HomeWork （ 一） P46：Interlude: Process API

总的来说这一页的习题就是让你看懂，我们有哪些API！以及这些API最基本的调用方法和特性。

Homework (Code)

In this homework, you are to gain some familiarity with the process management APIs about which you just read. Don’t worry – it’s even more fun than it sounds! You’ll in general be much better off if you find as much time as you can to write some code5, so why not start now?

Questions

1. Write a program that calls fork(). Before calling fork(), have the main process access a variable (e.g., x) and set its value to some- thing (e.g., 100). What value is the variable in the child process? What happens to the variable when both the child and parent change the value of x?

2. Write a program that opens a file (with the open() system call) and then calls fork() to create a new process. Can both the child and parent access the file descriptor returned by open()? What happens when they are writing to the file concurrently, i.e., at the same time?

3. Write another program using fork(). The child process should print “hello”; the parent process should print “goodbye”. You should try to ensure that the child process always prints first; can you do this without calling wait() in the parent?

4. Write a program that calls fork() and then calls some form of exec() to run the program /bin/ls. See if you can try all of the variants of exec(), including execl(), execle(), execlp(), execv(), execvp(), and execvP(). Why do you think there are so many variants of the same basic call?

5. Now write a program that uses wait() to wait for the child process to finish in the parent. What does wait() return? What happens if you use wait() in the child?

解答：

第 1／2 题 : 父进程和子进程共享“变量”、“文件”、“缓冲区”，但是文件这里是个坑，第一题变量就不说了，第二题中 fwrite 和 write的差别会导致输出不一样，比如

#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>int main(int argc, const char * argv[]) {char *hello="hello\n";char *world="world\n";char *welcome = "welcome\n";FILE *fp;if((fp  = fopen("/Users/air/Desktop/OS3pieces/Process API Program in The Book/HomeWork2/test.txt", "at+"))== NULL){printf("OPEN ERROR!");return 0;}if(fwrite(welcome, strlen(welcome), 1, fp) < 1)printf("WRITE ERROR");int pid = fork();if(pid  < 0){printf("Fork　created error!\n");}else if(pid == 0){printf("I am the child, my pid is %d\n",(int)getpid());if(fwrite(hello, strlen(hello), 1, fp) < 1)printf("WRITE ERROR");}else{printf("I am the parent, my child is %d, my own pid is %d\n", pid, (int)getpid());if(fwrite(world, strlen(world), 1, fp)< 1)printf("WRITE ERROR");}fclose(fp);return 0;
}

因为子进程的缓冲区内已经有了welcome，所以打印的时候会一起打印出来；

比如：把 fwrite 换成了 write之后：

#include <string.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/wait.h>int main(int argc, const char * argv[]) {char *hello="hello\n";char *world="world\n";char *welcome = "welcome\n";FILE *fp;if((fp  = open("/Users/air/Desktop/OS3pieces/Process API Program in The Book/HomeWork22/test.txt", O_WRONLY))== NULL){printf("OPEN ERROR!");return 0;}if(write(fp,welcome, strlen(welcome)) < 1)printf("WRITE ERROR");int pid = fork();if(pid  < 0){printf("Fork　created error!\n");}else if(pid == 0){printf("I am the child, my pid is %d\n",(int)getpid());if(write(fp,hello, strlen(hello)) < 1)printf("WRITE ERROR");}else{printf("I am the parent, my child is %d, my own pid is %d\n", pid, (int)getpid());if(write(fp,world, strlen(world))< 1)printf("WRITE ERROR");}fclose(fp);return 0;
}