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Unix System Programming Overview Outline What Is a Process? What Is Overview Last Week: ● How to program with directories ● Brief introduction to the UNIX file system Unix System Programming This Week: ● How to program UNIX processes (Chapters 7-9) Processes » Follow the flow of Ch 8. Process control sprinkled with reflections from Ch 7 (e.g., exit, process/program memory layout). ● fork() and exec() Maria Hybinette, UGA 1 Maria Hybinette, UGA 2 Outline What is a Process? ● What is a process? ● Review: A process is a program in execution (an active ● fork() entity, i.e., it is a running program , this was also on the ● exec() » Basic unit of work on a computer ● wait() » Examples: ● Process Data – compilation process, ● Special Exit Cases – word processing process – a.out process ● Process Ids – Shell process ● I/O Redirection – (we just need to make sure the program is running) ● User & Group ID real and effective (revisit) ● getenv & putenv ● ulimit Maria Hybinette, UGA 3 Maria Hybinette, UGA 4 What is a Process? What is a Process? ● It has both time, and space ● Each user can run many processes at once (e.g., by using &) » A container of instructions with some resources ● A process: » cat file1 file2 & ● Process reads, and writes (or updates) ● Two processes started on the command line. machine resources » ls | wc -l » e.g., CPU time (CPU carries out the instructions), ● A time sharing system (such as UNIX) run several processes » memory, by multiplexing between them » files, Processes » I/O devices (monitor, printer) to accomplish its task n-1 0 1 2 … OS (schedules process) Maria Hybinette, UGA 5 Maria Hybinette, UGA 6 What Makes up a Process? Formal Process Definition size? ● Program code (text) User Mode 3GB » Compiled version of the text Address Space ● Data (cannot be shared) A process is a program in execution, a stack sequential execution characterized by trace. It » global variables – Uninitialized (BSS segment) sometimes has a context (the information or data) and listed separately. – Initialized routine1 this context is maintained as the process var1 ● Process stack (scopes) var2 progresses through the system. » function parameters heap » return addresses arrayB[10] ; » local variables and functions data arrayA[10] = {0} ● <<Shared Libraries >> ● Heap: Dynamic memory (alloc) main ● OS Resources, environment routine1 text » open files, sockets routine2 » Credential for security 0x0 ● Registers address space are the shared resources of a(ll) thread(s) in a » program counter, stack pointer Maria Hybinette, UGA 7 Maria Hybinette, UGA program 8 Info about a process (running and foot print) 0xFFFFFFFF 4GB ● ● longsleepHelloW (binary with long sleep) Example: Process with Kernel Space 4GB Virtual Address Virtual Addresses ● size a.out. (foot print) Space (32 bit ● ps ! architectures) 0xC0000000 3GB » 61542 pts/5 00:00:00 longsleepHelloW! ● User Space (focused on ● cat /proc/61542/status! earlier, lower address ● space) User Space cat /proc/61542/maps! Virtual Addresses ● Kernel Space ● ppp 0x0 0x0 Maria Hybinette, UGA 9 Maria Hybinette, UGA 10 What is needed to keep track of a Process? Process Representation Process P2 Information System Memory ● Memory information: Memory Limits » Pointer to memory segments needed to run a process, i.e., pointers to the Page tables Memory base Kernel Process Table address space -- text, data, stack Initial P0 segments. Process Number Program counter P0 : HW state: resources ● Process management information: Program Counter Process P3 … » Process state, ID Registers P1 : HW state: resources » Content of registers: Process P P : HW state: resources – Program counter, stack pointer, Process State 2 2 process state, priority, process ID, CPU time used List of opened files P3 : HW state: resources Memory mappings ● File management & I/O information: I/O Devices allocated » Working directory, file descriptors … open, I/O devices allocated Accounting Pending requests Process P1 ● Accounting: amount of CPU used. Process control … Block (PCB) Maria Hybinette, UGA 11 Maria Hybinette, UGA 12 System Control: OS View: Process Control Block Process Attributes (PCB) ● How does an OS keep track of the state of a ps and top command can be used to look at process? current processes » Keep track of some information in a structure. – Example: In Linux a process information is kept in a ● PID - process ID: each process has a unique ID structure called struct task_struct declared in #include linux/sched.h! ● PPID - parent process ID: The process that – What is in the structure? forked to start the (child) process struct task_struct ● nice value - priority (-20 highest to 19 lowest) pid_t pid; /* process identifier */ long state; /* state for the process */ ● TTY associated with terminal (TTY teletype unsigned int time_slice /* scheduling information */ terminal) struct mm_struct *mm /* address space of this process */ – Where is it defined: ● not in /usr/include/linux – only user level code ● usr/src/kernel/2.6.32-431.29.2.elf6.x86_64/include/linux Maria Hybinette, UGA 13 Maria Hybinette, UGA 14 Back to user-level Other Process Attributes ● Finding PIDs ● Real user ID » At the shell prompt ● Effective user ID – ps u, ps, ps aux, ● Current directory ● ps no args # your process ● ps –ef # every process ● File descriptor table ● ps -p 77851 # particular process ● Environment – top interative » In a C program: int p = getpid(); // more later ● Pointer to program code, data stack and heap ● Execution priority ● Signal information Maria Hybinette, UGA 15 Maria Hybinette, UGA 16 Process ID conventions, and the 3 General Process Types in UNIX Process Life Cycle Interactive – foreground (shell must wait until complete [takes user input], or – background (&) [no user input] ● PID 0 – initiated an controlled terminal session » is usually the scheduler process (swapper), a system process – can accept input form user as it runs and output to the terminal (does not correspond to a program stored on disk, the grandmother of all processes). Daemons – server processes running in the background (e.g., listening to a port) ● init - Mother of all user processes, init is started at – Not associated with the terminal boot time (at end of the boot strap procedure) and is – typically started by init process at boot time responsible for starting other processes – Examples: ftpd, httpd, …, mail » It is a user process with PID 1 – If user wants to creates one, detach it from the terminal, kill its parent. (init adopts) » init uses file inittab and directory /etc/rc?.d Batch (at, cron, batch) » brings the user to a certain specified state (e.g. multiuser) – Jobs that are queued and processed one after another ● – recurrent tasks scheduled to run from a queue getty - login process that manages login sessions – periodic, recurrent tasks run when system usage is low, cron-jobs (administered by the daemon crond). – Examples: backups, experimental runs. » ZombiesMaria Hybinette, UGA… don’t count. 17 Maria Hybinette, UGA 18 Hierarchical Processes Tree on a (historical) UNIX System Display Process Hierarchy mother of all user processes OS Kernel pstree (processes) ● Syntax: pstree | more (all process) Process 0 Process 1 Process 2 (BSD) ● Syntax: pstree <PID> (sched - ATT, swapper - BSD) (init) pagedaemon ● Syntax: pstree <username> tree (directory) deamon (e.g. httpd) getty getty ● -d (directories), -a (hidden), -s (size), -p (permissions) ● tree –H . login login bash ksh Maria Hybinette, UGA 19 Maria Hybinette, UGA 20 Daemon Processes PID and Parentage ● Print out status information of various processes in the system: ps -axj (BSD) , ps -efjc (SVR4) , switches / flags varies ● A process ID or PID is a positive integer that uniquely identifies a running process and is stored in a variable ● process status (ps) of type pid_t ● Daemons (d) run with root privileges, no controlling terminal, parent process is init ● Example: print the process PID and parents PID {atlas:maria} ps -efjc | sort -k 2 -n | more // solaris below UID PID PPID PGID SID CLS PRI STIME TTY TIME CMD {saffron} print-pid root 0 0 0 0 SYS 96 Mar 03 ? 0:01 sched #include <sys/types.h> My PID is 3891 root 1 0 0 0 TS 59 Mar 03 ? 1:13 /etc/init -r #include <unistd.h> MY PPID is 3794 root 2 0 0 0 SYS 98 Mar 03 ? 0:00 pageout root 3 0 0 0 SYS 60 Mar 03 ? 4786:00 fsflush #include <stdio.h> PID COMMAND %CPU TIME root 61 1 61 61 TS 59 Mar 03 ? 0:00 /usr/lib/sysevent/syseventd int main(void) root 64 1 64 64 TS 59 Mar 03 ? 0:08 devfsadmd 3891 print-ids 0.0% 0:00.00 root 73 1 73 73 TS 59 Mar 03 ? 30:29 /usr/lib/picl/picld { 3874 top 13.6% 0:19.71 3794 ksh 0.0% 0:00.04 root 256 1 256 256 TS 59 Mar 03 ? 2:56 /usr/sbin/rpcbind pid_t pid, ppid; root 259 1 259 259 TS 59 Mar 03 ? 2:05 /usr/sbin/keyserv root 284 1 284 284 TS 59 Mar 03 ? 0:38 /usr/sbin/inetd -s printf( My PID is %d\n, (pid = getpid()) ); daemon 300 1 300 300 TS 59 Mar 03 ? 0:02 /usr/lib/nfs/statd printf( My PPID is %d\n\n, (pid = getppid()) ); root 302 1 302 302 TS 59 Mar 03 ? 0:05 /usr/lib/nfs/lockd root 308 1 308 308 TS 59 Mar 03 ? 377:42 /usr/lib/autofs/automountd } rootMaria Hybinette, 319 UGA 1 319 319 TS 59 Mar 03 ? 6:33 /usr/sbin/syslogd 21 Maria Hybinette, UGA 22 ● Linux processes ● pstree ● ps -efjc | sort -k 2 -n | more ● pidstat {nike:maria:125} ps -efjc | sort -k 2 -n | more # linux Oct 2014 below UID PID PPID PGID SID CLS PRI STIME TTY TIME CMD ● top, htop root 1 0 1 1 TS 19 Oct01 ? 00:02:11 /sbin/init root 2 0 0 0 TS 19 Oct01 ? 00:00:04 [kthreadd] ● mpstat root 3 2 0 0 FF 139 Oct01 ? 01:23:55 [migration/0] root 4 2 0 0 TS 19 Oct01 ? 00:01:10 [ksoftirqd/0] ● jobs root 5 2
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