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Chapter 3: ProcessesChapter 3: Processes
3.2 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Chapter 3: ProcessesChapter 3: Processes
Process Concept
Process Scheduling
Operations on Processes
Cooperating Processes
Interprocess Communication
Communication in Client-Server Systems
3.3 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Process ConceptProcess 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 execution must progress in sequential fashion
A process includes:
program counter which represents current activity
Stack which holds temporary data such as function parameters, local variables and return addresses.
data section which contains global variables
Heap which contains memory that is dynamically allocated during process runtime.
Text includes the application's code, might be shared by a number of processes?
The process's address space.
3.4 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Process in MemoryProcess in Memory
3.5 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Process StateProcess State
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
3.6 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Diagram of Process StateDiagram of Process State
3.7 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Process Control Block (PCB)Process Control Block (PCB)
Information associated with each process
Process state
Program counter indicates the address of next instruction.
CPU registers
CPU scheduling information includes process priority and pointers to scheduling queues.
Memory-management information includes value of limit and base registers
Accounting information includes amount of CPU time used, time limits etc.
I/O status information includes the list of I/O devices allocated to processes, list of open files
3.8 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Process Control Block (PCB)Process Control Block (PCB)
3.9 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
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PCBPCB
Kernel User
text
data
heap
stack
PSW
IR
PC
SP
General Purpose Registers
.
RAM CPU
StateMemoryFilesAccountingPriorityUserCPU register storage
OS Data Structures
Process Address Space
3.10 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
CPU Switch From Process to ProcessCPU Switch From Process to Process
3.11 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Process Scheduling QueuesProcess 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.
It is stored as a linked list. It contains pointers to the first and final PCB’s.
Each PCB includes a pointer field that points to the next PCB in the
ready queue.
Device queues – set of processes waiting for an I/O device
Processes migrate among the various queues
3.12 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Ready Queue And Various I/O Device QueuesReady Queue And Various I/O Device Queues
3.13 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Representation of Process SchedulingRepresentation of Process Scheduling
3.14 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
SchedulersSchedulers
Long-term scheduler (or job scheduler) – selects which processes should be brought into the ready queue
Short-term scheduler (or CPU scheduler) – selects which process should be executed next and allocates CPU
Medium term Scheduler (Swaper)
3.15 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Addition of Medium Term SchedulingAddition of Medium Term Scheduling
3.16 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Schedulers (Cont.)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
If all processes are I/O bound, The ready queue will almost always be empty
If all processes are CPU bound, The I/O waiting queue will almost always be empty, devices will
go unused System will again be unbalanced
3.17 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Context SwitchContext 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
Context-switch time is overhead; the system does no useful work while switching
Time dependent on hardware support
Part of OS responsible for switching the processor among the processes is
called Dispatcher
3.18 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
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Two-state process modelTwo-state process model
EnterQueue
Dispatch
Pause
ExitProcessor
Dispatcher is now redefined:• Moves processes to the waiting queue• Remove completed/aborted processes• Select the next process to run
3.19 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
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a := 1 b := a + 1 c := b + 1 read a file a := b - c c := c * b b := 0
a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0
a := 1 read a file b := a + 1 c := b + 1 a := b - c c := c * b b := 0
Ready Ready Ready
How process state changes1How process state changes1
3.20 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
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a := 1 b := a + 1 c := b + 1 read a file a := b - c c := c * b b := 0
a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0
a := 1 read a file b := a + 1 c := b + 1 a := b - c c := c * b b := 0
Running Ready Ready
a := 1 b := a + 1 c := b + 1 read a file a := b - c c := c * b b := 0
a := 1 b := a + 1 c := b + 1 read a file a := b - c c := c * b b := 0
TimeoutTimeout
How process state changes2How process state changes2
3.21 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
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a := 1 b := a + 1 c := b + 1 read a file a := b - c c := c * b b := 0
a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0
a := 1 read a file b := a + 1 c := b + 1 a := b - c c := c * b b := 0
Ready Running Ready
a := 1 read a file b := a + 1 c := b + 1 a := b - c c := c * b b := 0
I/OI/O
How process state changes3How process state changes3
3.22 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
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a := 1 read a file b := a + 1 c := b + 1 a := b - c c := c * b b := 0
a := 1 b := a + 1 c := b + 1 read a file a := b - c c := c * b b := 0
a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0
Ready Blocked Running
TimeoutTimeout
a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0
a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0
How process state changes4How process state changes4
3.23 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
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a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0
a := 1 read a file b := a + 1 c := b + 1 a := b - c c := c * b b := 0
a := 1 b := a + 1 c := b + 1 read a file a := b - c c := c * b b := 0
Running Blocked Ready
I/OI/O
How process state changes5How process state changes5
3.24 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
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a := 1 read a file b := a + 1 c := b + 1 a := b - c c := c * b b := 0
a := 1 b := a + 1 c := b + 1 read a file a := b - c c := c * b b := 0
Blocked Blocked Running
a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0
How process state changes6How process state changes6
The Next Process to Run cannot be simply selected from the front
3.25 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
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a := 1 read a file b := a + 1 c := b + 1 a := b - c c := c * b b := 0
a := 1 b := a + 1 c := b + 1 read a file a := b - c c := c * b b := 0
Blocked Blocked Running
a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0
a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0
Suppose the Green process finishes I/OSuppose the Green process finishes I/O
How process state changes7How process state changes7
3.26 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
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a := 1 read a file b := a + 1 c := b + 1 a := b - c c := c * b b := 0
a := 1 b := a + 1 c := b + 1 read a file a := b - c c := c * b b := 0
Blocked Ready Running
a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0
a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0
a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0
TimeoutTimeout
How process state changes8How process state changes8
3.27 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
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a := 1 b := a + 1 c := b + 1 a := b - c c := c * b b := 0 c := 0
a := 1 b := a + 1 c := b + 1 read a file a := b - c c := c * b b := 0
Blocked Running Ready
a := 1 read a file b := a + 1 c := b + 1 a := b - c c := c * b b := 0
a := 1 read a file b := a + 1 c := b + 1 a := b - c c := c * b b := 0
a := 1 read a file b := a + 1 c := b + 1 a := b - c c := c * b b := 0
TimeoutTimeout
How process state changes9How process state changes9
3.28 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Process CreationProcess Creation
Reasons to create a process Submit a new batch job/Start program
User logs on to the system
OS creates on behalf of a user (printing)
Spawned by existing process
Parent process create children processes, which, in turn create other processes, forming a tree of processes
Resource sharing
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
3.29 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Process Creation (Cont.)Process Creation (Cont.)
Address space
Child duplicate of parent
Child has a program loaded into it
UNIX examples
fork system call creates new process
exec system call used after a fork to replace the process’ memory space with a new program
3.30 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Process CreationProcess Creation
3.31 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
C Program Forking Separate ProcessC Program Forking Separate Process
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) { /* child process */
execlp("/bin/ls", "ls", NULL);}else { /* parent process */
/* parent will wait for the child to complete */
wait (NULL);printf ("Child Complete");exit(0);
}}
3.32 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
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The fork() system callThe fork() system call
At the end of the system call there is a new process waiting to run once the scheduler chooses it
A new data structure is allocated
The new process is called the child process.
The existing process is called the parent process.
The parent gets the child’s pid returned to it.
The child gets 0 returned to it.
Both parent and child execute at the same point after fork() returns
3.33 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
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Unix Process ControlUnix Process Control
int pid;int status = 0;
if (pid = fork()) {/* parent */…… pid = wait(&status);
} else {/* child */……exit(status);
}
Parent uses wait to sleep until the child exits; wait returns child pid and status.
Wait variants allow wait on a specific child, or notification of stops and other signals
Parent uses wait to sleep until the child exits; wait returns child pid and status.
Wait variants allow wait on a specific child, or notification of stops and other signals
Child process passes status back to parent on exit, to report success/failure
Child process passes status back to parent on exit, to report success/failure
The fork syscall returns a zero to the child and the child process ID to the parent
The fork syscall returns a zero to the child and the child process ID to the parent
Fork creates an exact copy of the parent process
Fork creates an exact copy of the parent process
3.34 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
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Process TerminationProcess Termination
Batch job issues Halt instruction User logs off Process executes a service request to terminate Parent terminates so child processes terminate Operating system intervention
such as when deadlock occurs Error and fault conditions
E.g. memory unavailable, protection error, arithmetic error, I/O failure, invalid instruction
3.35 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Process TerminationProcess Termination
Process executes last statement and asks the operating system to delete it (exit)
Output data from child to parent (via wait)
Process’ resources are deallocated by operating system
Parent may terminate execution of children processes (abort)
Child has exceeded allocated resources
Task assigned to child is no longer required
If parent is exiting
Some operating system do not allow child to continue if its parent terminates
– All children terminated - cascading termination
3.36 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Inter process CommunicationInter process Communication
Processes executing concurrently in the operating system may be either independent processes or cooperating processes.
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
3.37 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Advantages of Process CooperationAdvantages of Process Cooperation
Information sharing: Allow concurrent access to same piece of information that several users may be interested in it.
Computation speed-up: break a task to subtasks, each of which will be executing in parallel with the others. (Should be multiple processing elements such as CPUs)
Modularity: divided the system functions into separate processes or threads.
Convenience: for user, user may work on many tasks at the same time.
3.38 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
MODELS OF IPCMODELS OF IPC
Cooperating processes require an inter process communication mechanism that will allow them to exchange data and information.
1. Shared Memory
2. Message Passing
3.39 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
SHARED MEMORYSHARED MEMORY
1. Region of memory that is shared by cooperating processes is established.
2. Processes can then exchange information by reading and writing data to shared region
3.40 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
MESSAGE PASSINGMESSAGE PASSING
Communication takes place by means of messages exchanged between the cooperating processes.
3.41 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Communications Models Communications Models
3.42 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Advantages & Disadvantages of Message Advantages & Disadvantages of Message Passing & Shared MemoryPassing & Shared Memory
1. Message passing is useful for exchanging smaller amounts of data
2. Message Passing is easier to implement than shared memory for IPC
3. Shared Memory allows maximum speed and convenience as it can be done at memory speeds within a computer
4. Shared memory is faster than message passing as message passing is implemented using system calls.
3.43 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Contd….Contd….
1. Message passing requires the more time consuming task of kernel intervention.
2. Shared memory system calls are required only to establish shared memory regions, no assistance from the kernel is required.
3.44 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Producer-Consumer ProblemProducer-Consumer Problem
Paradigm for cooperating processes, producer process produces information that is consumed by a consumer process
A compiler may produce assembly code, which is consumed by an assembler. The assembler, in turn, may produce object modules, which are consumed by the loader
A buffer of items that can be filled by the producer and emptied by the consumer. should be available
A producer can produce one item while the consumer is consuming another item.
The producer and consumer must be synchronized
the consumer does not try to consume an item that has not yet been produced.
the consumer must wait until an item is produced
3.45 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Producer-Consumer ProblemProducer-Consumer Problem
unbounded-buffer places no practical limit on the size of the
buffer
the consumer may have to wait for new items, but
the producer can always produce new items
bounded-buffer assumes that there is a fixed buffer size
the consumer must wait if the buffer is empty, and
the producer must wait if the buffer is full.
3.46 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Bounded-Buffer – Shared-Memory SolutionBounded-Buffer – Shared-Memory Solution
Shared data
#define BUFFER_SIZE 10
Typedef struct {
. . .
} item;
item buffer[BUFFER_SIZE];
int in = 0; % in: the next free position in the buffer
int out = 0; % out: the first full position in the buffer The buffer is empty when in == out ;
The buffer is full when ((in + 1) % BUFFERSIZE) == out
Solution is correct, but can only use BUFFER_SIZE-1 elements
3.47 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Bounded-Buffer – Insert() MethodBounded-Buffer – Insert() Method
The producer process has a local variable nextproduced in which the new item to be produced is stored:
while (true) { /* Produce an item */
while (((in = (in + 1) % BUFFER SIZE count) == out)
; /* do nothing -- no free buffers */
buffer[in] = item;
in = (in + 1) % BUFFER SIZE;
{
3.48 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Bounded Buffer – Remove() MethodBounded Buffer – Remove() Method
while (true) {
while (in == out)
; // do nothing -- nothing to consume
// remove an item from the buffer
item = buffer[out];
out = (out + 1) % BUFFER SIZE;
return item;
{
The consumer process has a local variable nextconsumed in which the item to be consumed is stored:
3.49 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Message PassingMessage Passing
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
Implementation of communication link physical (e.g., shared memory, hardware bus) logical (e.g., logical properties) e.g. send(), receive()
3.50 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Implementation QuestionsImplementation Questions
How are links established?
Can a link be associated with more than two processes?
How many links can there be between every pair of communicating processes?
What is the capacity of a link?
Is the size of a message that the link can accommodate fixed or variable?
Is a link unidirectional or bi-directional?
3.51 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Methods for logical implementation of linkMethods for logical implementation of link
1. Direct or Indirect Communication
2. Synchronous and Asynchronous Communication
3. Automatic or explicit Buffering
3.52 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Direct CommunicationDirect 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 because the processes need to know only the identities to communicate.
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
3.53 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Indirect CommunicationIndirect 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
3.54 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Indirect CommunicationIndirect 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
3.55 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Indirect CommunicationIndirect 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.
3.56 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Indirect CommunicationIndirect Communication
If the mailbox is owned by a process (that is, the mailbox is
part of the address space of the process), then we distinguish between the owner (who can only receive
messages through this mailbox) and the user (who can only send messages to the mailbox).
Since each mailbox has a unique owner, there can be
no confusion about who should receive a message sent to this mailbox.
When a process that owns a mailbox terminates, the mailbox disappears.
Any process that subsequently sends a message to this mailbox
must be notified that the mailbox no longer exists.
3.57 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Indirect CommunicationIndirect Communication
If a mailbox owned by the operating system is independent and is not attached to any particular process: The operating system then must provide a mechanism
that allows a process to do the following:
Create a new mailbox.
Send and receive messages through the mailbox.
Delete a mailbox
Note: the ownership and receive privilege may be passed to other processes through appropriate system calls.
3.58 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
SynchronizationSynchronization
Message passing may be either blocking or non-blocking
Blocking is considered synchronous
Blocking send has the sender block until the message is received
Blocking receive has the receiver block until a message is available
Non-blocking is considered asynchronous
Non-blocking send has the sender send the message and continue
Non-blocking receive has the receiver receive a valid message or null
3.59 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
BufferingBuffering
Queue of messages attached to the link; implemented in one of three ways
1.Zero capacity – 0 messagesSender must wait for receiver (rendezvous)
2.Bounded capacity – finite length of n messagesSender must wait if link full
3.Unbounded capacity – infinite length Sender never waits
3.60 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Client-Server CommunicationClient-Server Communication
Sockets
Remote Procedure Calls
Remote Method Invocation (Java)
3.61 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
SocketsSockets
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
3.62 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Socket CommunicationSocket Communication
3.63 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Remote Procedure CallsRemote Procedure Calls
Remote procedure call (RPC) abstracts procedure calls between processes on networked systems.
Stubs – client-side proxy for the actual procedure on the server.
The client-side stub locates the server and marshalls the parameters.
The server-side stub receives this message, unpacks the marshalled parameters, and performs the procedure on the server.
A stub is a small program routine that substitutes for a longer program, possibly to be loaded later or that is located remotely. For example, a program that uses Remote Procedure Calls ( RPC ) is compiled with stubs that substitute for the program that provides a requested procedure. The stub accepts the request and then forwards it (through another program) to the remote procedure. When that procedure has completed its service, it returns the results or other status to the stub which passes it back to the program that made the request.
3.64 Silberschatz, Galvin and Gagne ©2005Operating System Concepts
Execution of RPCExecution of RPC
End of Chapter 3End of Chapter 3