1. CSE451 Section 2: Spring 2006 Kurtis Heimerl, YongChul Kwon kheimerl@cs, yongchul@cs

2. Office hour Prof. John Zahorjan –Wed 3:30 – 4:20 @ CSE534 YongChul Kwon –Mon & Thr 2:30-3:20 @ CSE216 –By appointment Kurtis Heimerl –Mon & Wed & Fri 12:30-1:20 @ CSE002 –By appointment

3. Reminders Section leader –AA: Kurtis Heimerl –AB: YongChul Kwon Read & start project 1 –Project 1 due Apr 14 11:00am Be familiar with VMware ® Check access to –forkbomb.cs.washington.edu

4. Proj1: Shell & system call Sharpening your C edge! Understand & practice –Library call & system call –Stack of H/W to user Apps –Working at kernel level By –Design & implement simple shell & system call

5. Shell? 9. Computer Science A program that works with the operating system as a command processor, used to enter commands and initiate their execution. -- American Heritage ® Dictionary of the English Language

6. Why do we call it ‘shell’? For Linux? For Windows®? H/W Operating System Shell Firefox Powerpoint

7. Library call & system call Library call –Not using any OS service –Provide a high level interface for OS service System call –Using some OS service –Process/Signal/File/Network/IPC/… What happens when we call –strncpy(3) –fgets(3)

8. Key C functions for shell String –strcmp, strncpy, strtok, atoi I/O –fgets Error report –perror Process control –fork, wait or waitpid, execv or execvp Environment variable –getenv Don’t forget to read manual pages of these!

9. Typical skeleton for shell Bool exit = false; while ( !exit ) { /* read command line from user */ /* parse command line */ if ( it is internal command ) { /* execute internal command */ } else { pid_t pid = fork(); /* spawn a new process */ if ( pid == 0 ) { /* call exec to launch requested program */ } else if ( pid < 0 ) { /* print error message: fork failed */ } else { /* wait until launched program terminates */ } } }

10. Internal Vs. External command Internal command –By executing routines embedded in a shell –Manage states of the shell –Shell scripts Shell has its own states –Current working directory –Alias to a command line –Environment variable –Processes spawned by the shell –…–… Then what does external command do?

11. New system call: execcounts Count # of process invocation calls –fork –vfork –clone –execve executed for a specific process and all of its descendants

12. Writing our system call Write a library for the new system call stub code –Refer the project description Modify your PCB definition(struct task_struct) –include/linux/sched.h(in kernel source) Modify code to maintain values correctly –A specific process and all of its descendants Implement a system call to retrieve this information Modify your library code

13. Tips Read the project instructions thoroughly! Find some useful functions in –include/linux/sched.h –inlcude/asm/current.h Don’t forget to you are working in Kernel mode –Be careful in translating addresses Learn by example –Investigate how other system calls are implemented –Recommended: getpid(), kill(), write()