CS电机资讯工程实习Linux开机流程.ppt

Introduction to Linux (II) Prof. Chung-Ta King Department of Computer Science National Tsing Hua University CS1103 電機資訊工程實習 Linux 開機流程 BIOS開機完成後根據設定的開機硬碟載入Bootloader BootLoader根據設定載入所指定的作業系統 PowerOn bootloader BIOS Hardware 開機 Linux kernel init System ready Layers of Unix (system calls: entries to kernel from user-space application) Processes A process is a program in execution Ethernet Printer Disk Terminal swapper lpd init inetd getty csh ls ps Low-level Process I/O All communication of a process with outside is done by reading or writing files? a single interface File descriptor: A non-negative integer for reference to a file Three descriptors are created at process creation: stdin (0), stdout (1), stderr (2)all are connected to the terminal/keyboard by default More descriptors can be created: fd = open(“outfile”, O_WRONLY, 0644); Descriptor table: with a limit on # of open files /man/2/syscalls Low-level Process I/O: Example main(int argc, char *argv[]) { /* copy f1 to f2 */ int f1,f2,n; char buf[BUFSIZ]; if ((f1=open(argv[1],O_RDONLY)) == -1) /* error if non-exist */ error(“can’t open %s\n”, argv[1]); if ((f2 = creat(argv[2],0644)) == -1) error(“can’t create %s”,argv[2]); while ((n = read(f1,buf,BUFSIZ)) 0) /* return 0-EOF; -1-error; nBUFSIZ-OK; */ /* read will return up to end of line */ if (write(f2,buf,n) != n) error(“write error”, (char *) 0); } Unix Process Creation Switch to another program: execve(“/usr/bin/rsh”,“rsh”,”cs20”, “date”,0,0); Replaces current process image Split a process: fork() and wait() fork() produces two identical processes Child process returns 0 and parent returns child’s pid if (fork() == 0)execve(“/bin/sh”, “sh”, “-c”, commandline,(char *) 0,0); Unix Process Creation Given fork(), execve(), and wait(), it is easy to understand how shell operates: repeat get next command fork a child to run command (fork() execve()) wait for the child to terminat