编写两个不同的可执行程序，名称分别为a和b，b为a的子进程。在a程序中调用open函数打开a.txt文件。在b程序不可以调用open或者fopen，只允许调用read函数来实现读取a.txt文件。（a程序中可以使用 fork与execve函数创建子进程）。

//fork共享文件标识符#include <stdio.h>#include <stdlib.h>#include <string.h>#include <errno.h>#include <unistd.h>#include <sys/types.h>#include <sys/wait.h>#include <sys/stat.h>#include <fcntl.h>int main(int arg, char * args[]){    pid_t child=0;    int status;    child=fork();    if(child==-1)    {        printf("system is game over\n");        return -1;    }    //open the file in read mode    int fd=open("a.txt",O_RDONLY);    if(fd==-1)    {        printf("open the file failed ! \n error msg:%s",strerror(errno));        return -1;    }    if(child==0)    {        char buf[10]={0};        sprintf(buf,"%d",fd);        char * argv[]={"../2/tec",buf,NULL};        execve("../2/tec",argv,NULL);    }else    {        printf("i am parent, i am wait!\n");        wait(&status);        printf("child msg is:%d\n",WEXITSTATUS(status));        printf("parent is end!\n");    }    return 0;}

//execve共享文件标识符#include <stdio.h>#include <stdlib.h>#include <string.h>#include <errno.h>#include <unistd.h>#include <sys/types.h>#include <sys/wait.h>int main(int arg, char * args[]){    if(arg<2)    {        printf("请输入一个参数！\n");        return -1;    }    int fd=atoi(args[1]);    if(fd<3)    {        printf("缺少文件标识符！\n");        return -1;    }    //read the file    char buf[50]={0};    read(fd,buf,sizeof(buf));    printf("the message is:%s\n",buf);    return 2;}