C/线程，返回值

Question

我创建了一个线程，该线程应该返回发送给它的整数的 sqrt，它在返回 int 值时工作正常，但是当我想返回 double 或 float 值时，它返回一些疯狂的数字，如何更改？

这是运行良好的代码:

int* function(int* x) {

printf("My argument: %d \n", *x);
int *y = malloc(sizeof(int));
*y=sqrt(*x);
return y;
}

int main(int argc, char* argv[])
{
pthread_t thread;
int arg = 123;
int *retVal;


pthread_create(&thread, NULL, (void * ( * ) (void *))function, &arg);

pthread_join(thread, (void **) &retVal);
printf("Sqrt of our argument: %d\n", * retVal);
free(retVal);
return 0;

}

但是当我将其更改为:

double* function(int* x) {


double *y = malloc(sizeof(double));
*y=sqrt(*x);
printf("My argument: %d \n", *x);
return y;
}

int main(int argc, char* argv[])
{
pthread_t thread;
int arg = 123;
double *retVal;


pthread_create(&thread, NULL, (void * ( * ) (void *))function, &arg);

pthread_join(thread, (void **) &retVal);
printf("Sqrt of our argument: %d\n", * retVal);
free(retVal);
return 0;
}

它返回 1076244058

Answer 1

您的更改是错误的

printf("Sqrt of our argument: %d\n", * retVal);

必须是

printf("Sqrt of our argument: %f\n", * retVal);

我猜你的编译器会告诉你类似的事情

warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘double *’ [-Wformat=]

顺便说一句，您的实现调用了未定义的行为转换函数:看看 this SO answer

正如已经建议的，您可以使用 arg 将值传递回 main，而不是从任务函数返回它。

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

void* function(void* x)
{
    double *y = x;

    *y = sqrt(*y);

    return x;
}

int main(void)
{
    pthread_t thread;
    double arg = 123;
    void *retVal = NULL;

    pthread_create(&thread, NULL, function, &arg);

    pthread_join(thread, &retVal);
    printf("Sqrt of our argument using arg   : %f\n", arg);

    if (retVal != NULL)
    {
        printf("Sqrt of our argument using retVal: %f\n", *((double *)retVal));
    }

    return 0;
}