Programming Interface
Main Points
• Process Concept
• Creating and managing processes
– fork, exec, wait
• Communicating between processes
• Example: implementing a shell
Objectives
• To introduce the notion of a process -a program in execution, which forms
the basis of all computation
• To describe the various features of
processes, including scheduling,
creation and termination, and
communication
Process vs. Program
• Review:
– Program
• Process is an instance of an executing program.
• Relationship:
– Program can create many processes
– Many processes can be running the same program
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
Operating System View of Process
• At start of process, the Operating System’s
duties include:
– Load program into memory
– Allocate memory for program data
– Set up kernel bookkeeping:

Process ID

Priority

User IDs

Process State

Address of executing
instruction

Address of return instruction
Typical Process Layout
• Libraries provide the
glue between user
processes and the OS
– libc linked in with all C
programs
– Provides printf, malloc,
and a whole slew of
other routines necessary
for programs
Activation Records
Stack
OBJECT1
OBJECT2
Heap
HELLO WORLD
GO BIG RED CS!
Data
printf(char * fmt, …) {
create the string to be printed
SYSCALL 80
}
malloc() { … }
strcmp() { … }
Library
Text
main() {
printf (“HELLO WORLD”);
printf(“GO BIG RED CS”);
!
Program
Stack and Stack Frames
square (int x)
{ return x*x }
doCalc(int val)
{
printf (“square is %d\n”,
square(val) }
main (int x, int y)
{ key = 9999
doCalc (key) }
STACK
Frames for C run-time start up
functions
Frame for main
Frame for doCalc
Frame for square
Process in Memory
Full System Layout
• The OS is omnipresent and
steps in where necessary to
aid application execution
Kernel Activation Records
USER OBJECT1
OBJECT2
LINUX
syscall_entry_point() { … }
– Typically resides in high
memory
• When an application needs
to perform a privileged
operation, it needs to
invoke the OS
OS Stack
OS Heap
OS Data
OS Text
Activation Records
OBJECT1
OBJECT2
HELLO WORLD
GO BIG RED CS!
printf(char * fmt, …) {
Stack
Heap
Data
Library
main() { … }
Program
Process 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
Diagram of Process State
Process Creation
 Parent process create children processes,
which, in turn create other processes, forming a tree of
processes
 UNIX examples
 fork system call creates new process
Child process is almost an exact duplicate of parent process
 exec system call used after a fork to replace the
process’ memory space with a new program
Process Creation (Cont)
• Generally, process identified and managed via a process
identifier (pid)
• 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
• Address space
– Child duplicate of parent
– Child has a program loaded into it
Process 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
Process Termination
Parent Process
It is useful for the parent to know how and when the child
terminates
 Waiting on a child process:
Wait() system call waits for one (any one) of the child processes to
end
Returns the process ID (PID) of the child being terminated
Returns status of the child
Kernel adds the process CPU times and resource statistic to the
running total for all children of the parent process
Releases resources held by child to system pool
SIGCHLD signal
Q.E.D.
Orphans and Zombies
Orphan
 Parents ends before Child process
 Who becomes the parent?
INIT, the ancestor of all processes. INIT will periodically clean up
orphans
Zombie
 Child terminates before the parent performs wait()
Kernel turns process into ZOMBIE
 Most resources held by child are returned to the system pool
 Un-killable processes – Are waiting for their parent to issue
wait()
Wait for parents to exit when they will become orphans
 Big problem for daemons.
Process Control Block (PCB)
Bookkeeping/Management of a process
Includes:
•
•
•
•
•
•
•
•
•
Process ID
Program Counter
CPU Registers
CPU scheduling information
Priority
Process state
Memory management information
Accounting information
List of I/O devices allocated to process
Process Control Block (PCB)
UNIX Process Management
• UNIX fork – system call to create a copy of the
current process, and start it running
– No arguments!
• UNIX exec – system call to change the program
being run by the current process
• UNIX wait – system call to wait for a process to
finish
• UNIX signal – system call to send a notification
to another process
Process Creation
Parent my
perform other
actions here
Child statuses
passed to
parent
Memory of
parent copied
to child
Kernel restarts
parent
C 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);
}
}
UNIX Process Management
Question: What does this code print?
int child_pid = fork();
if (child_pid == 0) {
// I'm the child process
printf("I am process #%d\n", getpid());
return 0;
} else {
// I'm the parent process
printf("I am parent of process #%d\n", child_pid);
return 0;
}
Implementing UNIX fork
Steps to implement UNIX fork
– Create and initialize the process control block
(PCB) in the kernel
– Create a new address space
– Initialize the address space with a copy of the
entire contents of the address space of the parent
– Inherit the execution context of the parent (e.g.,
any open files)
– Inform the scheduler that the new process is
ready to run
Implementing UNIX exec
• Steps to implement UNIX fork
– Load the program into the current address space
– Copy arguments into memory in the address
space
– Initialize the hardware context to start execution
at ``start''
Windows CreateProcess
• System call to create a new process to run a
program
– Create and initialize the process control block (PCB) in
the kernel
– Create and initialize a new address space
– Load the program into the address space
– Copy arguments into memory in the address space
– Initialize the hardware context to start execution at
``start'’
– Inform the scheduler that the new process is ready to
run
Windows CreateProcess API
(simplified)
if (!CreateProcess(
NULL,
// No module name (use command line)
argv[1],
// 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
)
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 via a context switch
• Context of a process represented in the PCB
• Context-switch time is overhead; the system does no useful
work while switching
• Time dependent on hardware support
CPU Switch From Process to Process
CPU Switch From Process to Process
Process Scheduling
• Multiprogramming
• To have CPU always active, we switch between
processes.
• Process Scheduler
– Select an available process from a set of processes for
program execution.
– May force a process to become idle, so another can
run.
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
• Device queues – set of processes waiting for
an I/O device
• Processes migrate among the various queues
Representation of Process Scheduling
Interprocess Communication
• Processes within a system may be independent or
cooperating
• Cooperating process can affect or be affected by other
processes, including sharing data
• Reasons for cooperating processes:
–
–
–
–
Information sharing
Computation speedup
Modularity
Convenience
• Cooperating processes need interprocess communication
(IPC)
• Two models of IPC
– Shared memory
– Data Transfer
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
• Advantages of process cooperation
– Information sharing
– Computation speed-up
– Modularity
– Convenience
Communications Models
Message Passing
Shared Memory
Producer-Consumer Problem
• Paradigm for cooperating processes,
producer process produces
information that is consumed by a
consumer process
IPC Data Transfer –
Message 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)
Implementation 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?
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 bidirectional
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.
Synchronization
• Message passing may be either blocking or nonblocking
• 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
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
Examples of IPC System
PIPES
 Data streaming
 Direct communication
 Ordinary pipes
Producer – Consumer fashion
Cannot be accessed outside the process that creates it.
Usually Parent – Child communication
Deleted when process terminates
 Named pipes
 Alias: FIFO in unix
 Appear as typical files in the system
Examples of IPC Systems
Shared Memory
• POSIX Shared Memory
– Process first creates shared memory segment
segment id = shmget(IPC PRIVATE, size, S
IRUSR | S IWUSR);
– Process wanting access to that shared memory must attach to it
shared memory = (char *) shmat(id, NULL, 0);
– Now the process could write to the shared memory
sprintf(shared memory, "Writing to shared
memory");
– When done a process can detach the shared memory from its
address space
shmdt(shared memory);
Examples of IPC Systems - Mach
• Mach communication is message based
– Even system calls are messages
– Each task gets two mailboxes at creation- Kernel
and Notify
– Only three system calls needed for message
transfer
msg_send(), msg_receive(),
msg_rpc()
– Mailboxes needed for commuication, created via
port_allocate()
Examples of IPC Systems – Windows XP
• Message-passing centric via local procedure call
(LPC) facility
– Only works between processes on the same system
– Uses ports (like mailboxes) to establish and maintain
communication channels
– Communication works as follows:
• The client opens a handle to the subsystem’s connection
port object
• The client sends a connection request
• The server creates two private communication ports and
returns the handle to one of them to the client
• The client and server use the corresponding port handle to
send messages or callbacks and to listen for replies
Examples of IPC Systems - POSIX
POSIX Message Passing
Processes can exchange messages by using four system calls:
 msgget(mailbox_name, IPC_CREAT)
Converts a mailbox name to a message queue ID (msqid). It will
create the mailbox if necessary. Returns the msqid.
 msgsnd(msqid, message@, message_size)
Sends the message to the mailbox
 msgrcv( msqid, message@, message_size, priority, synch/asynch)
Receives the message from the mailbox
 msgctl(msqid, IPC_RMID, dummyParam@)
Release the mailbox from process resources
Communications in Client-Server Systems
• Sockets
• Remote Procedure Calls
Sockets
• 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
Socket Communication
Socket Communication
Application
Application
Socket
Transport
layer
Socket
Transport
layer
Network
layer
Network
layer
Link
Layer
Link
Layer
Physical
Layer
Physical
Layer
Remote 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 peforms
the procedure on the server
Execution of RPC
System Call : fork()
• Create a child process
• Returns to both the child and parent
System Call : exec()
• Run application in current process
exec(prog, args)
Execl( pathname, arg[0], arg[1]…. 0)
Excelp(arg[0], arg[1]…. 0)
http://www.yolinux.com/TUTORIALS/ForkExecPr
ocesses.html
System Call: wait()
• Pause until child process has exited
wait(): Blocks calling process until the child process
terminates. If child process has already teminated,
the wait() call returns immediately. if the calling
process has multiple child processes, the function
returns when one returns.
waitpid(): Options available to block calling process
for a particular child process not the first one
• http://www.yolinux.com/TUTORIALS/ForkExecProcesse
s.html
System call : system()
• Invokes the command processor to execute
a command.
• Uses fork() exec() and wait()
• The call "blocks" and waits for the task to be
performed before continuing.
system(“ls –l”);
UNIX Signal
• Facility for one process to send another
instant notification
• Send an interrupt to a process
signal(processID, type)
• Signal Handling example
– http://www.yolinux.com/TUTORIALS/C++Signals.h
tml
System Call : dup2()
• Replace the tofileDesc file descriptor with a
copy of the fromfiledesc file descriptor. Used
for replacing stdin or stdout or both in a child
process
dup2(fromFileDesc, toFileDesc)
• Example:
http://www.cs.loyola.edu/~jglenn/702/S2005/Exam
ples/pipe.html
Shell
• A shell is a job control system
– Allows programmer to create and manage a set of
programs to do some task
– Windows, MacOS, Linux all have shells
• Example: to compile a C program
cc –c sourcefile1.c
cc –c sourcefile2.c
ln –o program sourcefile1.o sourcefile2.o
Implementing a Shell
char *prog, **args;
int child_pid;
// Read and parse the input a line at a time
while (readAndParseCmdLine(&prog, &args)) {
child_pid = fork(); // create a child process
if (child_pid == 0) {
exec(prog, args);
// I'm the child process. Run program
// NOT REACHED
} else {
wait(child_pid);
// I'm the parent, wait for child
return 0;
}
}
POSIX
• POSIX, an acronym for "Portable Operating
System Interface", is a family of standards
specified by the IEEE for maintaining
compatibility between operating systems.
POSIX defines the application programming
interface (API), along with command line
shells and utility interfaces, for software
compatibility with variants of Unix and other
operating systems.
http://en.wikipedia.org/wiki/POSIX