COMP 3100 Operating Systems

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1 Programming Interface» A process is an instance of a running program. COMP 3100 Operating Systems» Functionality that an OS provides to applications» Process Management» Input/Output Week 3 Processes and Threads Process Management» System call interface for process operations» Process Creation» Process Termination Process Creation» A running process may create a new process via a system call.» The process creator is called the parent process» The process being created is called the child process» The OS manages these processes using the process identifier (pid) which is an integer number.

2 Process Management (Windows)» Approach to process management:» System call to create a process» System calls for other process operations» Simple in theory, complex in practice.» The parent process controls the privileges, scheduling priorities, file contents, and I/O for the program Process Creation - Windows In Windows, there is a routine called CreateProcess boolean CreateProcess (char *prog, char *args)» This routine accepts 10 parameters which are used to set up the process context.» Some specify the program and its arguments» Some deal with the process runtime environment Process Management- Windows In Windows, the kernel needs to:» Create and initialise the process control block (PCB) in the kernel.» Create and initialise a new address space.» Load the program prog into the address space.» Copy arguments args into memory in the address space.» Initialise the hardware context to start execution at start.» Inform the scheduler that the new process is ready to run. Process Management (UNIX)» Approach to process management:» UNIX splits CreateProcess into two steps: fork exec» Complex in theory, simple in practice.

3 Process Creation (UNIX)» UNIX fork» Creates a complete copy of the parent process into the child process.» The child process sets up privileges, priorities, and I/O for the program that is about to be started.» This sets up the context for the process.» fork takes no arguments, and returns an integer.» UNIX fork UNIX fork» returns once in the parent process with the child s process ID.» returns once in the child with 0 if the process was successfully created.» getpid() can be used to access the process ID of current process UNIX fork The steps for implementing UNIX fork in the kernel are:» Create and initialise the process control block (PCB) in the kernel.» Create a new address space.» Initialise the address space with a copy of the entire contents of the address space of the parent.» Inherit the execution context of the parent (i.e. any open files)» Inform the scheduler that the new process is ready to run. Process Creation (UNIX)» UNIX exec» Brings the new executable image into memory and starts running it.» Takes two arguments» The name of the program to run.» An array of arguments to pass to the program.

4 UNIX exec The parent process may need to pause until the child process completes. The wait system call pauses the parent until the child finishes, crashes, or is terminated.» wait is parametized with the process ID of the child since a parent process could create many child processes.» Optional call in UNIX. Example 1 int child_pid = fork(); if (child_pid == 0){ //I am the child process printf( I am process #%d\n, getpid()); return 0; } else{ //I am the parent process printf ( I am the parent of process #%d\n, child_pid ); return 0; } Possible output: I am the parent of process 495 I am process 495 Example 1 UNIX signal int child_pid = fork(); if (child_pid == 0){ //I am the child process printf( I am process #%d\n, getpid()); return 0; } else{ //I am the parent process printf ( I am the parent of process #%d\n, child_pid ); return 0; } UNIX allows one process to send another process an instant notification, or upcall. In UNIX, this notification is sent by calling signal. Signals are used for terminating an application, suspending it temporarily or a host of other reasons. Possible output (less likely): I am process 495 I am the parent of process 495

5 Process Termination» A process terminates when it finishes executing its final statement, and it asks the OS to delete it by using the exit system call.» All of the resources of a terminated process are deallocated by the OS. E.g. memory, open files, I/ O buffers. Process Termination» When a child process terminates, it may return a status value (integer) to its parent process.» If a parent process terminates, then all of its children must be terminated. This is called a cascading termination.» The PCB (process control block) in Windows implementation is only reclaimed when both the parent and child processes finish. Process Termination» A parent may terminate execution of one of its children because:» A child process has exceeded its usage of allocated resources» A task assigned to a child process is no longer required.» The parent process is exiting, and the OS does not allow a child process to continue if its parent terminates. Input/Output (I/O)» There is a wide diversity of input and output devices: keyboard, mouse, disk, camera etc.» The API for each device is specialised and customised to the device s characteristics.» Specialising an interface to a device has a significant downside:» Every new type of hardware device will require the system call interface of the OS to be upgraded to handle that device.

6 UNIX I/O Interface The UNIX I/O interface regularised all device input and output to a single common interface.» This interface was used for» All device input and output» Reading and writing to files» Interprocess communication UNIX I/O Interface The basic ideas in the UNIX I/O interface are:» Uniformity Use same set of system calls: open, close, read, write» Open before use» Returns a file descriptor for use in other system calls» Byte-oriented» Devices accessed via byte arrays» Kernel-buffered reads and writes» Decouples the application from the device» Explicit close» Garbage collect any unused kernel memory File Descriptor A file descriptor is a handle (abstract reference) to a device, file, or communication channel. It is used to identify a data source for subsequent system calls such as read, write and close. UNIX I/O Interface For interprocess communication: 1. Pipes 2. Replace file descriptor 3. Wait for multiple reads

7 Pipe A pipe is a temporary kernel buffer connecting a process producing data with a process consuming the data. It requires two file descriptors. One for writing to put data into the pipe. One for reading to pull data out of the pipe. Pipe» Data is read in the exact sequence it is written.» Data is buffered in the pipe» Processes sharing data can be decoupled» Reduces waiting» The pipe terminates when either endpoint closes the pipe or terminates. Replace File Descriptor Wait for Multiple Reads» A child process can read from or write to a file or pipe rather than interacting directly with an actual device.» The child process is therefore unaware of the source/destination of its I/O.» The file descriptors of the child process are manipulated using a special system call: dup2(from, to) For client-server computing, a server may have a pipe open to multiple client processes.» Read will block if there is no data to be read. Inefficient for the server to poll each pipe.» UNIX system call allows the server to wait for input from any of a set of file descriptors: select(fd[ ], number)

8 Interprocess Communication Interprocess Communication Various programs and users may need to communicate with each other. This is usually handled with system calls. A kernel upcall is needed if a process generates an event that needs the instant attention of another event. Constructing a complex application from simpler modules is a common practice. For this to work, processes need to communicate with each other. Three widely used forms of interprocess communication are: 1. Producer-consumer 2. Client-server (On the same machine) (On the same machine/distributed computing) 3. File system (Distributed Computing) Producer-Consumer Producer-Consumer» Programs are constructed to accept the output of other programs as input. 5 3» Communication is 1-way: The producer only writes data The consumer only reads data Can be implemented using pipes. 2 Interprocess communication between a producer application and a consumer. The producer uses the write system call to put data into the buffer, the consumer uses the read system call to take data out of the buffer.

9 Client-Server Communication Client-Server Communication» Communication is 2-way: Clients send requests to the server. These are written into one pipe. Server reads a request from the first pipe, executes the request, and writes the response onto a second pipe. Interprocess communication between a client process and a server process. Two separate buffers are used for requests. Shared Memory Model» Interprocess communication using shared memory requires communicating processes to establish a region of shared memory shmget()» A shared memory region resides in the address space of the process creating the shared memory segment.» Other processes that wish to communicate using this shared memory segment must attach it to their address space: shmat()» A region of memory shared by cooperating processes is established. Processes can then exchange information by reading and writing data from/to the shared region. Shared Memory - System Calls» int shmget(key_t key, size_t size, int shmflag)» void *shmat(int shmid, const void *shmaddr, int shmflg)» int shmdt (const void *shmaddr)» int shmctl (int shmid, int cmd, struct shmid_dis * buf)

10 Message Passing Model Message Passing - System Calls» Message passing provides mechanism to allow processes to communicate and to synchronize their actions without sharing the same address space and is particularly useful in a distributed environment,where communicating processes may reside on different computers connected by a network.» Message-passing provides at least two operations send(message) and receive(message)» int msgget(key_t key,int msgflg);» int msgsnd(int msqid,const void *msgp,size_t msgsz,int msgflg);» ssize_tmsgrcv(int msqid,void *msgp,size_t msgsz,long msgtyp,int msgflg);» int msgctl(int msqid,int cmd,struct msqid_ds *buf);

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