mit-6.s081-2020 | Lab1 Utilities

info

MIT 6.S801 xv6 book 2020版 中文翻译

https://zhuanlan.zhihu.com/p/272199762

https://fanxiao.tech/posts/MIT-6S081-notes/#16-lab-1-unix-utilities

sleep (easy)

code

#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"

int main(int argc, char *argv[]) {

    if (argc < 2) {
        fprintf(2, "Usage: sleep <sleepNum>... \n");
        exit(1);
    }

    int sleepNum = atoi(argv[1]);
    if (sleepNum < 0) {
        fprintf(2, "Negative sleep num, invalid! \n");
        exit(1);
    }

    sleep(sleepNum);
    exit(0);

}

pingpong (easy)

info

关于为什么需要close()

code

#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"


int main(int argc, char *agrv[]) {

    // 管道是单向通信的。所以当需要在两个进程之间通信的时，需要一个关闭读端也就是p[0]，
    // 一个关闭写端也就是p[1]
    int p_parent_to_child[2];
    int p_child_to_parent[2];
    pipe(p_parent_to_child);
    pipe(p_child_to_parent);

    char buf[5];

    int pid = fork();
    if (pid == 0) {
        close(p_parent_to_child[1]);  // 关闭写端
        read(p_parent_to_child[0], buf, 4);
        printf("%d: received %s\n", getpid(), buf);
        close(p_child_to_parent[0]);
        write(p_child_to_parent[1], "pong", 4);
    } else {
        close(p_parent_to_child[0]);
        write(p_parent_to_child[1], "ping", 4);
        close(p_child_to_parent[1]);
        read(p_child_to_parent[0], buf, 4);
        printf("%d: received %s\n", getpid(), buf);
    }

    exit(0);
    
}

primes (moderate)/(hard)

info

贝尔实验室与 CSP 线程

code

#include "kernel/types.h"
#include "kernel/stat.h"
#include "user/user.h"
#include <stdbool.h>

#define N 36

int main(int argc, char *argv[]) {

    int nums[N];
    for (int i = 0; i < N; i++) {
        nums[i] = i;
    }
    
    while (true) {
        int p[2];
        pipe(p);
        int pid = fork();
        if (pid > 0) {  // 父进程往管道里面写数据
            close(p[0]);
            int writebuf[N];
            int cnt = 0;
            for (int i = 2; i <= 35; i++) {
                if (nums[i] != 0) {
                    writebuf[cnt++] = i;
                    write(p[1], &writebuf[cnt - 1], sizeof writebuf[cnt - 1]);
                }
            }
            close(p[1]);
            wait(0);
            exit(0);
        } else {  // 子进程从管道里面读数据
            close(p[1]);
            int prime = 0;
            if (read(p[0], &prime, sizeof prime) == 0) {
                exit(0);
            }
            printf("prime %d\n", prime);
            for (int i = prime; i <= 35; i += prime) {
                nums[i] = 0;
            }
            close(p[0]);
        }
    }

    exit(0);

}

find (moderate)

info

The C library function void memmove(void str1, const void *str2, size_t n) copies n characters from str2 to str1, but for overlapping memory blocks, memmove() is a safer approach than memcpy().

void *memmove(void *str1, const void *str2, size_t n)