Introduction to Linux (II)

1
Introduction to Linux (II)
CS1103 ????????

• Prof. Chung-Ta King
• Department of Computer Science
• National Tsing Hua University

2
Linux ????

• BIOS????????????????Bootloader
• BootLoader??????????????

bootloader
PowerOn
BIOS
Hardware ??
Linux kernel
init
System ready
3
Layers of Unix
(system calls entries to kernel from user-space
application)
4
Processes

• A process is a program in execution

init
swapper
csh
ls
ps
Disk
getty
Terminal
inetd
lpd
Printer
Ethernet
5
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

http//linux.die.net/man/2/syscalls
6
Low-level Process I/O Example

• main(int argc, char argv)
• / copy f1 to f2 /
• int f1,f2,n char bufBUFSIZ
• if ((f1open(argv1,O_RDONLY)) -1)
• / error if non-exist /
• error(cant open s\n, argv1)
• if ((f2 creat(argv2,0644)) -1)
• error(cant create s,argv2)
• while ((n read(f1,buf,BUFSIZ)) gt 0)
• / return 0-gtEOF -1-gterror nltBUFSIZ-gtOK /
• / read will return up to end of line /
• if (write(f2,buf,n) ! n)
• error(write error, (char ) 0)

7
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
  childs pid
• if (fork() 0)execve(/bin/sh, sh, -c,
  commandline,(char ) 0,0)

8
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 terminate (wait())

9
Examine Process Status (ps al)
10
Processes and Descriptors

• char argu "rsh","cs20",date,0
• fd open(outfile,O_RDWR,0644)
• dup2(fd,1) / dup fd to 1 /
• / 1 now link to outfile /
• if (fork() 0) / the child /
  execve(/usr/bin/rsh, argu,0)
• else / the parent /
• fprint(stderr, child working \n)
• wait(status)
• system(ps al)

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0
2
0
2
ex1
ex1
1
1
3
open()
dup2()
0
2
0
2
ex1
1
3
date
1
fork()
ex1 rsh
0
2
0
2
ex1
1
3
1
3
system()
0
2
0
2
cs20
csh
ps
1
3
1
3
fork()
outfile
cs21
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Pipes for Inter-process Comm.

• Pipe a unidirectional byte stream
• e.g., ls pr -2 lpr
• int sk2 / sk0 read-end sk1 write-end
  /
• pipe(sk) / create a pipe /
• if (fork()) / the parent /
• close(sk1)
• while(read(sk0,buf,SIZE)gt0) printf("s",buf)
  
• else / the child /
• close(sk0) fdpopen("ps -l","r")
• while((sread(fd,buf,SIZE))gt0)
  write(sk1,buf,s)

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3
ex2
3
ex2
4
3
(a)
ex2
4
3
ex2
csh
fork()
0
ex2
4
ps
1
(b)
(c)