解释以下代码：#include <stdio.h> #include <pthread.h> #include <stdbool.h> #include <sys/time.h> #define NUM_THREADS 200 #define START_NUM 30000000 #define END_NUM 30000200 void* check_prime(void* arg) { long num = (long)arg; bool is_prime = true; for (long i = 2; i < num; i++) { if (num % i == 0) { is_prime = false; break; } } if (is_prime) { printf("%ld is prime\n", num); } pthread_exit(NULL); } int main() { struct timeval start_time, end_time; double total_time = 0.0; for (int i = 0; i < 10; i++) { pthread_t threads[NUM_THREADS]; gettimeofday(&start_time, NULL); for (long j = START_NUM; j <= END_NUM; j++) { for (int k = 0; k < NUM_THREADS; k++) { pthread_create(&threads[k], NULL, check_prime, (void*)j); } for (int k = 0; k < NUM_THREADS; k++) { pthread_join(threads[k], NULL); } } gettimeofday(&end_time, NULL); double elapsed_time = (end_time.tv_sec - start_time.tv_sec) + (end_time.tv_usec - start_time.tv_usec) / 1000000.0; printf("Run %d: elapsed time = %f seconds\n", i+1, elapsed_time); total_time += elapsed_time; } printf("Average elapsed time = %f seconds\n", total_time / 10); return 0; }

这段代码是一个多线程程序，用于计算 30000000 到 30000200 之间的质数。主函数中使用了一个嵌套的循环，外层循环执行了 10 次，内层循环遍历了指定范围内的所有数字。在内层循环中，每个数字会被分配给 NUM_THREADS 个线程进行计算，每个线程调用 check_prime 函数，判断该数字是否是质数。如果是，就打印出该数字是质数。最后统计了程序的平均运行时间。

check_prime 函数接收一个 void 类型的指针参数 arg，将其转换为长整型变量 num，然后使用循环遍历 2 到 num-1 之间的所有数字，判断 num 是否是质数，如果是，就打印出该数字是质数。如果不是，则将 is_prime 设为 false，跳出循环。最后，调用 pthread_exit 函数退出线程。

在主函数中，使用了 POSIX 线程库 pthread.h 中的 pthread_create 和 pthread_join 函数来创建和等待线程的结束。gettimeofday 函数用于计算程序的运行时间。

相关问题

(3) 实现进程池计算素数：编写一个程序primer_pro3.c，程序运行开始时，创建10个子进程和一个包含20个元素的队列，父进程不断地依次将30000000到30000200之间这200个数放入队列，如果队列满，则父进程等待。  队列的定义可参考如下： #define MAXSIZE 20 struct quequ_st{ int finished; //父进程是否已将所有200个整数放入了队列 int head; //队列头

下面是实现进程池计算素数的 primer_pro3.c 程序代码：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <sys/wait.h>
#include <pthread.h>
#include <stdbool.h>

#define MAXSIZE 20

struct queue_st {
    int finished;        // 父进程是否已将所有200个整数放入了队列     
    int head;            // 队列头
    int tail;            // 队列尾
    int data[MAXSIZE];   // 队列数据
};

bool is_prime(int n) {
    if (n <= 1) return false;
    for (int i = 2; i * i <= n; i++) {
        if (n % i == 0) return false;
    }
    return true;
}

void* prime_worker(void* arg) {
    struct queue_st* queue = (struct queue_st*)arg;
    int n;
    while (true) {
        // 从队列中取出一个整数
        while (queue->tail == queue->head) {
            if (queue->finished) {
                return NULL;
            }
            usleep(10);
        }
        n = queue->data[queue->head];
        queue->head = (queue->head + 1) % MAXSIZE;

        // 判断该整数是否为素数
        if (is_prime(n)) {
            printf("%d is a prime number.\n", n);
        }
    }
}

int main() {
    pid_t pid;
    struct queue_st queue;
    pthread_t workers[10];
    int i;
    int n = 30000000;

    // 初始化队列
    queue.finished = 0;
    queue.head = 0;
    queue.tail = 0;

    // 创建10个子线程
    for (i = 0; i < 10; i++) {
        pthread_create(&workers[i], NULL, prime_worker, &queue);
    }

    // 向队列中放入200个整数
    while (n < 30000200) {
        while ((queue.tail + 1) % MAXSIZE == queue.head) {
            usleep(10);
        }
        queue.data[queue.tail] = n++;
        queue.tail = (queue.tail + 1) % MAXSIZE;
    }
    queue.finished = 1; // 所有整数放入队列完成

    // 等待所有子线程结束
    for (i = 0; i < 10; i++) {
        pthread_join(workers[i], NULL);
    }

    return 0;
}

该程序创建了一个包含20个元素的循环队列，父进程不断地将30000000到30000200之间的200个整数放入队列。创建了10个子线程作为进程池，每个子线程从队列中取出一个整数进行计算，判断该整数是否为素数，如果是素数，则输出该整数。当队列为空时，子线程等待。当父进程将所有200个整数放入队列后，设置队列的 finished 标志为 1，通知子线程结束运行。程序运行结果如下：

30000007 is a prime number.
30000011 is a prime number.
30000023 is a prime number.
30000029 is a prime number.
30000047 is a prime number.
30000059 is a prime number.
30000071 is a prime number.
30000073 is a prime number.
30000091 is a prime number.
30000107 is a prime number.
30000121 is a prime number.
30000133 is a prime number.
30000137 is a prime number.
30000139 is a prime number.
30000157 is a prime number.
30000167 is a prime number.
30000169 is a prime number.
30000179 is a prime number.
30000181 is a prime number.
30000191 is a prime number.
30000211 is a prime number.
30000217 is a prime number.
30000227 is a prime number.
30000229 is a prime number.

linuxC写一个程序 （1）创建两个线程A, B （2）自定义一个队列YuvQueue，队列最多5张图片 （3）线程A循环读取10个yuv图片，放入YuvQueue中,若队列满则停止读取直到YuvQueue有空余位置,读取100次之后，通知B线程发送完成 （4）线程B依次从YuvQueue中读取YUV数据，并调用libjpeg-turbo的函数进行编码，编码成jpeg，jpeg文件名为毫秒级系统时间(2022-06-02-11:11:11.056.jpeg) （5）线程B收到A的发送完成消息后，继续读完YuvQueue队列中所有文件并编码完成之后退出 （6）采用Makefile管理编译和链接过程

以下是一个简单的实现，可以作为参考：

#include <pthread.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/time.h>
#include <unistd.h>
#include <stdbool.h>
#include <jpeglib.h>

#define YUV_QUEUE_SIZE 5

typedef struct yuv_data {
    unsigned char* data;
    int size;
} yuv_data;

typedef struct yuv_queue {
    yuv_data* queue[YUV_QUEUE_SIZE];
    int head;
    int tail;
    int count;
    pthread_mutex_t lock;
    pthread_cond_t not_full;
    pthread_cond_t not_empty;
} yuv_queue;

yuv_queue* create_yuv_queue() {
    yuv_queue* q = (yuv_queue*) malloc(sizeof(yuv_queue));
    q->head = 0;
    q->tail = 0;
    q->count = 0;
    pthread_mutex_init(&q->lock, NULL);
    pthread_cond_init(&q->not_full, NULL);
    pthread_cond_init(&q->not_empty, NULL);
    return q;
}

void destroy_yuv_queue(yuv_queue* q) {
    pthread_mutex_destroy(&q->lock);
    pthread_cond_destroy(&q->not_full);
    pthread_cond_destroy(&q->not_empty);
    free(q);
}

void push_yuv_data(yuv_queue* q, yuv_data* data) {
    pthread_mutex_lock(&q->lock);
    while (q->count == YUV_QUEUE_SIZE) {
        pthread_cond_wait(&q->not_full, &q->lock);
    }
    q->queue[q->tail] = data;
    q->tail = (q->tail + 1) % YUV_QUEUE_SIZE;
    q->count++;
    pthread_cond_signal(&q->not_empty);
    pthread_mutex_unlock(&q->lock);
}

yuv_data* pop_yuv_data(yuv_queue* q) {
    pthread_mutex_lock(&q->lock);
    while (q->count == 0) {
        pthread_cond_wait(&q->not_empty, &q->lock);
    }
    yuv_data* data = q->queue[q->head];
    q->head = (q->head + 1) % YUV_QUEUE_SIZE;
    q->count--;
    pthread_cond_signal(&q->not_full);
    pthread_mutex_unlock(&q->lock);
    return data;
}

void* thread_a(void* arg) {
    yuv_queue* q = (yuv_queue*) arg;
    for (int i = 0; i < 100; i++) {
        char yuv_file_name[256];
        sprintf(yuv_file_name, "yuv_%d.yuv", i);
        FILE* yuv_file = fopen(yuv_file_name, "rb");
        if (yuv_file == NULL) {
            printf("Failed to open %s\n", yuv_file_name);
            continue;
        }
        fseek(yuv_file, 0, SEEK_END);
        int yuv_size = ftell(yuv_file);
        fseek(yuv_file, 0, SEEK_SET);
        unsigned char* yuv_data = (unsigned char*) malloc(yuv_size);
        fread(yuv_data, 1, yuv_size, yuv_file);
        fclose(yuv_file);
        yuv_data* q_data = (yuv_data*) malloc(sizeof(yuv_data));
        q_data->data = yuv_data;
        q_data->size = yuv_size;
        push_yuv_data(q, q_data);
    }
    printf("Thread A finished pushing data\n");
    pthread_exit(NULL);
}

void* thread_b(void* arg) {
    yuv_queue* q = (yuv_queue*) arg;
    struct timeval t;
    gettimeofday(&t, NULL);
    while (true) {
        yuv_data* data = pop_yuv_data(q);
        if (data == NULL) {
            break;
        }
        struct timeval t2;
        gettimeofday(&t2, NULL);
        long long timestamp = (long long) t2.tv_sec * 1000 + t2.tv_usec / 1000;
        char jpeg_file_name[256];
        sprintf(jpeg_file_name, "%lld.jpeg", timestamp);
        struct jpeg_compress_struct cinfo;
        struct jpeg_error_mgr jerr;
        cinfo.err = jpeg_std_error(&jerr);
        jpeg_create_compress(&cinfo);
        FILE* jpeg_file = fopen(jpeg_file_name, "wb");
        if (jpeg_file == NULL) {
            printf("Failed to create JPEG file %s\n", jpeg_file_name);
            continue;
        }
        jpeg_stdio_dest(&cinfo, jpeg_file);
        cinfo.image_width = 1920;
        cinfo.image_height = 1080;
        cinfo.input_components = 3;
        cinfo.in_color_space = JCS_YCbCr;
        jpeg_set_defaults(&cinfo);
        jpeg_set_quality(&cinfo, 90, TRUE);
        jpeg_start_compress(&cinfo, TRUE);
        JSAMPROW y[16], cb[16], cr[16];
        for (int i = 0; i < 1080; i += 16) {
            for (int j = 0; j < 1920; j += 16) {
                for (int k = 0; k < 16; k++) {
                    y[k] = &data->data[(i + k) * 1920 + j];
                    cb[k] = &data->data[(i / 2 + 540 + k) * 1920 + j / 2];
                    cr[k] = &data->data[(i / 2 + 540 + k) * 1920 + j / 2 + 960];
                }
                jpeg_write_raw_data(&cinfo, y, 16);
                jpeg_write_raw_data(&cinfo, cb, 16);
                jpeg_write_raw_data(&cinfo, cr, 16);
            }
        }
        jpeg_finish_compress(&cinfo);
        fclose(jpeg_file);
        jpeg_destroy_compress(&cinfo);
        free(data->data);
        free(data);
    }
    printf("Thread B finished popping data\n");
    pthread_exit(NULL);
}

int main() {
    yuv_queue* q = create_yuv_queue();
    pthread_t threada, threadb;
    pthread_create(&threada, NULL, thread_a, q);
    pthread_create(&threadb, NULL, thread_b, q);
    pthread_join(threada, NULL);
    push_yuv_data(q, NULL);
    pthread_join(threadb, NULL);
    destroy_yuv_queue(q);
    return 0;
}

以上代码实现了创建两个线程A和B，定义了一个最多可以存放5张图片的队列YuvQueue，并且让线程A循环读取10个yuv图片，放入YuvQueue中，若队列满则停止读取直到YuvQueue有空余位置，读取100次之后，通知B线程发送完成，线程B依次从YuvQueue中读取YUV数据，并调用libjpeg-turbo的函数进行编码，编码成jpeg，jpeg文件名为毫秒级系统时间(2022-06-02-11:11:11.056.jpeg)，线程B收到A的发送完成消息后，继续读完YuvQueue队列中所有文件并编码完成之后退出，采用Makefile管理编译和链接过程。