
    struct thread_result *res = malloc(sizeof *res);

    res->x  = 10 + args->a;
    res->y = args->a * args->b;
    return res;
}

int main()
{
    pthread_t threadL;
    struct thread_args in = { .a = 10, .b = 3.141592653 };
    void *out_void;
    struct thread_result *out;

    pthread_create(&threadL, NULL, thread_func, &in);
    pthread_join(threadL, &out_void);
    out = out_void;
    printf("out -> x = %ld\tout -> b = %f\n", out->x, out->y);
    free(out);

    return 0;
}

在許多情況下，不必以這種方式傳遞返回值 - 例如，引數 struct 中的空格也可用於返回結果，或者指向共享資料結構的指標可以傳遞給執行緒並且結果儲存在那裡。