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Processes Process Concept Process Scheduling Operations on Processes Cooperating Processes Interprocess Communication Communication in Client-Server Systems Process Concept An operating system executes a variety of programs: Batch system – jobs Time-shared systems – user programs or tasks Textbook uses the terms job and process almost interchangeably. Process – a program in execution; process A process includes: program counter stack data section Stack Temporary data: Function parameter, return addresses and local variables Memory: that is dynamically allocated during process run time Heap Data A process is more than the Text program code –text section Contains global variables Process in memory Contents of processor’s registers Value of Program counter Relationship b/w process and program Is process same as a program? No Program is sequence of statements that a programmer writes. A single program may contain more than one processes Many processes may be derived from a single program When you perform “Copy” at the Command Prompt of Windows XX, several processes are derived from that. Copy source_File destination_File Multiprogramming VS Uniprogramming Some systems allow only one process called Uni-programming Systems. To invoke a new process other processes must be finished. Most systems now a days allow more than one process called Multiprogramming Systems. In case of uni-programming you don’t need to store information about the process as much as you would need to store information about a process being executed in multiprogramming system. Process State In user view processes are running simultaneously In computers view these processes are in contention for the CPU and I/O As a process executes, it changes state new: The process is being created. running: Instructions are being executed. waiting: The process is waiting for some event to occur. ready: The process is waiting to be assigned to a processor. terminated: The process has finished execution. Diagram of Process State The state of a process is defined in part by the current activity of that process Process Control Block (PCB) 1/2 Saves Information associated with each process. Remember that we studied in interrupts that the process state is saved in memory. PCB provides data structure for saving process state Process state New, Wait, Ready etc. Program counter Next instruction to be executed. CPU registers EAX, EBX, SP etc. Process Control Block (PCB) CPU Switch From Process to Process Process Control Block (PCB) CPU scheduling information Process priority, pointers to scheduling queues Memory-management information Information about base and limit registers (remember the Memory-protection chapter 2?), page tables etc. Accounting information Amount of CPU time, Time limits (CPU protection), process numbers etc I/O status information List of I/O devices being used by the process Processes Process Concept Process Scheduling Operations on Processes Cooperating Processes Interprocess Communication Communication in Client-Server Systems Process Scheduling Objective of multi-programming is to have some processes running at all the time Maximization of CPU utilization It is achieved by time sharing Process Scheduler is used to control the “flow” of processes Process Scheduling Queues Job queue – set of all processes in the system. Ready queue – set of all processes residing in main memory, ready and waiting to execute. –Linked List Device queues – set of processes waiting for an I/O device.—A device queue for each device Monitor, HDD, Floppy and other I/O devices have separate Queues. Process migrates between the various queues. Ready Queue And Various I/O Device Queues Head is to tell the start of the queue where as Tail is to facilitate the entry of new Process in the queue Representation of Process Queue Scheduling Resource Schedulers More processes are submitted than can be executed immediately -- spooling Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue. Might take Seconds to be activated Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU. Is happening in milliseconds Long term and Short term Schedulers The long term scheduler may need to be invoked only when a process leaves the system Degree of multiprogramming -- number of processes in memory Long Term scheduler New process Ready Queue Short Term Scheduler CPU Schedulers cont. I/O bound –Process that spends more of its time doing I/O than it spends doing computation CPU bound –Process that generates I/O requests infrequently, using more of its time doing computations. Long term scheduler should select a good process mix of I/O bound and CPU bound processes –balance in queues. Long term scheduler may be absent or minimal on some systems –(e.g. Time sharing systems (UNIX, Microsoft windows). Swapping – good to improve the process mix. Addition of Medium Term Scheduling Temporarily swaps out the process to reduce the degree of multiprogramming Schedulers (Cont.) Short-term scheduler is invoked very frequently (milliseconds) (must be fast). Long-term scheduler is invoked very infrequently (seconds, minutes) (may be slow). The long-term scheduler controls the degree of multiprogramming. Processes can be described as either: I/O-bound process – spends more time doing I/O than computations, many short CPU bursts. CPU-bound process – spends more time doing computations; few very long CPU bursts. Long-term scheduler must strike a balance between these two types of processes Current Trends… Long-term scheduler is absent in almost all the latest operating systems like Windows and Unix ; Due to time-sharing we don’t need to have long-term schedulers. Degree of multiprogramming is in user’s hands Greater the degree of multi-programming , slower the computer gets Context Switch When CPU switches to another process, the system must save the state of the old process and load the saved state for the new process. The context is represented in the PCB of the process Context-switch time is overhead; the system does no useful work while switching. Useful work? Useful with respect to users point of view… Time dependent on hardware support. Processes Process Concept Process Scheduling Operations on Processes Cooperating Processes Interprocess Communication Communication in Client-Server Systems Process Creation Parent process create children processes, which, in turn create other processes, forming a tree of processes. Resource sharing in parent and child processes ( 3 Types) Parent and children share all resources. Children share subset of parent’s resources. Parent and child share no resources. Execution Parent and children execute concurrently. Parent waits until children terminate. Process Creation (Cont.) Address space Child duplicate of parent. (Linux) Child has a program loaded into it. (Windows) UNIX / Linux examples fork system call creates new process exec system call used after a fork to replace the process’ memory space with a new program. Windows 2000/XP examples CreateProcess system call Several others also present to control the execution of process. Unix Process Implementation Parent creates the process using Fork() call Child is created, and is exact copy of parent Child does not execute till it is issued Exec() call As parent and child are same the distinction of being a parent or child is made by ProcessID number UNIX FORK and EXEC functions #include <sys/types.h> #include<stdio.h> #include<unistd.h> int main() { pid_t pid; //fork another process pid = fork(); If (pid < 0) { //error occurred fprintf(stderr, “Fork failed”); exit(-1); } else (if pid ==0) { execlp(“/bin/ls”,”ls”); } //ls is “Dir” equivalent of linux Else { wait(NULL); //wait till child process is complete printf(“Child Complete”); POSIX Exit(0): } resumes parent wait fork() child exit() exec() Win16 function WinExec() (Now based on CreateProcess() function of Win32) #include “windows.h” #include “iostream.h” int main(int argc, char* argv[]) { WinExec("Calc",NULL); return 0; } Documentation (ref. MSDN) Creates a new process and runs it in separate memory space Processes are created in Win32 API using CreateProcess() function CreateProcess() is similar to fork() fork() is passed no parameters, CreateProcess() expects no fewer than ten Address space difference --loading another program in child’s address space Process Termination Process executes last statement and asks the operating system to delete it (exit()). Following steps are performed on exit Output data from child to parent (via wait). Process’ resources are deallocated by operating system. Parent may terminate execution of children processes (abort ()). Why would parent do that? Child has exceeded allocated resources. Task assigned to child is no longer required. Parent is exiting. • Operating system does not allow child to continue if its parent terminates. • Cascading termination. Interprocess Communication (IPC) Mechanism for processes to communicate and to synchronize their actions. Message system – processes communicate with each other without resorting to shared variables. IPC facility provides two operations: send(message) – message size fixed or variable receive(message) If P and Q wish to communicate, they need to: establish a communication link between them exchange messages via send/receive Inter Process Communication Independent process cannot affect or be affected by the execution of another process. Cooperating process can affect or be affected by the execution of another process Advantages of process cooperation Information sharing Computation speed-up (breaking a task into subtasks) Modularity (Divide the system functions into separate processes or threads) Convenience (editing, printing and compiling at the same time) Outline Process Concept Process Scheduling Operations on Processes Process Creation Process Termination Shared Memory Systems Message Passing Systems Naming Synchronization Buffering Communication in Client-Server Systems Sockets Remote Procedure Calls Remote Method Invocation Examples of IPC Systems Shared memory systems Communicating processes need to establish a region of shared memory. Shared memory region resides in the address space of process creating SM seg. producer process produces information that is consumed by a consumer process. Compiler, assembler, loader Producer is Server and a Consumer is client We must have a buffer of items that can be filled by the producer and emptied by the consumer. Consumer shouldn’t try to consume that has not yet been produced Shared memory systems (2) unbounded-buffer places no practical limit on the size of the buffer. Consumer may have to wait; producer can always produce bounded-buffer assumes that there is a fixed buffer size. Consumer must wait if the buffer is empty; producer must wait if the buffer is full Shared buffer is implemented as a circular array with two logical pointers in and out Buffer is empty when in==out Bufffer is full when ((in+1)%BUFFER_SIZE)==out Shared memory systems (3) # define BUFFER_SIZE 10 item nextconsumed; typedef struct { while (true) { ….. while(in==out) }item; ;//do nothing Item buffer[BUFFER_SIZE]; nextconsumed = buffer[out]; int in =0; out=(out+1)%BUFFER_SIZE; int out=0; } item nextproduced; while (true){ while ((in +1)%BUFFER_SIZE)==out) /*do nothing*/ buffer[in] = nextproduced; in = (in +1)% BUFFER_SIZE; } Outline Process Concept Process Scheduling Operations on Processes Process Creation Process Termination Shared Memory Systems Message Passing Systems Naming Synchronization Buffering Communication in Client-Server Systems Sockets Remote Procedure Calls Remote Method Invocation Examples of IPC Systems Message Passing Systems Mechanism for processes to communicate and to synchronize their actions. Message system – processes communicate with each other without resorting to shared variables. send(message) – message size fixed or variable receive(message) If P and Q wish to communicate, they need to: establish a communication link between them exchange messages via send/receive Several methods for logically implementing send()/ receive() operations: Direct & indirect communication Synchronous or asynchronous communication Automatic or explicit buffering Lets do an example of Direct message passing #include <windows.h> Its simple to execute this program: Program asks for the window caption #include <iostream.h> Enter the caption e.g. Calculator void main(void) Calculator program must be running { This program will simply close the char str[100]; program cin >> str; /*this sleep() is not required. Just wanted to tell you another function that delays the execution. Basically this functions puts the current thread to sleep for specified amount of time in milliseconds */ Sleep(3000); //wait for 3 seconds.. //Return the handler to the window with the specified caption HWND hWnd = FindWindow(NULL, str); //Send message to window with handle hWnd and ask it to close SendMessage(hWnd,WM_CLOSE,0,0L); //Guess what would happen if SetWindowText(hWnd,"Calc2000"); is used //instead of SendMessage } Direct Communication Processes must name each other explicitly: send (P, message) – send a message to process P receive(Q, message) – receive a message from process Q Properties of communication link Links are established automatically. A link is associated with exactly one pair of communicating processes. Between each pair there exists exactly one link. The link may be unidirectional, but is usually bi-directional. In MS Windows examples of such communication enabling functions (system calls) are SendMessage(…) //Function used to send message to other window PostMessage(…) //posts a message to a queue Hard-Coding ----disadvantage Indirect Communication (1) Messages are directed and received from mailboxes (also referred to as ports). Each mailbox has a unique id. Processes can communicate only if they share a mailbox. Properties of communication link Link established only if processes share a common mailbox A link may be associated with many processes. Each pair of processes may share several communication links. Link may be unidirectional or bi-directional. Indirect Communication (2) Operations create a new mailbox send and receive messages through mailbox destroy a mailbox Primitives are defined as: send(A, message) – send a message to mailbox A receive(A, message) – receive a message from mailbox A POSIX message queues use an integer value to identify a mailbox Indirect Communication (3) Mailbox sharing P1, P2, and P3 share mailbox A. P1, sends; P2 and P3 receive. Who gets the message? Solutions Allow a link to be associated with at most two processes. Allow only one process at a time to execute a receive operation. Allow the system to select arbitrarily the receiver. Sender is notified who the receiver was. Indirect Communication (4) Mailbox may be owned by a process or OS Owner – Process Owner –who can only receive messages through mailbox User –Who can only send messages to the mailbox No confusion about who should receive a message sent to this mailbox Owner –OS mailbox is independent –not attached with any particular process OS must provide a mechanism crate a new mailbox, send and receive messages, delete ownership and receiving privilege may be passed to other processes –multiple receivers for each mailbox. MS Windows API for Inter process Indirect communications MailSlots A mailslot is a mechanism for one-way interprocess communications (IPC). A Win32-based application can store messages in a mailslot. The owner of the mailslot can retrieve messages that are stored there. Good for a process broadcasting its message to many processes. CreateMailSlot ( … ) ReadFile( … ) WriteFile ( … ) Must be synchronized to some extent. The creater can read or write the mail slot but the client can only send data. Not like the description of previous slide void main(void) { Mailslot Server HANDLE hMS; hf ; BOOL ret=1; Process that receives messages DWORD bWritten, b, c, d, e; (Server.cpp) char a[10]; //lets create mail slot (kind of mail server) hMS= CreateMailslot("\\\\.\\mailslot\\my_mailslot",0,MAILSLOT_WAIT_FOREVER,NULL); if (hMS == INVALID_HANDLE_VALUE) { cout << "Failed to create mailslot" << endl; } /*open abstract file (this file is not in hard disk but in RAM). we will read or write to this file. NOTE: Client cannot read this file, only thing client can do is to write to this file.. just like email.. sender cannot view email on your end */ hf = CreateFile("\\\\.\\mailslot\\my_mailslot",GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ | FILE_SHARE_WRITE,NULL,OPEN_EXISTING,NULL,NULL); /*dummy wait for another process to send any message. In real life a process might be involved with any other computations and still other process can post mail message to it which this process can read upon return... when you will do some visual programming (event driven) this will become more clear.*/ cin >> a; //just to show you another API call //Read the documentation to be clear about it! :) GetMailslotInfo(hMS,&b,&c,&d,&e); a[0] = '\0'; //initalize the array ret = ReadFile(hMS,&a[0],c,&bRead,(LPOVERLAPPED)NULL); cout << a; GetMailslotInfo(hMS,&b,&c,&d,&e); ret = ReadFile(hMS,&a[0],c,&bRead,(LPOVERLAPPED)NULL); cout << a; cin >> a; //dummy wait again! } Mailslot Client #include <iostream.h> Process that receives messages #include <windows.h> (Client.cpp) void main(void) { HANDLE hfile; hfile = CreateFile("\\\\.\\mailslot\\my_mailslot",GENERIC_READ | GENERIC_WRITE, FILE_SHARE_READ | FILE_SHARE_WRITE,NULL,OPEN_EXISTING,NULL,NULL); DWORD bWritten; char a[100]; cout << endl <<"Enter message to send to other process: "; cin >> a; BOOL ret = WriteFile(hfile,a,strlen(a), &bWritten,NULL); //variable length ret = WriteFile(hfile,"OS!",3, &bWritten,NULL);//hardcoded } Create client.cpp and server.cpp. Compiler them and execute them separately First execute server.exe. Don’t do any thing with this executable Execute client.cpp and enter a message less than 10 chars Then continue the server.exe my pressing enter key! MS Windows API for Inter process Indirect communications Named pipes Protocol used to transfer data between processes, usually across a network. Typically, a process creates a named pipe (conceptually, a conduit) with a well-known name this process is the server process. Processes that can get the name of the pipe can open the other end of the pipe and can exchange data by performing read and write operations on the pipe. Outline Process Concept Process Scheduling Operations on Processes Process Creation Process Termination Shared Memory Systems Message Passing Systems Naming Synchronization Buffering Communication in Client-Server Systems Sockets Remote Procedure Calls Remote Method Invocation Examples of IPC Systems Synchronization Message passing may be either blocking or nonblocking. Blocking is considered synchronous Non-blocking is considered asynchronous Blocking send: The sender process is blocked until the message is received by the receiving process or by the mail box. Nonblocking send, Blocking receive, Nonblocking receive Producer – consumer problem ? Buffering Queue of messages attached to the link; implemented in one of three ways. 1. Zero capacity – 0 messages Sender must wait for receiver (rendezvous). 2. Bounded capacity – finite length of n messages Sender must wait if link full. 3. Unbounded capacity – infinite length Sender never waits. Outline Process Concept Process Scheduling Operations on Processes Process Creation Process Termination Shared Memory Systems Message Passing Systems Naming Synchronization Buffering Communication in Client-Server Systems Sockets Remote Procedure Calls Remote Method Invocation Examples of IPC Systems --Self study Client-Server Communication Sockets Remote Procedure Calls Remote Method Invocation (Java) Sockets BICSE (not to be covered as it is part of your C language course) A socket is defined as an endpoint for communication. Concatenation of IP address and port The socket 161.25.19.8:1625 refers to port 1625 on host 161.25.19.8 Communication consists between a pair of sockets. No programming practice for sockets because you will do it in either C or in Advanced Java Socket Communication Following code creates a process (e.g. Calculator) and executes it as a child process. WaitForSingleObject() halts the execution of parent process till the time child process is finished. #include <iostream.h> #include <windows.h> void main(void) { STARTUPINFO si; Reference MSDN PROCESS_INFORMATION pi; //create process wants this structure to be initilized ZeroMemory( &si, sizeof(si) ); //fills the process with all zero’s si.cb = sizeof(si); // Start the child process. Returns false if child process failed to execute if( !CreateProcess( NULL, // No module name (use command line). "calc", // Command line. NULL, // Process handle not inheritable. NULL, // Thread handle not inheritable. FALSE, // Set handle inheritance to FALSE. 0, // No creation flags. NULL, // Use parent's environment block. NULL, // Use parent's starting directory. &si, // Pointer to STARTUPINFO structure. &pi ) // Pointer to PROCESS_INFORMATION structure. ) { cout << "CreateProcess failed." ; } cout << "Child is running with parent on timeshared bases" << endl; cout <<"Now i am going to block the parent till child exits" << endl; // Wait till child process exits. WaitForSingleObject( pi.hProcess, INFINITE ); cout << "Child process exited" << endl; How to create child process in Win32 using CreateProcess() Do Read Documentation of CreateProcess() STARTUPINFO PROCESS_INFORMATION WaitForSingleObject() #include <Windows.h> #include <iostream.h> int main(void) { DWORD dwThreadId, dwThrdParam = 1; HANDLE hThread; hThread = CreateThread( NULL, // no security attributes 0, // use default stack size ThreadFunc, // thread function – The function that executes as a thread &dwThrdParam, // argument to thread function 0, // use default creation flags &dwThreadId); // returns the thread identifier // Check the return value for success. if (hThread == NULL) { cout << " Thread creation failed" << endl; return 0; } int x=0; while ( x < 50) Creating a Thread in { MS Platforms cout << "i am parent ---" << x << endl; Thread executes as a x ++; } Separate process CloseHandle( hThread ); } DWORD WINAPI ThreadFunc( LPVOID lpParam ) { char szMsg[80]; unsigned int x; x = 0; while (x < 50) { cout << x << " --- Thread is being executed" << endl; x++; } return 0; } This function runs as a concurrent thread. Output: The output may vary from computer to computer PARENT CHILD int main() { //fork another process Int pid = fork(); If (pid < 0) { //error occurred fprintf(stderr, “Fork failed”); exit(-1); } else (if pid ==0) { execlp(“/bin/ls”,”ls”); } Else { wait(NULL); //wait till child process is //complete printf(“Child Complete”); Exit(0): } int main() { //fork another process Int pid = fork(); If (pid < 0) { fprintf(stderr, “Fork failed”); exit(-1); } else (if pid ==0) { execlp(“/bin/ls”,”ls”); } Else { wait(NULL); printf(“Child Complete”); Exit(0): } NOTE: Same code runs for both child and parent (Identified by pid returned by fork() Interprocess Communication (IPC) Mechanism for processes to communicate and to synchronize their actions. Message system – processes communicate with each other without resorting to shared variables. IPC facility provides two operations: send(message) – message size fixed or variable receive(message) If P and Q wish to communicate, they need to: establish a communication link between them exchange messages via send/receive Direct Communication Processes must name each other explicitly: send (P, message) – send a message to process P receive(Q, message) – receive a message from process Q Properties of communication link Links are established automatically. A link is associated with exactly one pair of communicating processes. Between each pair there exists exactly one link. The link may be unidirectional, but is usually bi-directional. In MS Windows examples of such communication enabling functions (system calls) are SendMessage(…) //Function used to send message to other window PostMessage(…) //posts a message to a queue Lets do an example of Direct message passing #include <windows.h> Its simple to execute this program: Program asks for the window caption #include <iostream.h> Enter the caption e.g. Calculator void main(void) Calculator program must be running { This program will simply close the char str[100]; program cin >> str; /*this sleep() is not required. Just wanted to tell you another function that delays the execution. Basically this functions puts the current thread to sleep for specified amount of time in milliseconds */ Sleep(3000); //wait for 3 seconds.. //Return the handler to the window with the specified caption HWND hWnd = FindWindow(NULL, str); //Send message to window with handle hWnd and ask it to close SendMessage(hWnd,WM_CLOSE,0,0L); //Guess what would happen if SetWindowText(hWnd,"Calc2000"); is used //instead of SendMessage } Interesting Program Read the documentation carefully (ref msdn) and guess what is this program doing! A QUIZ MIGHT BE COMING YOUR WAY!!!!!!!!!!!!!!!!!! #include <windows.h> #include <iostream.h> void main(void) { FILETIME a,b,c,d; HANDLE h =OpenProcess(PROCESS_ALL_ACCESS,TRUE,1920); GetProcessTimes(h,&a,&b,&c,&d); //READ THE DOCUMENTATION SYSTEMTIME aa,bb,cc,dd; FileTimeToSystemTime(&c,&cc); cout << cc.wMilliseconds <<endl; } Indirect Communication Messages are directed and received from mailboxes (also referred to as ports). Each mailbox has a unique id. Processes can communicate only if they share a mailbox. Properties of communication link Link established only if processes share a common mailbox A link may be associated with many processes. Each pair of processes may share several communication links. Link may be unidirectional or bi-directional. Indirect Communication Operations create a new mailbox send and receive messages through mailbox destroy a mailbox Primitives are defined as: send(A, message) – send a message to mailbox A receive(A, message) – receive a message from mailbox A Indirect Communication Mailbox sharing P1, P2, and P3 share mailbox A. P1, sends; P2 and P3 receive. Who gets the message? Solutions Allow a link to be associated with at most two processes. Allow only one process at a time to execute a receive operation. Allow the system to select arbitrarily the receiver. Sender is notified who the receiver was.