1.6 Operating Systems / Processes 1.6 Operating Systems ...

1.6 Operating Systems / Processes
1.6.1 Definition of the Term Process [Task]
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Execution of a Program controlled by the operating system.
(Hint: one program can be started many times. On each start an new process is created)
– The operating system creates a virtual Computer for each
program it runs. i.e. Before execution of program instructions the MMU setup
is changed to restrict the memory access to the parts of the RAM which belong to the
program.
● The program addresses are mapped to the real memory adresses by
a table and calculations in the MMU. The program addresses are
also called virtual addresses because they refer to a virtual
computer.
● The set of all virtual addresses is called process address space or
virtual address space.
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1.6.2
1.6 Operating Systems / Processes
stack
heap
Uninitialized Data
initialialized read write
initialialized read only
Text Segment
(program instructions)
Datensegment
(Programmdata)
Kernel data
Kernel Kontext
(data to contol the execution
used by the operating system
This part is read from
the program file on
hard disk
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1.6 Operating Systems / Processes
1.6.3
#include
integer k = 7;
integer a[1000];
char * fmt = "Pleas enter n";
int factorial( int n ) { stack: Memory for
int e,i;
functions calls
e = 1;
for (i=2; i

Process A
stack
heap
uninitialized
Data
initialized
read write
initialized
read only
Text Segment
(Programmcode)
Create process:
fork();
Start
1.6 Operating Systems / Processes
stack B
stack A 1
heap A
stack A 2
heap B
uninitialized
Data A
uninitialized
Data B
initialized A
read write
initialized B
read write
initialized
read only
Text Segment
(Programmcode)
Main memory
Process B
stack
heap
uninitialized
Data
initialized
read write
initialized
read only
Text Segment
(Programmcode)
1.6 Operating Systems / Processes
1.6.4 Attributes of a Process (continued)
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Hint: The memory
mapping some time after
the new process started
is displayed here.
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– Status: sleeping (waiting), running, Kernel-Call (Operating system
function call), trace (Debug),
Prozess terminated (Runtime information is still needed):
Assign CPU
running
Revoke assignment of CPU
Wait for an
event
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End
(Termination)
1.6.4 Attributes of a Process
Values to characterize a process
● Porcess ID (pid) unique number (short integer), is assigned to each
process by the operating system (kernel)
(query in a C program: function call: getpid())
●
●
●
●
1.6 Operating Systems / Processes
Parent Process ID (Number of the process which generated this process.
Function (system) calls: fork(), getppid().)
User ID (Number to identify a user)
– Real (uid, short integer) getuid()
– Effektiv (valid for file access) (euid, short integer) geteuid()
Group ID (Number to identify a group of users)
– Real (gid, short integer) getgid()
– Effective (valid for file access) (egid, short integer) getegid()
Priority (nice value) (Attribute controls runtime assignment to processes)
1.6 Operating Systems / Processes
1.6.4 Attributes of a Process (continued)
– Working Directory (see exercises)
– Table of Resources (for example: all data to access open files on hard disk)
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– Process-Group ID (on process can become the leader of a set of processes. This set
is called the process group. With some system calls all processes of a process group can be
adressed)
– Terminal-Group ID (/dev/tty) (group of processes using the same
input/output channel, for example the same PuTTY window)
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– Environment Variables (pair of stings: one string is used as index to store a string
value: to set the language use LANG=EN, the first string LANG is the index, value is EN )
ready
Event occurred
waiting
– Umask (set permissions for the creation of new files)
– User Limits (ulimit) (set limits for process: execution time, memory,
files (number, size), number of (sub)processes created, ...)
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1.6 Operating Systems / Processes
1.6.5 Signals
– Can be received by a process (and handled or ignored. This is controlled by
the operating system and is distinct from events/interrupts received by the CPU from
electronic devices)
– Sources (for example: all data to access open files on hard disk)
●
●
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Hardware (if the CPU receives an interrupt while executing instructions of a user
program due to some program error the execution is interrupted and the interrupt handler
program part of the operating systems is run. As a result the process can receive a signal)
– Divide by zero (Source: Floating point unit; Signal: Floating Point
Exception SIGFPE)
– Illegal memory access (Source: MMU; wrong address i.e. Not assigned to
the process, attempt to write to a protected region; Signal: Segment
Violation SIGSEG)
– ...
Operating System (Timer to wake up a process: SIGALRM,
user enters +C: SIGINT, ...)
● Other processes (System call kill(pid, signal), signals: SIGKILL, SIGTERM, SIGUSR1, SIGUSR2, ...)
1.6 Operating Systems / Processes
1.6.6 Critical Regions (German: Kritische Abschnitte)
– Lock Variable (Stores the value of the signal)
– Test and Set must be performed in one interruptible (atomic)
operation
– Picture p. 92, [1] (C. Vogt: Betriebssysteme, Spektrum
Akademischer Verlag, Heidelberg Berlin, 2001)
Stop if signal
is red
Stop if signal
is red
Change
signal to red
Change
signal to red
critical
region
Change signal
to green
Change signal
to green
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– Handler (is started when the signal is received, system call to install the handler:
signal(signal, handler); Definition handler: void handleSig(int sig_no){ ... }
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1.6.6 Critical Regions (continued)
– Semaphore (Generalization of Lock Variable) (Stores the value of
the signal)
Possible Operations:
●
●
●
1.6 Operating Systems / Processes
Initialize S(set a start value for example to the number of available resources)
Pass (request one resource, if not available wait until the resource becomes available)
Release (the resource)
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1.6 Operating Systems / Processes
1.6.7 Dead Locks (German: Verklemmungen)
– Example: Traffic jam
Prerequisite for a Resource:
●
●
●
Exclusive usage
Cannot be withdrawn
1 < n Resources
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Picture p. 105, [1] (C. Vogt: Betriebssysteme, Spektrum
Akademischer Verlag, Heidelberg Berlin, 2001)
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