Operating System Services
„
Loading and Running program?
■
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How disks are used? Opening, storing files?
„
How CPU/memory/IO device is used?
„
How programs talk with other programs on the
same or other computers?
■ File System Manipulation
OS: System calls
■
Based on slides © OS Concepts by Silberschatz, Galvin and Gagne, 2008
Additional material by Diana Palsetia
Resource Management
■ Communication
„
Who logs in ? has access? What went wrong?
■
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Security, protection and error detection
How does the user interact ?
■
A view of OS Services
User Interface
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System Call
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„
„
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Programming interface to the services provided by
the OS
Typically written in a high-level language
Mostly accessed by programs via a high-level
Application Program Interface (API) rather than direct
system call use
Three most common APIs are
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Process Execution, I/O Operation
3
Win32 API for Windows,
POSIX API for POSIX-based systems
y
(including
(
g virtually
y all
versions of UNIX, Linux, and Mac OS X),
Java API for the Java virtual machine (JVM)
Why use APIs rather than system calls?
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1
API over System Call
„
Are system dependent
■
„
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Not be a good idea to directly use system calls
when portability cannot be neglected
S t
System
calls
ll are also
l quite
it complex
l
■
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API over System Call
■
Involves the duo of TRAP and RET (or some
variations of those two)
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To implement system call
■
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■
The caller know nothing about how the
system call is implemented
One would need specialized knowledge of I/O
registers
O d off operations
Order
i
needed
d d to use them
h
Implement enough protection because I/O
resources are generally shared among multiple
users and/or processes
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¾Managed by run-time support library (set of
functions built into libraries included with
compiler)
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Abstraction and Portability
Call a library function written in assembly
assembly, which
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Calls a TRAP function written in assembly, which
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Unix System Calls for File I/O
If you want to call a C function to draw a pixel,
you …
■
JJustt needs
d to
t obey
b API and
d understand
d t d
what OS will do as a result call
Most details of OS interface hidden from
programmer by API
„
There are 5 basic system calls that Unix
provides for file I/O:
¾ Moves args from stack to regs and …
1. int open(char *path, int flags , mode_t mode);
¾ Moves args from regs to stack and …
■
Calls an OS function written in C, which …
■
Returns
R
t
tto the
th TRAP and
d…
Returns to the library function and …
Returns to you
2. int close(int fd);
3. ssize_t read(int fd, void *buf, int size);
4 ssize
4.
ssize_tt write(int fd
fd, void *buf
buf, int size);
¾ Finally draws the pixel and …
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5. off_t lseek(int fd, off_t offset, int whence);
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2
File Descriptor
int open(char *path, int flags , mode_t mode)
„
An index for an entry in a os/kernel data structure (file
descriptor table) containing the details of all open
files
„
User application passes the abstract key to the
os/kernel through a system call
„
OS will access the file on behalf of the application,
based on the key
■
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Open makes a request to the operating system to use
a file
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path argument specifies what file you would like to use
„
flags and
fl
d mode
d arguments
t specify
if how
h
you would
ld lik
like
to use it
■
■
The application itself cannot read or write the file descriptor
table directly
¾ Use mode when creating file for first time
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„
Th
Three
file
fil d
descriptors
i t
predefined:
d fi d
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int close(int fd)
close() tells the operating system that you
are done with a file descriptor
„
There is a limit on how many files a program
can open
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ssize_t read(int fd, void *buf, int size);
because it takes resources to store all information
needed to correctly handle an opened file
So, close all files you don't currently need to
save some resources
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This is a non-negative integer
If it returns -1, then you have been denied access
You can open the same file more than once
¾ a different fd each time. If you open the same file for writing more
than once at a time, you may get bizarre results
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On success, it will return a file descriptor to you
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File descriptor 0 is standard input
File descriptor 1 is standard output
File descriptor 2 is standard error
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E.g. flags: O_APPEND, O_CREAT, O_WRONLY(defined in
fcntl.h )
E.g. modes: S_IRWXU, S_IRUSR (defined in stats.h)
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read size bytes from the file opened in file
descriptor fd, and to put those bytes into the
location pointed to by buf
„
Returns how many bytes were actually read
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write syscall writes data and returns the
number
b b
bytes
t written
itt
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3
Mechanics to open a file: Streams vs. File
Descriptors
off_t lseek(int fd, off_t offset, int whence)
„
„
All open files have a file pointer associated
with them
File descriptors are represented as
objects of type int
„
Streams are represented as
FILE * objects
„
Provide a primitive, low-level
p and output
p
interface to input
operations
„
Streams provide a higher-level
interface, layered on top of the
primitive file descriptor facilities
„
Interface provides only simple
functions for transferring blocks of
characters
„
Implemented in the C run-time
library
„
Can extract the file descriptor from a
stream and perform low-level
operations directly on the file
descriptor
„
„
Can initially open a connection as a
file descriptor and then make a
stream associated with that file
descriptor (using fdopen())
Can move the file pointer manually
■
■
How much to move is provided by offset
The whence variable specifies how the seek is to
be done
¾ from the beginning of the file
¾ from the current value of the pointer
¾ from the end of the file
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Upon successful completion, returns the
resulting offset location as measured in
bytes from the beginning of the file
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API is provided in <stdio.h>
Provides powerful formatted
input and output functions (printf
and scanf) as well as functions
for charactercharacter and line-oriented
line oriented
input and output
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Usage of streams and file descriptor
Standard C Library Example
„
Can also represent a connection to a device or a pipe
or socket for communicating with another process
■
„
„
Unless there is some specific operation you want to perform
that can only be done on a file descriptor
Portability of your programs to systems other than
GNU systems
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Operations that are specific to a particular kind of device use a file descriptor
Use streams rather than file descriptors
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Be aware that file descriptors are not as portable as streams
You can expect any system running ISO C to support
streams
But non-GNU systems may not support file descriptors at all,
or may only implement a subset of the GNU functions that
operate on file descriptors
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4
perror
„
„
How do I know what .h files to include ?
Usually when an error occurs in a system or
library call, a special return value comes
back, and a global variable "errno" is set to
say what the error is
For example, suppose you try to open a file
that does not exist:
Man pages are you best friend
„
man –s# functionname
■ 2: System calls
■ 3: Library calls
„
Search functions but don’t know there name:
■
fd = open("in1", O_RDONLY);
if (fd < 0) {
perror(“open syscall failed because");
exit(1);
}
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„
■
Keyword search:
man -k regexp
Examples:
¾ man -k “memory”
¾ man -k “locate.*string”
¾ Unix Specification:
http://www.opengroup.org/onlinepubs/009695399/
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