Processes Jinkyu Jeong ([email protected]) Computer Systems Laboratory Sungkyunkwan University http://csl.skku.edu

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

OS Internals shell User space

shell

ps

ls

trap

Kernel space

System Call Interface

Memory Management

I/O Management

scheduler IPC synchronization

(device drivers)

Protection

File System Management

Process Management

Hardware Control (Interrupt handling, etc.)

Hardware SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

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What is a Process? • An instance of a program in execution • The basic unit of protection • A process is identified using its process ID (PID) • A process includes – CPU context (registers) – OS resources (address space, open files, etc.) – Other information (PID, state, owner, etc.) SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

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Process vs. Program Memory

PC

Disk

Code Data Heap

code data program

SP Stack

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

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Running a Process

Fetch I  Mem[PC] Decode I Execute I Update PC

PC

Code

Data

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

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Running Multiple Processes Code A

Data A Code B

Data B Code C

Data C SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

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Interleaving Multiple Processes Code A

Data A Code B

Data B Code C

Data C SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

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Virtualizing the CPU Code A

Data A

OS creates the illusion that each process has its own CPU (and memory)

Code B

Data B OS Code

OS Data SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

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Example #include #include

int main() { int pid; if ((pid = fork()) == 0) /* child */ printf (“Child of %d is %d\n”, getppid(), getpid()); else /* parent */ printf (“I am %d. My child is %d\n”, getpid(), pid); } SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

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Example Output

% ./a.out I am 31098. My child is 31099. Child of 31098 is 31099. % ./a.out Child of 31100 is 31101. I am 31100. My child is 31101.

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

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Process Hierarchy • Parent-child relationship – One process can create another process – Unix calls the hierarchy a “process group” – Windows has no concept of process hierarchy

• Browsing a list of processes: – ps in Unix – Task Manager (taskmgr) in Windows SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

$ cat file1 | wc

sh cat

wc

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Process Creation • fork() – Creates a new process cloning the parent process • Parent inherits most of resources and privileges: open files, UID, etc. • Child also duplicates the parent’s address space

– Parent may either wait for the child to finish (using wait()), or it may continue in parallel – Shells or GUIs use this system call internally

• exec() – Replaces the current process image with a new program – Windows: CreateProcess() = fork() + exec() SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

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Process Termination • Normal exit (c) • Error exit (voluntary) • Fatal error (involuntary) – Segmentation fault – illegal memory access – Protection fault – Exceed allocated resources, etc.

• Killed by another process (involuntary) – By receiving a signal

• Zombie process: terminated, but not removed SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

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Process APIs • exit() – Caller process terminates its execution – Return from main() is identical – Parameter denotes whether this exit is normal (0) or error (1)

• kill() – Send signal to a process – Parameters are directives to go to sleep, to die, and other useful imperatives

• wait() – Wait for state changes in any child of the calling process • Usually termination of a child

– Release resources associated with the terminated child – waitpid() for a specific child SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

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Simplified Shell int main(void) { char cmdline[MAXLINE]; char *argv[MAXARGS]; pid_t pid; int status; while (getcmd(cmdline, sizeof(buf)) >= 0) { parsecmd(cmdline, argv); if (!builtin_command(argv)) { if ((pid = fork()) == 0) { if (execv(argv[0], argv) < 0) { printf(“%s: command not found\n”, argv[0]); exit(0); } } waitpid(pid, &status, 0); } } } SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

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Process State Transitions Created

exit Scheduled

Ready

Running Time slice exhausted

I/O or event completion

I/O or event wait

Blocked

SSE3044: Operating Systems, Fall 2016, Jinkyu Jeong ([email protected])

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Process State – Linux Example R: Runnable S: Sleeping T: Traced or Stopped D: Uninterruptible Sleep Z: Zombie