Lecture 3 Process API in UNIX systems

Transcription

1 Lecture 3 Process API in UNIX systems

2 Contents In this lecture, you will learn some process creation API, including: How to Create Wait for Execute A process, and Why in such the way.

3 The fork() System Call

4 The fork() System Call ODD?

5 The fork() System Call The process that is created by using the fork() system call is an (almost) exact copy of the calling process.

6 The fork() System Call Discussion: what is the output? int rc = fork(); if (rc < 0) { printf( A"); exit(1); } else if (rc == 0) { printf( B"); } else { printf( C"); } return 0; }

7 The fork() System Call

8 The fork() System Call Discussion: why not deterministic?

9 The fork() System Call Discussion: what is the output? for(loop=0;loop<2;loop++) { if((pid=fork()) < 0) printf( A"); else if(pid == 0) printf( B ); else printf( C ); }

10 Contents In this lecture, you will learn some process creation API, including: How to Create Wait for Execute A process, and Why in such the way.

11 The wait() System Call

12 The wait() System Call parent waits for child process to finish

13 Contents In this lecture, you will learn some process creation API, including: How to Create Wait for Execute A process, and Why in such the way.

14 The exec() System Call

15 The exec() System Call

16 The exec() System Call The process that is created by using the exec() system call can be a different program. Some details in exec() It does not create a new process; rather, it transforms the currently running program into a different running program.

17 Review Process creation APIs fork() wait() exec() What are the differences?

18 Contents In this lecture, you will learn some process creation API, including: How to Create Wait for Execute A process, and Why in such the way.

19 Why? Motivating The API Available to run code after the call to fork() but before the call to exec(). What kind of code? E.g. prompt> wc p3.c > newfile.txt

20 Thinking in Redirection How can the shell, which is just a user program, manage to do this? Some code between the calling of fork() and that of exec().

21 Thinking in Redirection

22 Pipes in Unix UNIX pipes are implemented in a similar way, but with the pipe() system call. The output of one process is connected to an inkernel pipe. The input of another process is connected to that same pipe. E.g., ls wc

23 Pipes in Unix

24 How to Create a Pipe with Parent Writes & Child Reads

25 Discussion What if the parent wants to write sth. to child, while child also wants to write sth. to parent? Hints, ordinary pipes are unidirectional

26 Contents In this lecture, you will learn some process creation API, including: How to Create Wait for Execute A process, and Why in such the way. And a little more interesting staff Communication (IPC) Termination, etc.

27 IPC in Unix Pipes and FIFO (Named Pipes) Signal

28 IPC in Linux Pipes and FIFO (named Pipes) Signal Message Shared Memory Semaphore (for synchronization) Socket (between machines)

29 Message Passing vs. Shared Memory

30 Message Passing vs. Shared Memory Message passing is useful for exchanging smaller amounts of data, because no conflicts need be avoided. Message passing is easier to implement in a distributed system than shared memory. Shared memory can be faster than message passing. Message-passing systems are typically implemented using system calls and thus require the more timeconsuming task of kernel intervention. In shared-memory systems, system calls are required only to establish shared memory regions

31 Message Passing vs. Shared Memory Message passing provides better performance than shared memory in multi-processing systems. Shared memory suffers from cache coherency issues As the number of processing cores on systems increases, it is possible that we will see message passing as the preferred mechanism for IPC.

32 Other Parts Of The API The kill() system call is used to send signals to a process, and terminate the process. Command-line tools Ps Top

33 Discussion 下列程序的输出是什么? #include <sys/wait.h> #include <sys/types.h> #include <stdio.h> #include <unistd.h> #include <stdlib.h> int value = 5; int main() { pid_t pid; pid = fork(); if (pid == 0) { printf("child process, value1 : %d\n", value); value += 15; printf("child process, value2 : %d\n", value);} } else if (pid > 0) { printf("parent process, value3 : %d\n", value); wait(null); printf("parent process, value4 : %d\n", value); exit(0); }

34 Answer 编译运行后有两种输出 ( 因为 fork 后, 子进程可能先执行, 父进程也可能先执行 ) (1) 子进程先执行 : child process, value1 : 5 child process, value2 : 20 parent process, value3 : 5 parent process, value4 : 5 (2) 父进程先执行 : parent process, value3 : 5 child process, value1 : 5 child process, value2 : 20 parent process, value4 : 5

35 Summary Process creation API Fork() Wait() Exec() IPC Pipe Message passing vs. shared memory Process termination Kill()