900字范文 > [第八章] 深入理解计算机系统第三版家庭作业参考答案

[第八章] 深入理解计算机系统第三版家庭作业参考答案

时间：2020-12-08 17:14:26

8.9

8.10

A.fork

B.longjmp execve

C.setjmp

8.11

4次，画画进程图就行了

8.12

8次

8.13

x=4

x=3

x=2

8.14

8.15

8.16

counter = 2

8.17

hello 0 1 Bye 2 Bye

hello 1 0 Bye 2 Bye

hello 1 Bye 0 2 Bye

8.18

8.19

2^n

8.20

8.21

abc 或 bac

8.22

#include <stdio.h>#include <stdlib.h>#include <stdarg.h>#include <unistd.h>#include <string.h>#include <ctype.h>#include <setjmp.h>#include <signal.h>#include <dirent.h>#include <sys/time.h>#include <sys/types.h>#include <sys/wait.h>#include <sys/stat.h>#include <fcntl.h>#include <sys/mman.h>#include <errno.h>#include <math.h>#include <pthread.h>#include <semaphore.h>#include <sys/socket.h>#include <netdb.h>#include <netinet/in.h>#include <arpa/inet.h>#define MAXARGS 128#define MAXLINE 8192 /* Max text line length */extern char **environ; /* Defined by libc *//* Linux C中environ 变量是一个char** 类型，存储着系统的环境变量 */void unix_error(char *msg) /* Unix-style error */{fprintf(stderr, "%s: %s\n", msg, strerror(errno));exit(0);}void app_error(char *msg) /* Application error */{fprintf(stderr, "%s\n", msg);exit(0);}pid_t Wait(int *status) {pid_t pid;if ((pid = wait(status)) < 0)unix_error("Wait error");return pid;}char *Fgets(char *ptr, int n, FILE *stream) {char *rptr;if (((rptr = fgets(ptr, n, stream)) == NULL) && ferror(stream))app_error("Fgets error");return rptr;}/* parseline - Parse the command line and build the argv array */int parseline(char *buf, char **argv) {char *delim; /* Points to first space delimiter */int argc; /* Number of args */int bg; /* Background job? */buf[strlen(buf)-1] = ' '; /* Replace trailing '\n' with space */while (*buf && (*buf == ' ')) /* Ignore leading spaces */buf++;/* Build the argv list */argc = 0;while ((delim = strchr(buf, ' '))) {argv[argc++] = buf;*delim = '\0';buf = delim + 1;while (*buf && (*buf == ' ')) /* Ignore spaces */buf++;}argv[argc] = NULL;if (argc == 0) /* Ignore blank line */return 1;/* Should the job run in the background? */if ((bg = (*argv[argc-1] == '&')) != 0)argv[--argc] = NULL;return bg;}pid_t Fork(void) {pid_t pid;if ((pid = fork()) < 0)unix_error("Fork error");return pid;}int mysystem(char *command) {char *Argv[MAXARGS];int status;if(Fork() == 0) {parseline(command, Argv);for(int i = 0; i < MAXARGS; i++) {if(Argv[i]==NULL) {break;}}execve("/bin/sh", Argv, environ);}Wait(&status); /*等待子进程结束*/return WEXITSTATUS(status);}int main(int argc, char **argv, char **env) {char str[12] = "/bin/sh -c ";char command[MAXLINE];char temp[MAXLINE-12];printf("\033[1;32m>\033[0m ");while(Fgets(temp, MAXLINE-12, stdin)) {strcpy(command, str);strcat(command, temp);printf("\033[1;32m mysystem 返回值为 %d \n\033[0m", mysystem(command));printf("\033[1;32m\n>\033[0m ");}return 0;}

我并没有使用 “csapp.h” 头文件，而是用到什么就复制出什么，一方面可以清楚被封装函数的调用方法，另一方面可以学习作者是如何对函数进行封装的。

我做的有些麻烦，虽然使用了 parseline 函数，但是我的代码只支持不带参数的 command，比如 “/bin/sh ls -a”，就只能识别到 “/bin/sh ls”，可以修改 parseline 函数，不过我懒 (ಥ _ ಥ)

对于几种不同的输入，有不同的返回值：

USER@NAME:~# ./mysystem> ls123.py 公共模板.........mysystem 返回值为 0 > ./exit12mysystem 返回值为 12 > lllll/bin/sh: 1: lllll: not foundmysystem 返回值为 127

exit12 程序非常简单，只有四行：

#include <stdio.h>#include <stdlib.h>int main() {exit(12);}

8.23

一个进程的待处理信号不会排队，只会被丢弃。

8.24

USER@NAME:~# gcc -o P8-24 P8-24.c USER@NAME:~# ./P8-24123123child 11345 terminated by signal 11: Segmentation fault22child 11344 terminated by signal 11: Segmentation fault