COM222 INTRODUCTION TO
OPERATING SYSTEMS
LESSON 2. SYSTEM CALLS
AND SYSTEM PROGRAMS
Unit introduction
• This unit aims at understanding what
system calls and interrupts are. It explains
basics of operating systems system calls
and introduces the aspect of interrupts and
interrupts handling.
Learning outcomes:
• By the end of this unit the student should
be able to:
• Define what a system call is
• Explain the difference between system
calls and system programs
• Explain the concept of interrupts and
interrupts handling
System Calls
• System calls provide an interface between the process
and the operating system.
• System calls allow user-level processes to request some
services from the operating system, which process itself
is not allowed to do.
• In handling the trap, the operating system will enter in
the kernel mode, where it has access to privileged
instructions, and can perform the desired service on the
behalf of user-level process. It is because of the critical
nature of operations that the operating system itself does
them every time they are needed. For example, for I/O a
process involves a system call telling the operating
system to read or write particular area and this request is
satisfied by the operating system.
System Calls for Process
Management
• The examples of system calls for process
management are FORK, WAITPID and EXEC.
• FORK is a system call for process management that
helps to create a new process.
• It creates an exact duplicate of the original process,
including all the file descriptors, registers and
everything. After the FORK, the original process and
the copy (the parent and child) go their separate
ways.
WAITPID system call
• This is one of the system calls for process
management. To wait for the child to finish executing a
command, the parent executes a WAITPID system
call, which just waits until the child terminates any
child if more than one exists.
• WAITPID can wait for a specific child or for any old
child by setting the first parameter to -1. When
WATPID completes, the address pointed to by the
second parameter will be set to the child's exit status.
EXEC System call
• This is another system call for process management.
The Exec System call is used by the child process to
execute the user command. When a command is typed,
a new process is generated. When this is done it causes
its entire core image to be replaced by the file named in
its first parameter.
• In the most general case, EXEC has three parameters:
the name of the file to be executed, a pointer to the
argument array, and a pointer to the environment array
(in a program).
System Calls for signaling
• Although most forms of inter-process communication are
planned, situations exist in which unexpected
communication is needed. For example, if a user
accidently tells a text editor to list the entire contents of a
very long file, and then realizes the error, some way is
needed to interrupt the editor.
• When a signal is sent to a process that has not
announced its willingness to accept that signal, the
process is instantly killed. To avoid this fate, a process
can use the SIGACTION system call to announce that it
is prepared to accept some signal type, and to provide
the address of the signal handling procedure and-a place
to store the address of the current one.
• Signals are handled by signal handlers that may run for
as long as they want to and perform any system calls
they want to.
• In practice, signal handlers are usually fairly short. When
the signal handling procedure is done, it calls
SIGRETURN to continue where it left off before the
signal. The SIGACTION call replaces the older SIGNAL
call, which is now provided as a library procedure,
however, for backward compatibility.
System Calls for File
Management
• Many system calls relate to the file system. In this
section we take a look at calls that operate on individual
files.
• To create a new file, the CREAT call is used.
• CREAT not only creates a new file but also opens it for
writing, regardless of the file's mode.
• The CREAT call is obsolete, as OPEN can now create
new files, but it has been included for backward
compatibility.
System Calls for Directory
Management
• The first two calls are MKDIR and RMDIR.
• The MKDIR is used to create an empty directory. The
RMDIR is used to remove an empty directory.
• The next call is LINK. Its purpose is to allow the same
file to appear under two or more names, often in different
directories. A typical use is to allow several members of
the same programming team to share a common file,
with each of them having the file appear in his own
directory, possibly under different names.
• Sharing a file is not the same as giving
every team member a private copy,
because having a shared file means that
changes that any member of the team
makes are instantly visible to the other
members-there is only one file.
Interrupts
• Interrupts are an unpleasant fact of life.
They should be hidden away, deep in the
bowels of the operating system, so that as
little of the system as possible knows
about them. The best way to hide them is
to have every process starting an I/O
operation has completed and the interrupt
occurs.
Interrupt Handling
• When an event occurs, the process is put on the ready
queue. When an event occurs, the system receives an
interrupt. An interrupt is a signal to the CPU. It is called
an interrupt. It is called an interrupt, because the CPU
stops whatever it is doing and looks to see what caused
the interrupt.
• The interrupt come from a hardware device (e.g. a timer,
disk drive or network card) stating that some action is
complete and data is available that the CPU needs to
use. It could arise from software – e.g. a run-time error
being trapped in a process.
• When an interrupt occurs, the CPU jumps to a special
interrupt service routine to deal with the interrupt. This
can involve rescheduling processes or moving
processes between states. For example, a mouse click
will generate an interrupt. This might make our process
runnable again. When servicing this interrupt the CPU
will therefore move the process Blocked state to Ready
state.
• After executing the interrupt service routine, the CPU
may resume the original process or it may reschedule a
new process. The decision depends on the event and
the scheduling policies.
References:
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Beaumont C, (2001): “Introducing operating systems” edge
hill, 1st Edition
Operating Systems, McGraw-Hill Book Company, New York
By S. E. Madnick and J. J. Donovan
Millenkovi C.M (2002). "Operating System; Concepts &
Design", Megraw Hill.
Tanenbaum A.S (1999). "Operating Systems Design and
Implementations", Prentice Hall NJ.