从文件里面读出1000个随机数，进行排序，再写到另一文件中。（考虑使用重定向dup/dup2）

//// Created by Lin on 2017/11/7.//#include <stdlib.h>#include <unistd.h>#include <sys/fcntl.h>#include <time.h>#include <sys/stat.h>#include <cstdio>#define N 10/** * 从文件读入数据 * @param fd * @param num */void readFromFile(int fd,int num[]){    if(fd<0) return;    int fid =dup(STDIN_FILENO);    if(dup2(fd,STDIN_FILENO)<0 && close(fd)<0){        perror("1");        exit(1);    };    for (int i = 0; i < N; ++i) {        scanf("%d\n",&num[i]);    }    if(dup2(STDIN_FILENO,fd)<0 && dup2(fid,STDIN_FILENO)<0 && close(fid)<0){        perror("2");        exit(1);    }}/** * 打印到文件 * @param fd * @param num */void printfToFile(int fd,int num[]){    int sout_fd = dup(STDOUT_FILENO);    if(dup2(fd,STDOUT_FILENO)<0 && close(fd)<0){        perror("3");        exit(1);    }    for (int i = 0; i < N; ++i) {        printf("%d\n",num[i]);    }    if(fflush(stdout)<0 && dup2(STDOUT_FILENO,fd)<0 && lseek(fd,0,SEEK_SET)<0){        perror("4");        exit(1);    }    dup2(sout_fd,STDOUT_FILENO);    close(sout_fd);}/** * 插入排序 * @param num */void sort(int num[]){    for (int i = 1; i < N; ++i) {        int t = num[i];        int j= i;        while(t < num[j-1] && j-1>=0){            num[j] = num[j-1];            j--;        }        num[j] = t;    }}int main(){    srand(time(NULL));    int nums[N];    int nin[N];    int fd = open("in", O_CREAT|O_RDWR,S_IRWXU);    for (int i = 0; i < N; ++i) {        nin[i] = rand()%2000;    }    printfToFile(fd,nin);    int saved_fd = open("out",O_CREAT|O_TRUNC|O_WRONLY,S_IRWXU);    if(fd<0 || saved_fd<0){        perror("Open");    } else{        printf("helloworld\n");        fflush(stdout);        readFromFile(fd,nums);        close(fd);        sort(nums);        printfToFile(saved_fd,nums);        close(saved_fd);    }}