lect07-sys-calls3.ppt

1、CO004 Projects on Operating Systems,System calls 3,Outline,Process Hierarchy exec() wait() exit(),Process Hierarchy,Process relationship: A parent process will have its parent process. Also, a child process will have its child process. This form a tree hierarchy.,Process A,Process B,Process C,Proces

2、s D,Process E,Process F,Program Execution,fork() is rather boring If a process can only duplicate itself and always runs the same program, then how can we execute other programs? We have another set of system calls to do this job. The exec() system call family.,exec(),Change program in process i.e.,

3、 launch a new program that replaces the current one Several different forms with slightly different syntax,status = execve(prog, args, env);,-1 on error. never see this if successful,name of file that should be executed,command line arguments - char* args,environment variables - char* args,Program E

4、xecution,A member of exec system call family execl(),Program: exec_example.c,$ ./exec_example_1 before execl .,Process 1234,int main(void) printf(“before execl n”); execl(“/bin/ls”, “/bin/ls”, NULL); printf(“after execl .n”); return 0; ,Program Execution,A member of exec system call family execl(),P

5、rogram: exec_example.c,$ ./exec_example_1 before execl . exec_example_1 exec_example_1.c,Process 1234,Result from “/bin/ls”,int main(void) printf(“before execl n”); execl(“/bin/ls”, “/bin/ls”, NULL); printf(“after execl .n”); return 0; ,Program Execution,A member of exec system call family execl(),P

6、rogram: exec_example.c,$ ./exec_example_1 before execl . exec_example_1 exec_example_1.c $ _,Process 1234,int main(void) printf(“before execl n”); execl(“/bin/ls”, “/bin/ls”, NULL); printf(“after execl .n”); return 0; ,Program Execution,The exec system call family is not simply a function that “invo

7、kes” a command. It is doing a very complex thing,Process 1234,int main(void) printf(“before execl n”); execl(“/bin/ls”, “/bin/ls”, NULL); printf(“after execl .n”); return 0; ,Originally, Im executing this code.,Program Execution,The exec system call family is not simply a function that “invokes” a c

8、ommand. It is doing a very complex thing,Process 1234,int main(void) printf(“before execl n”); execl(“/bin/ls”, “/bin/ls”, NULL); printf(“after execl .n”); return 0; ,file: /bin/ls,Now, I change to execute the program: /bin/ls.,Program Execution,The process is changing the code that it is executing

9、and never returns to the original code. The last two lines of codes are therefore not executed. The process that calls any one of the member of the exec system call family will throw away many things, e.g., Memory: including local variables, global variables, and allocated memory; Register value: e.

10、g., the program counter; But, the process will preserve something, such as PID; Process relationship; Running time, etc.,Program Execution,The exec system call family has 6 members and they are named as,Program Execution,Pathname vs Filename Argument list vs array,/home/henry/os/shell.c,execl(“/bin/

11、ls”, “/bin/ls”, “-l”, NULL);,execle(“/bin/ls”, array);,What you have learned?,fork() to create a child process. exec() to execute any commands. We can do a lot things based on the two system calls. For example, we want do implement a “shell”. But only fork() and exec() are not enough.,Example: imple

12、ment a system() function call,“system()” is a library function call, and it is built based on “fork()”, and exec system call family. (Thats why Ive told you that system calls are not programmer-friendly.),int main(void) printf(“before system n”); system(“ls”); printf(“after system n”); ,Program: sys

13、tem_example.c,We are now going to implement the system() function using system calls.,1 int system_ver_3150(const char *cmd_str) 2 if(cmd_str = NULL) 3 return -1; 4 5 if(fork() = 0) 6 execl(/bin/sh, /bin/sh, -c, cmd_str, NULL); 7 fprintf(stderr, 8 %s: command not foundn, 9 cmd_str); 10 exit(-1); 11

14、12 13 return 0; 14 15 16 int main(void) 17 printf(before system_ver_3150 .n); 18 system_ver_3150(/bin/ls); 19 printf(after system_ver_3150 .n); 20 return 0; 21 ,system() Implementation,$ ./system_implement_1 before system_ver_3150 . system_implement_1 system_implement_1.c after system_ver_3150 . $ _

15、,Program: system_implement_1.c,“/bin/sh” is the shell program. We actually want to execute the command: /bin/sh -c cmd_str,1 int system_ver_3150(const char *cmd_str) 2 if(cmd_str = NULL) 3 return -1; 4 5 if(fork() = 0) 6 execl(/bin/sh, /bin/sh, -c, cmd_str, NULL); 7 fprintf(stderr, 8 %s: command not

16、 foundn, 9 cmd_str); 10 exit(-1); 11 12 13 return 0; 14 15 16 int main(void) 17 printf(before system_ver_3150 .n); 18 system_ver_3150(/bin/ls); 19 printf(after system_ver_3150 .n); 20 return 0; 21 ,system() Implementation,Program: system_implement_1.c,However, when I run it for the 2nd time,$ ./syst

17、em_implement_1 before system_ver_3150 . after system_ver_3150 . system_implement_1 system_implement_1.c $ _,Whatd happened?!,1 int system_ver_3150(const char *cmd_str) 2 if(cmd_str = NULL) 3 return -1; 4 5 if(fork() = 0) 6 execl(/bin/sh, /bin/sh, -c, cmd_str, NULL); 7 fprintf(stderr, 8 %s: command n

18、ot foundn, 9 cmd_str); 10 exit(-1); 11 12 13 return 0; 14 15 16 int main(void) 17 printf(before system_ver_3150 .n); 18 system_ver_3150(/bin/ls); 19 printf(after system_ver_3150 .n); 20 return 0; 21 ,system() Implementation,Program: system_implement_1.c,Lets re-color the program!,Parent process,Chil

19、d process,Both processes,$ ./system_implement_1 before system_ver_3150 . after system_ver_3150 . system_implement_1 system_implement_1.c $ _,$ ./system_implement_1 before system_ver_3150 . after system_ver_3150 . system_implement_1 system_implement_1.c $ _,system() Implementation,$ ./system_implemen

20、t_1 before system_ver_3150 . after system_ver_3150 . system_implement_1 system_implement_1.c $ _,$ ./system_implement_1 before system_ver_3150 . system_implement_1 system_implement_1.c after system_ver_3150 . $ _,First Execution Sequence,Second Execution Sequence,Parent,fork(),Child,Parent,Parent,fo

21、rk(),Child,Parent,It is because of the OS process scheduling,system() Implementation,Dont forget that were trying to implement a system()-compatible function it is very weird to allow different execution orders. Our current problem is how to let the child to execute first? Sorrywe cant control the p

22、rocess scheduling of the OS to this extent. Then, our problem becomes How to suspend the execution of the parent process? How to wake the parent up after the child is terminated?,system() Implementation,We introduce you the wait() system call!,1 int system_ver_3150(const char *cmd_str) 2 if(cmd_str

23、= NULL) 3 return -1; 4 5 if(fork() = 0) 6 execl(/bin/sh, /bin/sh, -c, cmd_str, NULL); 7 fprintf(stderr, 8 %s: command not foundn, 9 cmd_str); 10 exit(-1); 11 12 13 return 0; 14 15 16 int main(void) 17 printf(before system_ver_3150 .n); 18 system_ver_3150(/bin/ls); 19 printf(after system_ver_3150 .n)

24、; 20 return 0; 21 ,Put something useful here!,Program: system_implement_2.c,wait(),When a process is done it can call exit(status). This is the status that echo $? can show you in the shell A parent can wait for its children (it blocks until they are done),status = waitpid(pid, ,-1 on error. otherwi

25、se, PID of process that exited,which PID to wait for; -1 means any child,exit code of process that has exited,check man page for details,system() Implementation with wait(),1 int system_ver_3150(const char *cmd_str) 2 if(cmd_str = NULL) 3 return -1; 4 5 if(fork() = 0) 6 execl(/bin/sh, /bin/sh, -c, c

26、md_str, NULL); 7 fprintf(stderr, 8 %s: command not foundn, 9 cmd_str); 10 exit(-1); 11 12 wait(NULL); 13 return 0; 14 15 16 int main(void) 17 printf(before system_ver_3150 .n); 18 system_ver_3150(/bin/ls); 19 printf(after system_ver_3150 .n); 20 return 0; 21 ,When wait() returns, the parameter can t

27、ell the parent about the current status of the child process, i.e., terminated or suspended. The parameter NULL means that “I dont care about the childs status”. Look up the man page of wait by yourself. In MINIX, the command is: “man 2 wait”,Program: system_implement_2.c,system() Implementation with wait(),