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LINUX
LAST UPDATED: 08 March 2009 17:02:35 -0600
Changes to this page are IN PROGRESS
AN OVERVIEW OF LINUX INSTALLATION TOOLS
Linux's command-line interface and configuration text files can be
confusing to new--and experienced--users. The introduction of the
menu-driven character mode and X Window System configuration
applications has helped many configure and customize Linux, but the
experience is still complicated.
Still, Linux need not trade in its text configuration and script
support for graphical support of the Windows variety, which has
spawned many specialized tools to manage groups of servers and
workstations, making administration on a mass scale very complex.
Linux administrators work wonders with just a few simple scripts; for
example, when they use awk and grep to manage dozens or even hundreds
of Linux servers and workstations remotely. However, setting up such
scripts requires significant expertise.
If you can't adjust to Linux configuration files, package systems,
and configuration applications, don't despair. An overview of Linux's
configuration tools at the OS, application, and desktop environment
levels--including YaST, Linuxconf, and RPM--is available at
FIRST COMMERCIAL MAINSTREAM DEVELOPMENT TOOLS FOR RED HAT
Metrowerks Inc. has released its CodeWarrior software development
tools for Linux. CodeWarrior aids cross-platform development. It is
the first widely used, commercially available Integrated Development
Environment to be ported to Red Hat Linux. Metrowerks has no plans at
this time to release into the open source community any source code
for CodeWarrior, although it has released its JIT compilers for Java
on PowerPC-based systems in the past.
Red Hat and Metrowerks believe that their willingness to stand
behind CodeWarrior's user-friendly development tools will help speed
the adoption of Linux in the enterprise market. The companies said
that their cooperation is a direct response to demands from the
developer community for mainstream, commercial software development
tools for Linux. For more information, see
A buffer overflow attack occurs when a hacker floods an input
buffer on the execution stack with more data than is expected by the
program. Security breaches caused by buffer overflow vulnerabilities
are one of the biggest security problems on the Internet. The worst
case scenario with buffer overflows is that you surrender root access
to the attacker, who can then do anything with your system. Buffer
overflow problems tend to be discovered, exploited, and patched on a
case-by-case basis, which means that many more could be hidden in our
servers and introduced daily in new software releases. What are these
vulnerabilities, why are they fixed retroactively, and what can be
done about them? Read more at
Managing partitions and drive space is one of the continual
challenges (and sometimes headaches) of working on Linux. The Logical
Volume Manager for Linux site offers a Linux implementation of the
UNIX standard Logical Volume Manager (LVM) software developed by Heinz
Mauelshagen that makes managing your drives much easier. LVM stands
between your physical hard drives and peripherals and the I/O
interface in the kernel and gives you a logical overview of your
disks, which are referred to as physical volumes. By organizing your
drives as pools (volumes) of data storage comprised of equally sized
extents rather than requiring you to divide them into physically
fixed-size sections, LVM enables you to change easily the size
allocated to a particular partition.
Here's how it works: An LVM system consists of arbitrary groups of
physical volumes that you organize into volume groups. You can have
multiple physical volumes in a given volume group; you can also have
multiple volume groups on your system. The volume group then becomes
the basic unit of data storage, not the physical group (e.g., the
disk). You can apportion the space on your volume group into logical
volumes of various sizes. A logical volume is the conceptual
equivalent of a logical partition on a hard drive, but at a higher
level as it can span multiple drives as well as represent only a
portion of a physical drive. The size of the logical volume is set by
its number of extents. Logical volumes can be used just like regular
disk partitions, but because they can span multiple drives, they
enable you to use your drive space much more efficiently and avoid
partition space jams. Find out more about this program and how to
download it at
http://linux.msede.com/lvm/
GET GRAPHICAL--GEOGRAPHICAL--WITH GRASS
Geographic Information Systems are transforming the way we gather
and analyze all kinds of geographic and spatial data. Linux developers
won't be surprised to know that UNIX has served as the primary
development environment for these efforts. The Geographic Resources
Analysis Support System (GRASS) was originally developed by the U.S.
Army Construction Engineering Research Laboratories (USA-CERL) as a
tool for land management and environmental planning. (Baylor
University took over management of the project in 1982.) GRASS has
evolved into a powerful utility with a wide range of applications in
many areas of scientific research and academic and commercial settings
around the world, as well as governmental agencies including NASA,
NOAA, USDA, the National Park Service, the U.S. Census Bureau, USGS,
and many environmental consulting companies. The program is in the
public domain, and development is continuous. Developers are
encouraged to make their own additions and modifications. If you're
interested, you may even be able to join the development team. You can
find visualization demos, information on downloading the program, a
recently released UNIX/Linux GUI developed for the program, and much
more at this Baylor University Web site:
http://www.baylor.edu/~grass/
RATE YOUR I/O PERFORMANCE WITH INDEPENDENT BENCHMARK PROGRAMS
Want to see just how great your configuration and coding might be?
At this Advanced Computer Network Corporation site, you can download
some nifty little I/O benchmark programs. The site has programs that
run on Windows NT, DOS, and Win95 as well as Linux and UNIX--always
nice if you want to prove the superiority of the Linux solution.
http://www.acnc.com/benchmarks.html
UNIX REFERENCE DESK
At root, Linux is, of course, a variant of the UNIX OS, and UNIX
sources can be helpful in understanding existing programs or dreaming
up new ones. You can find a wide range of information on useful
developer info at the UNIX Reference Desk. Some of the more helpful
links include those to the 4.4BSD (Berkeley Systems Design) Manuals,
including the UNIX Programmer's Supplementary Documents (PSD), the
UNIX System Manager's Manual (SMM), and the UNIX User's Supplementary
Documents (USD). Also useful is a link to an overview of UNIX variants
that lists the main UNIX standards. Go to
http://www.geek-girl.com/unix.html
and follow the links.
LOOKING FOR DOCS? LOOK NO MORE.
You can find links to the best quality Linux documentation on any
Linux topic you need on the home page of the Linux Documentation
Project. The Documentation Project (naturally) has its own guides
listed first (all available for free download), but it also has links
to a host of How-to's and FAQs, as well as links to a few LDP books
that are available for purchase. Several of LDP's own docs are
targeted specifically at developers and include source code examples,
including The Linux Kernel Module Programming Guide (version 1.0.1 by
Ori Pomerantz) and The Linux Programmer's Guide (version 0.4 by B.
Scott Burkett, Sven Goldt, John D. Harper, Sven van der Meer, and Matt
Welsh).
http://metalab.unc.edu/LDP/linux.html
GET THE MOST UP-T0-DATE KERNELS AND DISTRIBUTIONS
Want to know about all the available Linux online distributions?
Jump over to the FTP Sites list and you'll find links to both free
download sites and major distributors of commercial versions of
Linux--as well as links to a host of other useful Linux resources.
http://www.linux.org/dist/ftp.html
HELPSTUDIO 1.0 RELEASED
Looking for a comprehensive and user-friendly Java-based authoring
environment that enables you to write online help by pointing and
clicking? Then check out the latest release of HelpStudio by
Singapore-based MediaMix Technologies. HelpStudio 1.0 not only comes
with a small footprint (less than 205KB), it also promises to shorten
the process of creating online help. Using HelpStudio 1.0, you can
incorporate online help in your Java applications, applets, and
JavaBeans. Since it has been written in pure Java, you can use it on
any compliant Java Virtual Machine.
For details and purchasing information, go to
http://www.mediamixtech.com.sg/helpstudio.html
Linux is a worldwide phenomenon whose explosive growth lies partly
in its promise to bring the third-world technology "have-nots"
directly into the heart of the high-tech world--no artificial
commercial barriers stand in the way of hands-on learning and working
with source code. Interested in developing non-English language
versions of Linux and creating a revolution in political economic
relations on the side? Or just want to find interesting developer
colleagues? Check out the list of more than 60 USENET national Linux
news groups. Countries listed include Indonesia, the Philippines,
Croatia, South Africa, Swaziland, Chile, China, and Algeria.
http://www.linux.org/users/index.html
NEW LINUX DEVELOPMENT KERNEL RELEASED
The Linux kernel version 2.3.4 was released June 1. For a complete
list of changes in this release or to download the kernel, go to
http://linuxtoday.com/stories/6396.html
WEBMIN 0.72
Webmin 0.72 is a powerful Web-based administration interface for
Unix systems. Using Webmin 2.0, you can use any browser that supports
tables and forms to set up user accounts, Apache, DNS, NFS,
local/remote file systems, and more. Webmin consists of a simple Web
server and CGI programs that directly update system files such as
/etc/inetd.conf and /etc/passwd. The Web server and all CGI programs
are written in Perl version 5 and use no external modules, which means
you need only a Perl binary to run Webmin.
Interested in efficient process migration? EPCKPT is a kernel-based
checkpointing utility that enables you to save an image of the state
of a process (or group of processes) at a certain point during its
lifetime. Checkpoints are important to a wide range of applications,
including fault tolerance, application debugging, animated playback,
and process migration. Note that EPCKPT is restricted to the Intel x86
platform running Linux.
For more information, see
http://www.cs.rochester.edu/u/edpin/epckpt/
QMAILADMIN-0.19 BETA AVAILABLE
Qmail is a Y2K-compliant, secure replacement package for sendmail.
QmailAdmin is a free software package that provides a Web interface
for managing a qmail system. In its latest beta version,
QmailAdmin-0.19 works with both Qmail and Vchkpw virtual domain
packages. The package supports POP-only accounts, adding/deleting
users, aliases, forwards, mailing lists, and autoresponders.
For more info on QmailAdmin, including a demo, go to
http://www.inter7.com/qmailadmin/
For more details on qmail, check out
http://qmail.area.com/top.html
FINE-TUNE YOUR MEMORY
This site, run by Rik van Riel, is aimed at providing full
documentation on the Linux memory management system and giving an
overview of development efforts in the Linux memory management
community. It's updated consistently, if erratically, and contains
links to memory patches as well as other Linux sites useful for
developers.
Check it out at
http://humbolt.geo.uu.nl/Linux-MM/
HELP SET THE LINUX WORLD ON FIRE
Apple's new G3 desktop computers made clear that FireWire, the
IEEE-1394 standard (trademarked by Sony as iLink), is the future of
high-bandwidth peripherals ranging from video and digital cameras and
typesetters to heavy-duty industrial tools. A group of Austrian
developers is getting on the stick now to make this powerful
technology available on Linux by coordinating a driver development
project. Sign up on their developers' mailing list, or check out the
archives at
http://eclipt.uni-klu.ac.at/ieee1394/
FIND THE LATEST IDEAS IN LINUX DEVELOPMENT
The Linux Project Catalogue is just that, a catalog of current
Linux development projects. The site also hosts a discussion forum
where people explore ideas for new projects. The site is cleanly
organized with a decent search engine and sections on system tools,
utilities, applications, communication, multimedia, programming,
games, documentation, and distribution. Find out more at
http://www.aesia.com/lpc/
LINUX PROGRAMMING AND DEBUGGING TECHNIQUES
Most CPUs require that multibyte objects reside at particular
offsets in the system's RAM. For example, 32-bit processors require
that a 4-byte integer reside at a memory address that is evenly
divisible by 4. This requirement is called memory alignment. For
example, an int can be located at memory address 0x2000 or 0x2004 but
not at 0x2001. On most Unix systems, an attempt to use misaligned data
results in a bus error, which terminates the process altogether. On
Linux, this constraint was relaxed: the use of misaligned data is
handled by the kernel so the process can continue normally, albeit at
a substantial performance penalty. As a rule, you should avoid using
misaligned data in your Linux apps.
THE DIFFERENCES BETWEEN KERNEL MODE AND USER MODE
Linux has two execution modes: user mode and kernel mode. Ordinary
programs run in user mode, which means that they are protected from
damaging other processes or the rest of the system. For example, a
program that runs in user mode is not allowed to access memory outside
its address space. Conversely, device drivers and file systems run in
kernel mode. Code running in kernel mode has unrestricted access to
the system's resources. Applications, which run in user mode, often
need to perform operations that run in kernel mode such as accessing a
file or allocating memory. For this purpose, system calls are used.
UNDERSTANDING SYSTEM CALLS
A system call, or syscall, is the mechanism by which an application
that is running in user mode requests a protected code that executes
in kernel mode. For example, an application that allocates memory
invokes a memory allocation function, which, ultimately, executes a
syscall. Syscalls look very similar to ordinary C function calls.
However, they differ from C functions in several respects. First, all
the arguments passed to and from a syscall must have the same size (in
general, this is the size of a machine word). Smaller units, such as
short and char, are automatically promoted to a larger unit by C
before they are passed. Second, the return type of a syscall can only
be a signed word. In particular, structs may not be passed to or
returned from a syscall. However, you can pass a reference to a
struct.
PROCESS ATTRIBUTES
The two basic attributes of a process are its process id, or pid,
and the pid of its parent process. A pid is a positive integer that
serves as the unique identifier of a running process. When a process
creates a new process (such a process is called a child), the original
process is the parent process. When a child process dies, the parent
process is notified, and the kernel stores its exit status until the
parent process requests it.
WHAT ARE ZOMBIES?
When a process ends, the kernel stores its exit status in the
process table. Processes that have ended and are kept in the kernel
only to preserve their exit status are called "zombies." A zombie is
removed from the system's process table when its status has been read
from the process table. Thus, zombies can hang around for days and
weeks. Often, zombies are a result of a power fault or other failures;
their exit status is kept in the kernel so they can be analyzed
afterward.
ENVIRONMENT VARIABLES
An environment variable is a pair consisting of a name and its
associated value formed as a single string:
SYMBOL=VALUE
The set of environment variables makes up the program's
environment. The environment is made available to a program at startup
as a global pointer called environ (defined in the header file ).
Alternatively, the ANSI C function getenv() can be used to read the
value of an environment variable. You can use environment variables to
control and manipulate a program without having to modify it. For
example, a program can check the value of an environment variable
"USER" and adjust its behavior according to specific values.
USING CHECKER TO DETECT MEMORY LEAKS
Among other onerous bugs that C and C++ experience rather often is
memory leaks. To facilitate the task of detecting memory leaks, you
can use Checker. Checker is a utility that adds extra "scaffolding
code" to the user's program to detect such leaks. First, you compile
and link the program with Checker. After running the program, Checker
invokes a memory-leak detector that tells you exactly where the leaked
memory was allocated.
The Checker utility, as well as information on how to use it, is
available at
ftp://sunsite.unc.edu/pub/linux/devel/lang/c
WHAT YOU CAN LEARN FROM CORE DUMPS
Abnormal process termination occurs when the kernel terminates the
process because of an error or when the process receives a kill()
message that requests process termination. Usually, a process that
terminates abnormally dumps its core before dying. The core dump
listing provides a complete history of the state of the program when
it died, including the function call sequence, the memory address of
each function called, signal codes, and the code line numbers. The
core dump is automatically written into the processes working
directory in a file named "core". Many debugging tools can analyze
this file and provide you with valuable information about what the
process was doing when it died.
UNDERSTANDING FILE DESCRIPTORS
When a process opens a file, the kernel returns a corresponding
file descriptor that the process refers to in order to process the
file. A file descriptor is a low positive integer. The first three
file descriptors of a process--0, 1, and 2--are associated with the
standard input (stdin), the standard output (stdout), and the standard
error (stderr), respectively. Other files have higher descriptor
numbers. Standard C follows this convention too. Thus, the functions
scanf() and printf() use stdin and stdout, respectively. You can
override the default setting and redirect the processes I/O to
different files by using different file descriptors.
GARBAGE COLLECTOR FOR LINUX
Standard C and C++ do not have a garbage collector. If you find the
burden of explicitly releasing every chunk of dynamically allocated
memory by your programs too tedious and bug-prone, you may want to
consider installing an automatic garbage collector utility. The Boehm
garbage collector for C and C++ is available at
http://reality.sgi.com/boehm_mti/gc.html
and can be used on most operating systems, including Linux.
ADVANTAGES OF WRITING PORTABLE CODE
When writing Linux applications, it is advisable to stick to the
common denominator of Unix and Linux, so that the code can be ported
easily to other Unix platforms. This way, you ensure that improvements
made to the code by Unix developers can be ported back to Linux and
vice versa.
GCC EXTENSIONS TO ANSI C: INLINE FUNCTIONS
Gcc, the standard compiler used with Linux, adds several extensions
to ANSI C. Some of these extensions are now in the process of being
added to the new ANSI C standard, the C9X. However, other extensions
are Linux-specific. The following tips discuss some of these
extensions and their usefulness.
The first extension to be discussed is the keyword "inline". Many
system-specific header files declare functions as inline. An inline
function is expanded like a macro, thereby eliminating the overhead of
a function call. In addition, inline functions provide all the
type-checking available with ordinary function calls, so inline
functions are both safe and efficient.
GCC EXTENSIONS TO ANSI C: ATTRIBUTES
Another gcc extension is the keyword "attribute," which enables the
programmer to provide more information about a function, a
declaration, or a variable than is possible in strict ANSI C. The
additional information can help the gcc compiler to produce more
optimized code. In addition, attributes give the programmer tighter
control over the generation of object code, which is impossible
otherwise. For example, the attribute "noreturn" explicitly declares a
function that does not return and instructs the compiler to suppress
spurious warnings. Likewise, the "aligned" attribute tells the gcc
compiler how exactly to align a type or a variable.
GCC EXTENSIONS TO ANSI C: THE LONG LONG DATA TYPE
A new data type that is not part of ANSI C is long long. It is
large enough to store a 64-bit integer. Since most Linux and Unix
platforms are based on 32-bit architectures, long long variables are
used for storing very large numbers that exceed the 32-bit limit.
Similarly, unsigned long long can store a 64-bit unsigned integer.
ADVANTAGES OF SHARED LIBRARIES
Linux, like many other operating systems, supports shared
libraries. A shared code library offers several advantages over a
statically linked library. A shared library saves system memory
because the system shares memory among all the processes that use the
library, rather than allocating distinct memory buffers for each
process. In addition, a shared library saves disk space because
programs that use it do not contain distinct copies of the shared
code. Finally, a shared library can be upgraded more easily because
the processes that use it pick up the latest version of the library
automatically when they run, whereas programs that use statically
linked libraries have to be recompiled and linked.
The Unix kernel switches context among threads faster than it does
with full-blown processes. This is why programmers often prefer to use
multithreading instead of multiprocessing. Linux, however, works
differently than Unix in this respect. Its kernel performs context
switches among processes significantly faster than the Unix kernel.
Since multithreading has several inherent safety problems and since
multiprocessing is simple and fast on Linux, you should consider using
multiprocessing rather than multithreading to achieve concurrency on
Linux.
If you are looking for a specific application for Linux, you can
consult the Linux Software Map (LSM) at
http://www.execpc.com/~lsm
The LSM contains documentation of thousands of software products
that are readily available for Linux users. The documentation includes
product names, version numbers, a Web site where the product can be
downloaded from, and other useful information.
The command "man" activates the Linux Manual Pages (Man Pages)
utility, which provides succinct reference documentation about system
calls, devices, files, libraries, file formats, and others. To learn
how to use man, simply type the command "man man" and follow the
instructions.
On Linux, every file is associated with a 16-bit integer that
represents the file's type and its access permissions. This small
integer is called the "file mode." The file mode usually appears as a
sequence of six octal digits; the low order three digits represent the
file access, the next digit contains file permissions, and the last
two digits represent the file type. For example, the file mode 0041755
(note: the first zero indicates an octal number and is not included in
the file mode) indicates a file type 04, a permission modifier of 1,
and access permissions of 0755. The file mode can be altered to
control what processes may access the file.
Linux defines three categories of file users: the file owner, the
group of the file owner, and, finally, all other users. The term
"world permissions" refers to permissions that are granted to all
these three categories. The file access permissions contain three
digits, each of which corresponds to one of the three categories.
Linux, like all Unix systems and many other operating systems,
refers to midnight, January 1, 1970, Universal Time Coordinated (which
is roughly equivalent to Greenwich Mean Time) as its baseline for time
measurements. Midnight, January 1, 1970, is also called "the epoch."
The time on Linux systems is represented as a long integer that
contains the number of seconds elapsed since the epoch.
A HOWTO is a detailed document on a specific subject written in a
standard expository style. Such a document usually explains how to do
something (hence the name). A mini-HOWTO is a small-sized HOWTO that
is limited to a few pages. HOWTOs and mini-HOWTOs are available in
several formats including plain text, HTML, and PostScript.
The GNU General Public License (GPL) is one of the most restrictive
free software licenses. One of the GPL rules requires that if you
include source code that is licensed under the GPL terms in another
program, that program has to be licensed under the GPL rules, too. For
this reason, the GPL is sometimes called a virus.
The Linux Documentation Project (LDP) takes care of composing and
maintaining the standard online manual pages. The LDP also produces
books about Linux that are targeted at Linux users and system
administrators. In addition, it maintains a large collection of HOWTOs
and mini-HOWTOs. You'll find the LDP Web site at
The GNU Network Object Model Environment (GNOME) project attempts
to build a complete desktop based entirely on free software that
complies with the open source criteria. You can find more info about
the GNOME project and download GNOME software at
Gdb is a command-line debugger that was developed by the Free
Software Foundation. It can debug a program, attach to an existing
process for debugging purposes, and inspect a core file of a killed
process. Gdb enables you to define breakpoints in the debugged source
file, examine variables, and set break conditions. The three basic
commands are run, step, and break.
The Linux Counter is a nonprofit organization whose purpose is to
count the number of Linux users worldwide. You can register yourself
as a Linux user, register your machine, or view the current count of
Linux users at
A symbolic link, or symlink, is a special kind of file that
contains the path to another file. Linux recognizes symlinks, so when
a symlink is opened, the system automatically reads its contents and
opens the associated file.
On Linux, most physical devices are represented as files. There are
two types of device files: block device files and character device
files. A block device represents a hardware device from which the
system cannot read one byte at a time, but rather, it must read blocks
of bytes. CD-ROM drives are an example of block devices, and so are
disk drives. In contrast, device drives that can be read one byte at a
time are called character devices. These include printers, modems,
terminals, video and audio cards, and mice.
The . and .. (a dot and two dots) are special files that exist in
every directory. The . name is identical to the current directory
(note that directories are ordinary files). The .. file name is the
current directory's parent directories.
The Lothar project attempts to make hardware configuration easier
in Linux by providing Ethernet cards and sound cards autodetection
along with a nice graphical interface. A new release of Lothar is now
available. Users who would like to download the software or contribute
to this project can find more info at
Trinux is a small and portable version of Linux that has been
stripped of nonessential components. In addition, it is enhanced with
GPL security tools. The authors of Trinux are able to create a custom
Linux distribution that occupies less than 5MB. You can find more info
on Trinux at
One of the parameters of a process is called "resident set size,"
which is the maximum amount of RAM that the process can use at any
given time. Exceeding this limit causes paging, which degrades
performance. System administrators and users can change the resident
set size of a process to ensure optimal performance.
A major fault is a memory access that forces the kernel to access
the disk before the process can continue. Major faults occur, for
example, when the process attempts to read parts of an executable file
that have been swapped. Frequent major faults usually indicate that
the system has insufficient memory or that the process has
insufficient resident set size. Minor faults and major faults are
similar except that major faults force a disk access, whereas minor
faults do not result in a disk access. The runtime overhead of major
faults is significant.
Displaying a character on a terminal involves several steps of
translation. First, the character code is received as input from the
keyboard or a file. The second phase maps the character code to the
current font to determine what shape, or glyph, should be displayed on
the screen. For example, the decimal code 98 is translated into the
glyph "b" when an English font is used. However, when an Arabic or
mathematical font is used, the same code is translated to different
glyphs.
The K Desktop Environment (KDE for short) suite offers a true
graphical desktop that has advanced features, including drag-drop with
integrated HTML display. KDE can be used instead of legacy graphical
interfaces such as mwm, olwm, fvwm, and afterstep, which are based on
Unix's X Window System. You can download the KDE suite at
Pipes provide the simplest form of interprocess communication
(IPC). You can think of a pipe as a file that is shared by two
processes. In general, one process writes information to a pipe and
another reads from it. Pipes are also called "FIFOs" because their
data is organized in a first-in first-out layout.
There are two types of pipes: unnamed pipes and named ones. An
unnamed pipe is created whenever it is needed, and it is deleted when
both the writer and the reader have closed it. Unnamed pipes do not
exist in the file system (hence the name). In contrast, a named pipe
has a file name. The file name of a named pipe can be used by two
processes that do not share file descriptors to communicate through
it.
A new exams-based certification program has been initiated by Red
Hat, Inc. It is targeted at beginner to network professional levels.
Registrants will be required to demonstrate performance capabilities
in various areas, including diagnostic testing and troubleshooting,
installing and configuring Linux, performing essential administration,
and setting up common IP-based network services. It is estimated that
other vendors will follow suit. You can find more info on this
certification program at
System memory is divided into units that are called "pages." A page
consists of 4,096 bytes (4K) on Intel and SPARC architectures and
8,192 bytes (8K) on Alpha systems. A memory address is said to be
"page-aligned" if it's the address of a page beginning. Thus, on 4K
page size architecture, 4,096, 12,288, and 413,696 are instances of
page-aligned memory addresses.
Open source is one of those phrases that means different things to
different people. The Open Source Organization has defined the
criteria with which open source software must comply. These criteria
are available at
SuperAnt offers a free Pascal programming CD-ROM containing a
compiler, interpreter, and other tools for Pascal software developers.
In addition to free Pascal, it contains some files about Modula-2,
Modula-3, and Oberon programming languages. For more information about
SuperAnt Free Software compilers and interpreters, go to
VMware Virtual Platform is a software layer that allows multiple
operating systems to run concurrently on the same physical machine. It
accomplishes this by multiplexing hardware resources into multiple
virtual machines that run on the same PC. VMware under Linux simulates
a PC inside a PC and runs DOS, Windows NT, and Windows 2000. For more
information on the Virtual Platform technology, see
Linux International is a nonprofit association of groups,
corporations, and individuals who promote the Linux operating system
and the Linux community. The organization's Web site includes
introductory information on Linux, links to Linux resources, and Linux
news. You can visit the Linux International Web site at
Linux newsgroups frequently discuss the issue of Linux and Unix
differences. How different are these platforms in reality? From a
user's point of view, the differences aren't that big. In fact, all
the familiar Unix concepts, command lines, environment variables, file
system, authorizations, and applications are very much the same on
both operating systems. Indeed, Linux has ironed out a few wrinkles
and faulty specifications that exist in several Unix flavors and, in
general, it offers higher POSIX compliance. However, the crucial
difference is that traditional Unix flavors require dedicated (and
expensive) hardware, whereas Linux can run on Intel-based hardware.
The newly released GCC 2.95 is a free C/C++ compiler. GCC 2.95 has
several new optimizations, and it supports new platforms and languages
aside from C and C++. The new release offers higher ANSI/ISO C++
compliance. For example, operator new now throws a std::bad_alloc
exception when it fails, rather than returning a null pointer.
You can download GCC 2.95 from GNU's FTP server at
ftp://ftp.gnu.org
The site also provides installation instructions and extensive
documentation for GCC.
KDEVELOP 1.0--AN INTEGRATED DEVELOPMENT ENVIRONMENT
The Kdevelop 1.0 Integrated Development Environment for Unix
Systems is now available at
http://www.kdevelop.org
and will also be included in the next KDE release. KDevelop offers
many advantages to C/C++ application development, including a
wizard-based project generator and a coding environment with
syntax-highlighting, class browsing, various file viewers, and support
for easy internationalization.
MystBurn is a free CD-recording program that supports various CD
formats such as bootable CD, Iso9660, Audio CD, and more. You can
download MystBurn 1.1.3 from
The GuardianLinux line of network security systems is a security
management system offering packet filtering capabilities, IPSec
Virtual Private Network (VPN) hardware subsystem, forward and reverse
Network Address Translation (NAT), port forwarding, and proxy
services. All system software is licensed under GPL terms except for
the IPSec hardware configuration utilities, which are licensed from
the manufacturer. For more information about the GuardianLinux line of
products see
The LinuxPPC organization attempts to port Linux to the PowerPC
family of processors. The organization's Web site can be found at
http://www.linuxppc.org.
The site is now hosting the PowerPC FAQ, which is a major resource
for information about the PowerPC processor. The FAQ covers the
different types of PowerPC processors, operating systems, and
companies that work on PowerPC-related technology. The FAQ's URL is
Beowulf is a method of connecting many Linux-based computers to
share their computational powers in heavy-duty processing tasks such
as oil searching, structural analysis, and decryption. The
computational power achieved using this method can compete with
governmental supercomputers. In fact, the Beowulf method has proven to
be so efficient and useful that companies such as IBM and Compaq are
now in the process of adding it to their product lineup.
The Gtk+ Reflected Editor (GRE) is a simple bidirectional text
editor that was specially designed for languages that are written from
right to left, such as Hebrew and Arabic. The first public version of
GRE can now be downloaded from
http://publish.uwo.ca/~ualadl/grehtml/gre.html.
The download file contains the source code as well as the
executable file.
Phat Linux is a full Linux distribution that runs on an MS-DOS
partition. This eliminates the need for users to create a dedicated
partition for Linux, a process that deters many new people from using
Linux. The Phat Linux version was recently released. It now features
KDE and many popular programs pre-installed. (Phat Linux is maintained
by two students who felt that Linux was too hard to install.) The Phat
Linux Web site can be found at
The next version of Perl will be 5.6 (this version number comes
instead of 5.006 or 5.1). Release 5.6 provides many new features and
improves existing ones, including Unicode support, improved threading,
event loops, and improved compilation. You can find more info about
Perl 5.6 at
The homepage of Dennis Ritchie, the creator of C and Unix (as well
as a co-author of the legendary The C Programming Language) is
probably the best virtual museum of the C programming language. You
can find it at
http://cm.bell-labs.com/cm/cs/who/dmr
The site presents interesting historical documents and memos from
the early days of C and Unix. Amazingly enough, the original source
code files of the primeval C compiler (from 1972 or so) are available
online! The site also contains many pictures of the early Unix
machines and prototypes from the late 1960s.
GTK+ is an Open Source Free Software GUI Toolkit, primarily
developed for use with the X Window System. GTK+ enables you to
develop open software without having to pay for licenses or royalties.
You can download GTK+ 1.2 and get more information on GTK+ at
Qt (pronounced "cute") is a GUI multi-platform software toolkit
written in C++ that simplifies the task of writing and maintaining
graphical user interface applications. When developing software with
Qt, you can run it on the X Window System (Unix/X11) or Microsoft
Windows NT and Windows 95/98 simply by recompiling your source code on
the target platform. For more information on Qt, see
Code Fusion from Cygnus is an Integrated Development Environment
(IDE) for Linux application developers. Code Fusion supports C, C++,
and other languages. It is specifically optimized for Intel-based
architectures and supports all major Linux distributions. For more
info on Code Fusion, see
Samba is a software package that enables Linux to talk to the
Windows systems. You can download the entire package from
http://www.samba.org
but it's probably already included in your Linux distribution CD.
The programs in the Samba package enable you to share a directory with
Windows systems and access other Windows resources.
If you're familiar with Linux programming, you must have
encountered this error message before: "Segmentation fault (core
dumped)". What exactly does it mean?
A segmentation fault occurs when an invalid region of memory is
accessed, for example, when the program dereferences a null pointer or
when it attempts to access a memory region through a wild pointer.
Consequently, the kernel terminates the offending program and creates
a core dump file named "core" that is located in the current working
directory.
Zope is an Open Source application server and portal toolkit. It
can be installed on most Unix platforms as well as Windows NT and can
be used with most popular Web servers or its own built-in Web server.
Zope is written in Python and C and can be customized or extended by
its users.
Usually, machines run multiple concurrent TCP/IP connections.
Therefore, an IP address alone cannot provide a unique identifier for
every connection on a given machine. For this purpose, a connection is
identified by IP address and port number. A port number is a 16-bit
integer that identifies an endpoint connection on a host. Thus, the
combination of IP address and port number provides a worldwide unique
identification of a TCP/IP connection.
Port numbers range from 0 to 65535. However, Linux reserves port
numbers 0-1024. These ports are called "reserved ports" because they
may be used only by processes running as root. Ordinary processes
cannot use these port numbers.
Many common protocols such as ftp, http, and telnet are based on
"well-known port numbers": they are assigned an agreed-upon port
number, which is set by the Internet Assigned Number Authority (IANA).
For instance, http uses port 80 by default. Well-known ports enable
clients to access services easily from a server, since the particular
port number of a given service is already known. Another advantage in
using well-known port numbers is the ability to provide many services
on a single machine, when each service is identified by its port
numbers.
The GNU Image Manipulation Program (GIMP) is a freely distributed
image processing program capable of photo retouching, image
composition, and image authoring. This site contains information about
downloading, installing, using, and enhancing GIMP, including the
latest releases, plug-ins, and scripts:
Command line options come in two flavors: short options and long
options. A short option consists of a dash followed by a single
alphanumeric character. For example, the rm command, which deletes one
or more files, can take the short option -d to remove a directory:
rm -d
A long option is indicated by a sequence of two dashes followed by
a string. Thus,
rm --directory
is the long option equivalent of rm -d (both forms are
interchangeable). Long options are more readable, so Linux novices may
feel more comfortable with them.
In the mid 1980s, Unix development split into two schools: one at
the University of Berkeley and the other at AT&T Bell Labs. The
Berkeley version of Unix became known as Berkeley Software
Distribution (BSD). The AT&T version became known as System V. The two
Unix versions are incompatible in many ways. One of the most
noticeable differences between the two is their incompatible
networking interfaces. BSD uses sockets, whereas System V relies on
Transport Layer Interface (TLI). Linux adopted BSD sockets as its
networking interface. This is a fact to bear in mind when porting
legacy software that was originally written for System V Unix.
A region of mapped memory may be swapped by the kernel. When a
swapped
memory region is accessed, it first has to be re-read from a disk. In
time-critical applications, this latency can be unacceptable. To
ensure that a specific region of mapped memory is never swapped, you
can explicitly lock it. For that purpose, you use the MAP_LOCKED flag
when calling the memory-mapping function mmap().
----------------------------------------------
THE RPM PACKAGE
The Red Hat Package Manager (RPM) is an open packaging system
distributed under the GPL terms. RPM works on Red Hat Linux as well as
other Linux distributions and Unix systems. For an end user, RPM
provides many features that significantly simplify the process of
installing, uninstalling, and upgrading software packages. For a
developer, RPM packages source code for software into source and
binary packages.
You can read more about RPM and download its latest version from
http://www.rpm.org
----------------------------------------------
THE POPT LIBRARY
While the getopt() and getopt_long() functions perform the basic
operations of parsing command-line options, you can use the popt
library in applications that need more sophisticated manipulation of
command-line options. Popt is a free, open source library specifically
developed for that purpose. It enables users to add new command-line
options. In addition, it can read command-line options from any source
(such as a file). You can download the popt library from:
http://cvs.labs.redhat.com/lxr/source/popt
----------------------------------------------
THE GAWK SCRIPTING LANGUAGE
Gawk is GNU's version of the Awk scripting language, which is a
powerful tool for performing pattern matching, record processing, file
parsing, and other similar tasks. The following Web page provides
information on Gawk distributions, installation, and bug reports.
Amaya is a free browsing and authoring tool that can edit and
display
multiple documents simultaneously. Using its rich API, Amaya can be
easily extended to support new features as well. Amaya is a free, open
source tool. You can download it from
http://www.w3.org/Amaya
----------------------------------------------
THE ENLIGHTENMENT WINDOW MANAGER
Enlightenment is a free, configurable window manager for the Linux
desktop. Enlightenment enables you to control every aspect of how your
Linux desktop looks and acts. The last stable release is always
available at
http://www.enlightenment.org
----------------------------------------------
GCC EXTENSIONS: THE __FUNCTION__ MACRO
In addition to the macros __DATE__, __LINE__, and __FILE__, which
are
defined by ANSI C, GCC also defines the macro __FUNCTION__.
__FUNCTION__ is expanded as the actual name of the function in which
it is used, which makes it useful for reporting runtime errors and
assertions. For example,
void do_something()
{
if (successful != OK)
printf("failed in %s", __FUNCTION__);
else
{/*...*/}
}
----------------------------------------------
ADAPTIVE SERVER ENTERPRISE 11.9.2
Adaptive Server Enterprise (ASE) Development Kit version 11.9.2 is
now
available for Linux. The package includes the standard features of ASE
and all related connectivity components. Adaptive Server Enterprise
11.9.2 is offered free (but requires registration) from the Sybase ASE
11.9.2 download page:
Crystal Space is a free and portable 3D engine written in C++ and
distributed under GPL terms. It supports colored lights, portals,
mirrors, reflecting surfaces, 3D sprites, scripting, 8-bit, 16-bit,
and 32-bit display support, and various hardware accelerators. You can
find more detail about Crystal Space and download the latest release
from
http://crystal.linuxgames.com/
----------------------------------------------
SEEKABLE AND NONSEEKABLE FILES
Linux files can be divided into two major categories: seekable and
nonseekable. In a seekable file, read and write operations may occur
anywhere in the file. Furthermore, data can be re-read or overwritten
in a seekable file. In contrast, a nonseekable file is a
first-in/first-out data channel that does not support random access.
Data cannot be re-read or overwritten in a nonseekable file.
Sockets and pipes are instances of nonseekable files. Ordinary
files
and block devices are instances of seekable files.
----------------------------------------------
GIVE MEANINGFUL NAMES TO MAGIC NUMBERS
Magic numbers are arbitrary constant values that indicate array
bounds, maximal number of open files, the size of a memory page, and
similar environment and program-specific constants. Consider the
following example:
lseek(fd, n, 0); /* proceed n bytes from file's beginning */
The magical value 0 happens to indicate the beginning of the file
on a
particular platform. However, readers of this code have to guess what
this magic number represents. Furthermore, the use of a hard-coded
literal also compromises code portability, since on other
implementations, the file's beginning can be indicated by another
arbitrary number.
The preferred method of handling magic numbers is to give them a
meaningful name, which renders the code both more readable and easier
to maintain:
lseek(fd, n, SEEK_SET); /* SEEK_SET is defined in stdio.h */
----------------------------------------------
BLOCKED SIGNALS
A process can block incoming signals for a short time. This is
usually
done when the process is performing a sensitive task. If a signal is
sent to a process that is blocking that signal, the signal remains
pending until the process is ready again to handle it. Thus, blocked
signals are not lost.
----------------------------------------------
MYSQL GOES GPL
MySQL is a multiuser, multithreaded SQL database server. In
essence,
it is a client/server implementation that consists of a server daemon
and client programs and libraries. The latest version of MySQL,
3.20.32a, was released under GPL terms, which means that MySQL is now
an Open Source product. You can find more info about MySQL at
The init process is the first process invoked when you boot a Linux
machine. It is the only process started by the kernel, and it always
has a process ID (pid) 1. The init process is responsible for starting
other processes on the system. In addition, it automatically becomes
the parent of orphaned processes so that they don't turn into zombies.
----------------------------------------------
ORPHANED PROCESSES
A process that creates a new process is known as the parent of the
new
process. The parent process is notified when the child process ends.
When the parent process exits, its child process becomes an orphan
process. Orphan processes automatically become children of the init
process.
----------------------------------------------
CREATING A CHILD PROCESS
The system call fork() is the most popular method of creating a
child
process. Remember that unlike ordinary functions, every invocation of
fork() returns twice: once in the parent, and once in the child
process. The order of returns is unspecified. The return values
differ. In the parent process, fork() returns the pid of the newly
created child. In the child process, fork() always returns 0.
----------------------------------------------
LINUX CENTER
The Linux Center project maintains an index of Web pages about
Linux,
its applications, and general issues related to the Free Software
movement. The site also contains links to online magazines, search
engines, applications, and articles about Linux. The Linux Center Web
site is at
http://www.linux-center.org/en/
----------------------------------------------
REAL-TIME COMPUTING
The term "real-time" has become a buzzword, as in "real-time stock
quotes" or "real-time database queries." However, neither a
database-oriented system nor a NYSE stock ticker running on your
desktop is truly a real-time application. Contrary to common belief,
"real-time" doesn't mean "fast response time" or "low latency";
rather, it means a totally deterministic (predictable) response time.
If you execute the same database query twice, each execution time will
be different because of extraneous factors such as the number of users
connected to the database, system load, the distance of the disk
sector that contains the database records from the read/write magnetic
head, the size of available memory, and so on. In other words, a
database query is not a real-time operation, nor can it be.
On the other hand, a cellular phone must operate in real-time mode:
the chipset that receives the airborne digital signals must
demodulate, filter, and correct these signals within a fixed amount of
time, every time. Not on average, not most of the time, but every
time. Note that even a hundredth of second delay is unacceptable in
this case: the digital signals keep coming in. If they are not
processed in their allotted time slice, they are simply overrun by
incoming signals (think of a Web-based media player that skips frames
when displaying a video clip). To conclude, in real-time computing,
your primary concern is not average response time, but worst-case
response time.
Many operating systems, including Linux, are unsuitable for
real-time
applications because of their kernel design. In future tips, we will
explore real-time Linux variants.
----------------------------------------------
REAL-TIME COMPUTING--THE RTLINUX OPERATING SYSTEM
Linux is not a real-time operating system; it provides the best
average response time: when it is loaded down, it gracefully degrades
the performance of all running processes. In real-time computing, this
is unacceptable. RTLinux is a Linux-based operating system that
supports real-time computing. It is widely used as a desktop real-time
OS, but it is less favorable as an embedded system. RTLinux is an open
source product, distributed under the GPL terms. You can find more
info about it and download its source code from:
http://www.rtlinux.org/~rtlinux
----------------------------------------------
THE REAL TIME LINUX ORGANIZATION
The Real Time Linux Organization serves as a resource for the
real-time Linux community. It provides an arena for discussions among
users of the real-time Linux variants. The Real Time Linux
Organization Web site is at
http://www.realtimelinux.org/
----------------------------------------------
KPSQL
KPSQL is a tool similar to Oracle's Server Manager motif worksheet
and
Microsoft's SQL Server iSQL query. KPSQL is an open source product
distributed under the GPL license. Among the features of KPSQL are
syntax highlighting, support for drag and drop, HTML table output for
query results, and execution of multiple SQL statements. You can
download KPSQL and find more info about its features and requirements
at
http://www.mutinybaysoftware.com/products.html
----------------------------------------------
THE LINUXTOPIA WEB SITE
The Linuxtopia Web site was created for people who use Linux on the
desktop--newbies and experienced users alike. The main goal of
Linuxtopia is to provide a virtual community where users can learn
about Linux and find related links, news, articles, questions and
answers, and software. You can find the Linuxtopia Web site at
http://www.linuxtopia.com
----------------------------------------------
THE JED PROGRAMMER'S EDITOR
The Jed programmer's editor provides emulations for the following
editors: emacs, VAX/VMS EDT, Wordstar, Brief, and Borland's IDE. Jed
features include color syntax highlighting, folding, and drop-down
menus. Additional information about Jed is available at
http://space.mit.edu/%7Edavis/jed.html
Jed is an open source product. You can download its source code and
executable files from
ftp://space.mit.edu/pub/davis/jed/
----------------------------------------------
THE OMNIORB2 OBJECT REQUEST BROKER
omniORB2 is a CORBA 2.1-compliant object request broker (ORB)
developed by AT&T Laboratories, Cambridge. omniORB2 and its libraries
are open source products distributed under the GNU GPL terms. The C++
to CORBA mapping of omniORB2 conforms to CORBA 2.3. You can find more
info about omniORB2 and download it from
The Display GhostScript System (DGS) provides a device-independent
imaging model for displaying information and graphics on a screen,
regardless of display-specific details like screen resolution and
color issues. The imaging model of DGS uses the PostScript language,
which has powerful graphics capabilities.
DGS is composed of a PostScript language interpreter called
GhostScript, the client library, and the pswrap translator. The pswrap
translator enables you to take PostScript programs and wrap them with
a C function interface, thereby allowing your applications to call
them directly. DGS is available at
ftp://ftp.gnustep.org/pub/gnustep/dgs/
----------------------------------------------
THE BRAVE GNU WORLD ONLINE MAGAZINE
Brave GNU World is a multilingual monthly column that provides
information about internal developments of the GNU Project and GNU
community. It is available online at
http://www.gnu.org/brave-gnu-world/
Alternatively, you can subscribe to a mailing list to be informed
of
new issues as soon as they are released.
----------------------------------------------
THE MIDGARD PROJECT
The Midgard Project is responsible for the development of an open
source application--the Midgard Application Server. The product
consists of Midgard core libraries, a PHP-based Web Application Server
for Apache platform, and Web-based administration tools. For further
information and software download, visit the Midgard Web site at
http://www.midgard-project.org/
----------------------------------------------
THE COMMON UNIX PRINTING SYSTEM
The Common UNIX Printing System (CUPS) provides a portable printing
layer for various UNIX operating systems and Linux distributions. CUPS
supports both the System V and Berkeley command-line interfaces. CUPS
also includes a customized version of GNU GhostScript and an image
file RIP that can be used to support non-PostScript printers. CUPS can
be downloaded from
http://www.cups.org
----------------------------------------------
WINMODEM ON LINUX?
Although many Linux users balk at the idea of using Winmodem, it
still
has many useful applications on Linux: it can serve as an answering
machine, pager interface, phone emulation, etc. The following Web site
is dedicated to Winmodems on Linux. You can download related software,
view a list of Winmodem chipsets, and find related links:
http://linmodems.org/
----------------------------------------------
RESUMPTIVE AND NONRESUMPTIVE EXCEPTION HANDLING
In a resumptive exception handling model, after an exception has
been
handled, the program continues execution at the point where the
exception was thrown. This is how Structured Exception Handling (SEH)
works in C (note that SEH is a no-standard extension to ANSI C). On
the other hand, the C++ standard exception handling model is
nonresumptive. This means that program execution resumes at the next
statement following the catch-block. The C++ program does not return
to the point where the exception was thrown after that exception has
been handled. This is a source of confusion among C++ novices, who
mistakenly assume that the program automatically returns to the point
from which an exception was thrown, but it doesn't.
----------------------------------------------
THE PHP SCRIPTING LANGUAGE
PHP is an open source server-side, cross-platform, HTML-embedded
scripting language. It enables you to create dynamic Web pages that
are treated just like regular HTML pages. PHP's syntax is reminiscent
of C. However, it is much simpler than C and it is free from the
hassles of memory management and pointers. For more information, visit
the PHP Web site at
http://www.php.net/
----------------------------------------------
THE VIM EDITOR
VIM is an open source and enhanced version of vi, one of the
standard
text editors on UNIX. VIM features include unlimited undo, syntax
coloring, split windows, visual selection, GUI support, and more.
Besides Linux and Unix, VIM runs on many other operating systems,
including BeOS, DOS, MacOS, OS/2, RiscOS, VMS, and Windows32. You can
obtain further information about the VIM editor and download it from
http://www.vim.org/
----------------------------------------------
THE WINE COMPATIBILITY LAYER
Wine is a Windows compatibility layer that facilitates porting of
Windows sources to Unix and Linux. Wine also includes a program loader
that enables unmodified Windows 3.1/95/NT binaries to execute under
Intel Unix and Linux systems. Wine does not require Microsoft Windows,
since it consists of 100 percent free code. It supports many Windows
features, including multimedia and OLE DLLs reorganization, Clipboard,
and support for Win16 and Win32 function calls. You can find more
information about Wine and download it from
http://www.winehq.com
----------------------------------------------
PROJECT LINUX
Project Linux is aimed at Linux users and the Open Source community
in
general. Starting November 8, the organization will present the
following two Web sites: Get Linux and Open Talk. Get Linux provides
information for people new to Linux. It will include a list of
available distributions, reviews, and articles. The Open Talk section
will host forums of discussion about all things relating to open
source technologies. You can find the Project Linux Web site at
http://www.projectlinux.org
----------------------------------------------
REENTRANT AND NONREENTRANT SIGNALS
What happens when a signal is sent to a process that is already
handling that signal? In theory, the kernel can interrupt the process
and invoke the signal handler again. This approach requires that the
signal handler can function properly if it is invoked while it is
already executing. Such a handler is said to be reentrant. Writing
reliable and efficient reentrant handlers is not a trivial task.
Imagine what would happen if the signal handler locks a file, and at
that point, the kernel interrupts it and invokes that handler once
more. The second invocation will result in a deadlock because the
handler infinitely waits for the lock to be released. The lock,
however, cannot be released because the first handler call was
suspended.
In Linux, this situation doesn't occur, because the kernel
withholds
the signal until the process to which it is sent has finished handling
the previous signal (the second signal is said to be pending). Only
then is the pending signal sent to that process. This approach was
chosen to avoid the complexities of reentrant signal handlers.
----------------------------------------------
THE SAWMILL WINDOW MANAGER
Sawmill is a highly configurable window manager for Linux and
several
Unix flavors. Its major advantage over the Enlightenment window
manager is its minimal system requirements. Thus, it can run on Linux
machines that have slow processors and minimal memory.
Sawmill is distributed under the terms of the GNU GPL license. You
can
find more information on Sawmill and download it from
http://www.dcs.warwick.ac.uk/~john/sw/sawmill/
----------------------------------------------
THE QUANDARIES OF FLOATING POINT NUMBERS
People sometimes complain about the inaccuracy of floating point
arithmetic. To demonstrate that, try the following program:
#include <stdio.h>
int main()
{
float f1 = 2000.58;
float f2 = 2000.0;
printf("%f", f1 - f2);
}
On an Intel-based machine, this program prints 0.579956 instead of
0.58. More complex calculations yield higher inaccuracy. What is going
on here? First, remember that rounding, approximation, and truncation
are not the responsibility of a particular programming language or
compiler. Rather, they depend on the hardware that your machine uses.
Floating point numbers are merely an approximation based on the IEEE
standard. On most machines, the type float occupies 32 bits. The IEEE
standard for 32-bit floating point numbers requires that 1 bit be used
for the sign representation, 8 bits for exponent, and the remaining 23
bits are for the mantissa. Since a mantissa always has the form
1.nnnn..., the leading 1 can be dropped, so there are actually 24 bits
allocated for the mantissa. This 24-bit accuracy can have a deviation
of plus or minus %0.0000062 from the original number. For higher
accuracy, you can use the type double, which provides 53 bits of
mantissa. This is more accurate than 24 bits, but you can never get
absolute accuracy with built-in data types.
----------------------------------------------
LINUX C++ MAILING LISTS
The Linux C++ Mailing List was recently established. The purpose of
this mailing list is to help and support Linux programmers who use
C++. To subscribe to the list, send this e-mail with an empty subject
and message:
mailto:tuxCPProgramming-subscribe@listbot.com
You will receive a reply with further instructions on how to
confirm
your subscription, post messages, post replies, and unsubscribe from
the list.
----------------------------------------------
UNDERSTANDING COPY-ON-WRITE
When you invoke the fork() command, Linux creates a new process
that
is an exact copy of the existing one. When Linux instantiates the new
process, it uses an optimization method called "copy-on-write."
Instead of copying the memory block that contains the original
process's attributes, the new process's attributes are mapped to the
original memory block and marked as "read-only." When the new process
changes any of its attributes, it attempts to write to that read-only
memory block. Consequently, the exception handler allocates a new
block of memory, copies the data into it, marks the newly allocated
block as "write-enabled," and changes the mapping of the new process
to it. Thus, no actual copy takes place as long as the new process
doesn't change any of its attributes.
----------------------------------------------
LINUX 2000 ORGANIZATION
Linux 2000 Online is a resource site for Linux users--beginners and
experienced alike. The site contains forums, links to portals,
hardware and software vendors, and newsgroups from all over the world.
The Linux 2000 Online site is at
http://www.linux-2000.org/
----------------------------------------------
WHAT IS A SIDE EFFECT?
In standard C and C++, a side effect is defined as a "change in the
state of the execution environment." Modifying an object, accessing an
object that is declared volatile, invoking an I/O function, or calling
a function that does any of these operations are all side effects. The
validity of many operations depends on the existence of a side effect
(or lack thereof). The following are instances of side effects:
volatile int n = 0; int j = 0;
++j; /* object modification */
j = n; /* two side effects: accessing a volatile object and modifying
j */
printf( "hello world"); /* I/O function call */
----------------------------------------------
AVOID PASSING ARGUMENTS WITH SIDE EFFECTS
Many of the ANSI C functions are in fact macros in disguise.
<stdlib.h> routines, memset(), strcpy(), and other runtime library
functions are examples of such macros in disguise. You should beware
of passing expressions with a side effect as arguments to these
functions, because the results may be undefined. Consider the
following example:
char buff[12];
int n = 0;
memset(buff, ++n, sizeof(buff); /* ++n has a side-effect; bad idea */
If memset() happens to be a macro that passes its arguments to
another
function, the value of n may be incremented twice before it is used.
----------------------------------------------
THE UTOPIX WEB SITE
Utopix is a Web site dedicated to Linux and Windows users alike. It
provides news from the Linux industry and promotes the adoption of
Linux as a mainstream OS. The site offers a chat room, a forum, and
online polls. You can find Utopix at
http://utopix.cjb.net/
----------------------------------------------
THE REBOL PROGRAMMING LANGUAGE
REBOL (Relative Expression-Based Object Language) is a new,
portable,
general purpose, and efficient programming language for
intercommunications and interoperability. The language supports almost
40 hardware platforms, including Win32, Solaris, various Unix flavors,
Macintosh, and Linux. REBOL has a simple and intuitive structure with
English-like syntax. You can find further info on REBOL at
http://www.rebol.com
----------------------------------------------
THE GEEKNEWS ORGANIZATION
The Geeknews site is devoted to reporting Linux-related technology
news from around the Web. The site includes news flashes, reviews, and
downloads of open source software tools. The Geeknews Web site is at
http://www.Geeknews.org
----------------------------------------------
THE DIFF3 COMMAND
The diff3 is useful when two people change a common file and create
two independent versions thereof. Diff3 compares the original file, as
well as the two files derived from it, all at once. Diff3's syntax is
diff3 derived1 originalfile derived2
Where originalfile is the common ancestor from which derived1 and
derived2 are derived.
----------------------------------------------
LINUX VIRTUAL SERVERS
A virtual server is a scalable and highly available server built on
a
cluster of real servers. The architecture of the cluster is
transparent to the end users, who see only a single virtual server.
Thus, a virtual server offers a very high computation power as well as
exceptionally high fault tolerance. You can find more info on this
technology and its uses at
http://www.linuxvirtualserver.org/
----------------------------------------------
THE LINUX MICRO-CONTROLLER PORT
The uCLinux (Linux Micro-controller, the lowercase u stands for the
Greek letter "mu") project is a port of Linux 2.0 to embedded systems
that don't have a memory management unit. The end goal of this project
is to port Linux to handheld devices and embedded controllers that do
not have a built-in memory management unit. uCLinux is an Open Source
product distributed under the GNU Public License. For further info,
visit the uCLinux Web site at
Device drivers can be compiled and linked directly into the kernel.
This type of driver is called a statically linked device driver. Such
a driver is integrated into the kernel image and remains attached to
it until the next time the kernel is rebuilt. This is not the
preferred technique of writing device drivers because you need to
recompile, relink, and reinstall the kernel every time you want to
add, remove, or update a device driver. Still, in older versions of
Linux, this technique was rather popular.
A Loadable Kernel Module (LKM) can be added and removed while the
system is running. Modern device drivers are written as LKMs. This
form is a significant improvement over statically linked kernel device
drivers because it eliminates the need to recompile, relink, and
reinstall the kernel every time you modify any of its supported device
drivers. Another advantage of LKMs is that they are not subjected to
the GPL licensing rules because they are not linked into the kernel
image.
----------------------------------------------
LINUX KERNEL CRASH DUMP
The Linux Kernel Crash Dump project attempts to provide a more
reliable method of examining system failures after the machine
recovers. The project contains kernel and user level code that is
designed to save kernel memory information when the system crashes due
to a software failure. When the system is rebooted, the saved kernel
memory information can be recovered and analyzed to determine the
cause of the failure. The documentation, source code, and RPMs of
Linux Kernel Crash Dump are now available at
http://oss.sgi.com/projects/lkcd/
----------------------------------------------
THE RETURN STATUS OF MAIN()
When main() exits, either implicitly or explicitly (that is, by an
explicit call to exit() or due to a return statement), it returns to
the environment a status code that indicates successful or failed
termination. Remember, though, that the return code is truncated to
eight bits. Therefore, you should return only values in the range of
-127 through 127.
----------------------------------------------
BASIC INLINE ASSEMBLY
Standard C and C++ enable you to embed inline assembly code in your
program. The reserved keyword "asm" takes a quoted string that is a
native assembly directive. For example:
asm ("nop");
is a no-operation assembly directive, and
asm ("cli");
disables all interrupts. The __asm__ token is identical to asm. You
can use __asm__ instead of standard asm if the latter conflicts with
other identifiers in your program. Still, it is best to stick to the
standard asm keyword whenever possible to ensure compatibility with
other compilers and UNIX versions.
----------------------------------------------
DYNAMIC ALLOCATION OF STACK MEMORY
The function alloca() allocates memory at runtime. Note, however,
that
unlike malloc() and calloc(), alloca() allocates memory from the
stack, not from the heap. This is advantageous because the allocated
memory is automatically released when the function that invoked
alloca() exits. In the following example, alloca() is used to create a
memory buffer that is automatically released when the function exits:
int f()
{
char * p = (char *) alloca(12); /*allocate 10 bytes*/
if (p!=NULL)
strcpy(p, "hello world");
} /*memory is reclaimed automatically*/
Note that alloca() is not defined by ANSI C.
----------------------------------------------
BENEFITS OF RECOMPILING YOUR KERNEL
When upgrading your compiler, you may consider recompiling your
kernel. The new compiler version may provide better optimizations and
bug fixes so the overall performance gain can be significant. Note
that this tweak is recommended for experienced users and that you
should follow the documentation regarding kernel compilation that is
included in your Linux distribution.
----------------------------------------------
THE KDBG GRAPHICAL DEBUGGER INTERFACE
KDbg is a graphical debugger interface to the GNU gdb debugger. It
requires the KDE runtime libraries. Major features of KDbg include
inspection of variable values in a concise tree structure, debugging
of core dumps, attaching to running processes, conditional
breakpoints, and function keys for frequent debugging commands. KDbg
supports C and C++ and is an Open Source product. KDbg is available at
http://members.telecom.at/~johsixt/kdbg.html
----------------------------------------------
IMPLEMENTING A DEVICE DRIVER AS A SHARED LIBRARY
In addition to statically linked kernel device drivers and loadable
kernel modules, you can implement a device driver as a shared library.
This is useful when the driver has specific timing requirements or
privileges.
----------------------------------------------
THE LINUXONE DISTRIBUTION
LinuxOne Lite is a simplified version of Linux that has been
especially crafted for first-time users. It is targeted at curious PC
enthusiasts, educators, and students who are looking for a Linux
version that is easy to set up and use. For example, unlike other
distributions, LinuxOne Lite doesn't require that users know how to
partition their hard disk. For further info and a free download, see:
http://www.linuxone.net
----------------------------------------------
PSEUDO CHARACTERS IN NCURSES
Ncurses is an Open Source implementation of the traditional UNIX
curses library. It provides a uniform, simple, and high-level
interface for controlling the screen and capturing mouse and keyboard
input. Ncurses beginners should notice that many of the library's
functions take parameters of the type "chtype", rather than plain
char. Such a character is also called a "pseudo character" because it
is represented as an unsigned long integer, in which the low order
eight bits hold the original character and the higher order bits carry
additional information, such as video attributes.
----------------------------------------------
THE TINY COBOL PROJECT
The tiny COBOL project attempts to develop an open source COBOL
compiler for Linux that is as close as possible to the COBOL-74 ANSI
standard. For more information about this project, visit:
http://tiny-cobol.sourceforge.net/index.html
----------------------------------------------
DEFINING AN ALIAS
To avoid typing a long command that you use frequently, define an
alias for it. For example, the following line:
$ alias chkcode = 'gcc --fsynatx-only test.cxx'
defines an alias chkcode, which is synonymous with the long-winded
"gcc --fsynatx-only test.cxx" command.
----------------------------------------------
ADDING SHORTCUTS TO YOUR .PROFILE FILE
The bash command shell automatically executes the .profile file
that
is located in the user's default directory upon log in (.profile is
similar to the autoexec.bat file in DOS). You can add aliases,
environment variables, and hard links to your .profile file so that
they remain defined permanently.
----------------------------------------------
FESTIVAL SPEECH SYNTHESIS SYSTEM
Festival is a general framework for building speech synthesis
systems,
including full text to speech. The framework offers several APIs:
shell level, a Scheme command interpreter, a C++ library, an Emacs
interface, and others.
The Festival system is written in C++ and is distributed freely
with
full documentation and source code. For documentation and downloads,
see
----------------------------------------------
THE KERNEL JOURNAL
The Kernel Journal tracks announcements that are posted on the
Linux-kernel mailing list, which is the primary forum for Linux kernel
development. The Kernel Journal lists projects in order of the date of
their most recent announcement; it briefly describes the project and
its history of announcements, and it provides a URL from which the
source files of that project can be downloaded. The Kernel Journal
page can be found at:
http://www.linuxcare.com/developers/kernel/
----------------------------------------------
SWAPPING A RUNNING TASK FROM THE FOREGROUND INTO THE BACKGROUND
Suppose you've just started to run a long process (a compilation of
a
huge program, for example) and you forgot to append an ampersand to
that command. The task will run in the foreground, which means that
you cannot type other commands in the shell prompt until that task
finishes. You don't have to abort that task or wait for it to finish.
Simply redirect the running task to the background as follows: press
Ctrl-Z, then type
bg
(for background), and press Enter. The command prompt will appear
again, and the task will continue to run in the background.
----------------------------------------------
ABIWORD--A FREE WORD PROCESSOR FOR LINUX
AbiWord is a cross-platform word processor with a graphical user
interface that can import Word files. It is available on Linux, UNIX,
Win32, BeOS, and MacOS. AbiWord is an Open Source product distributed
under the GNU GPL terms. You can find more info on AbiWord and
download it from:
The bash command shell uses different prompts to indicate what your
user privileges are. For example, the default prompt for a root user
is #, whereas a regular user's default prompt is $. You should pay
attention to the bash prompt you get to avoid inadvertent deletion of
files when you're logged in with super user privileges--and, in
particular, after executing the su (switch user) command.
Intrusion detection is a relatively new methodology in the science
of
system security. In essence, it is based on the use of automated and
intelligent software tools that detect intrusion attempts in real-time
and then react accordingly. An Intrusion Detection System (IDS) can do
one or more of the following when it detects such an attempt:
- Page the system administrator.
- Distribute misinformation. Linux can pretend to be an NT server,
for
instance, and mislead the attacker.
- Act according to a set of predefined rules, for example:
"terminate
all processes and shut the system down when the attacker has a certain
IP address."
Linux regularly monitors the activity of every user and process and
logs that information. Crackers sometimes try to alter the system logs
to hide their malicious activity. chkwtmp is a software tool that
detects such alterations in wtmp and reports deleted entries in it.
You can find more info on chkwtmp and download it from:
Pluggable Authentication Modules (PAM) is a software library that
was
specifically designed for configuring authentication on a system. The
PAM library is implemented on Linux and most flavors of Unix. It has a
standard and rather intuitive interface for changing authentication
information (such as a user's password) and authentication checks. You
can read more about the PAM specifications and download the source
code from:
http://www.kernel.org/pub/linux/libs/pam
----------------------------------------------
TCP WRAPPERS AND NETWORK ACCESS CONTROL
TCP Wrappers is one of the most widely used toolkits for enforcing
network access control. If you own a recent Linux distribution
version, you probably already have TCP Wrappers installed on your
system. TCP Wrappers enables you to allow users and hosts to connect
to one another selectively. For example, you can force a user to
connect to a server from a particular machine by forcing him or her to
have a particular IP address. By combining this restriction with the
general username and password protection, you can get a higher level
of system security.
The bash shell command supports autocompletion. To complete the
full
name of a file that is located in the current directory, simply type
the first letters of the file that uniquely identify that file. For
example, suppose you have the following three files in the current
directory:
nightingale
knight
nite
and you want to delete the file nightingale. First, type
$ rm nig
and press Tab. Because nightingale is the only file in the current
directory that starts with the characters nig, the command shell
automatically extends what you've just typed to
$ rm nightingale
Now you can press Enter to delete that file.
----------------------------------------------
THE ENTROPY POOL
The entropy pool stores random information from external events
such
as mouse clicks and keyboard keypresses. These random data can later
be used by programs and scripts that need truly random and
nonreplicable numbers. You can read the random numbers in the entropy
pool from dev/random and dev/urandom devices. Whenever you read from
any of these devices, a hash algorithm first manipulates the random
data in the entropy pool and returns the resultant numbers. The random
driver's code (located at drivers/char/random.c) contains extensive
documentation on the generation of random numbers and the hash
algorithm.
----------------------------------------------
TODAY: Reflections on Multithreading
by Danny Kalev
While it is true that a multithreaded app is usually more
responsive than a single-threaded one, programmers new to
multithreading are often misled into thinking that a multithreaded
app executes faster than a single-threaded one. This is not always
the case, however. On a machine that has a single CPU, adding
threads will usually increase the overall processing time. This is
because the CPU instructions required to perform the scheduling and
switching between threads add more overhead in terms of execution
speed and application size.
Of course, in interactive applications such as word processing, Web
browsers, and graphical editors, multithreading is unavoidable;
otherwise, the application may become annoyingly nonresponsive.
However, processes that always run in the background usually
execute faster as multithreaded applications on machines that have
a single CPU. In some cases, even interactive applications can do
well without threads if they run on a machine that has a single
CPU. For instance, a text-based interactive calculator that accepts
an expression and displays the result doesn't necessarily have to
split the user interface logic and the number-crunching task into
two separate threads. The time wasted in performing the context
switch between those threads is probably longer than the time
needed for parsing the expression and computing the result.
Likewise, for applications that only run in the background, the use
of threads may not be necessary.
Only machines that have two or more processors offer true parallel
processing. In this case, the overall processing time can be
reduced by allocating a distinct processor to each thread.
Settling Into Gaming
by Eric Foster-Johnson
Continuing on gaming topics, a new game called Settlers of Catan
can
add to your Linux leisure activities. Settlers presents a multiplayer
strategy game similar to Civilization: Call To Power, from Activision
and Loki Entertainment Software. In Settlers, you build settlements
and cities in an attempt to dominate a remote island. You can pick
from
at least two Linux versions of Settlers, which was originally
developed as a board/card game by Klaus Teuber.
The first version is available from http://settlers.seul.org, and
is
one of the projects hosted under the SEUL, or Simple End User Linux
site at http://www.seul.org.
You can download the source code to Settlers, or a binary version
in
Red Hat Package Manager (RPM) format, from the site. This version
requires libsx, which is also available at the site in RPM format.
If you run a version of Linux that uses RPM natively, such as Red
Hat,
you can install libsx and then Settlers with no problems. For other
versions of Linux, you'll likely need to do more work.
With the binary RPM version of Settlers, the game-executable
programs
are located in /usr/games. If you build from the source code, you can
specify your own installation directory. The available documentation
is minimal, but enough to get you started.
This version of Settlers comes with two programs, SettlerServer,
which
runs the game, and SettlerClient, which individual players use to play
the game. You need at least one SettlerServer running to manage the
game, and two or more players, each running a copy of
SettlerClient. While you can play this game all on one system, this
isn't as much fun as playing with other users over a network.
To get going, you need to run the SettlerServer. Use a command such
as:
/usr/games/SettlerServer -p number &
Replace number with the number of players, from 2-4. Note that you
need at least two players to make a game, and if you connect up only
one player, you'll find the game interface just sits there
waiting for other players to connect.
Each player then needs to run SettlerClient using a command such
as:
/usr/games/SettlerClient &
The SettlerClient requires you to run the X Window System
for graphics. You'll be prompted for the hostname of the system
running the SettlerServer program, as well as for the name you want to
use when playing.
In addition to the SEUL version, you can also get a GNOME version
of
the same game, called gnocatan, which is available at
http://www.gnome.org/gnocatan.
Note that there appears to be no real connection between these
versions of the game.
If you like this game, you may wish to try Mayfair Games, at
http://www.coolgames.com, which sells the original board game.
C programmers often need to use callbacks. In this article I
discuss the fundamentals of function pointers and show how to use them
to implement callbacks. This discussion is a preview to next week's
newsletter, which will show how to write a program that sorts a huge
file.
A callback function is one that isn't invoked explicitly by the
programmer; the responsibility for its invocation is delegated to
another function that receives its address. To implement a callback,
you need to define an appropriate function pointer. Although the
syntax is a bit arcane, a function pointer declaration is very similar
to a function declaration. For example, the following declaration:
void f();
declares a function called f that takes no arguments and doesn't
return a value. A pointer to such a function has the following type:
void (*) ();
Let's parse it. The asterisk in parentheses is the nucleus of a
function pointer declaration. Two additional constituents are the
function's return type, which appears on the left and is "void" in our
example, and a parameter list enclosed in the rightmost parentheses.
In our case, the parameter list is empty because f() takes no
arguments. Note that we didn't create a pointer variable here -- we
only declared the type of such a variable. A pointer variable, of
course, has a name. Here is an example of a matching pointer variable:
void (*p) ();
p is a pointer to a function that returns void and takes no
arguments. The name of a pointer variable appears on the right of the
asterisk, inside the parentheses. We now assign a value to p. A value
is simply a name of a function that has a matching signature (a
function's signature consists of its return value and parameter list),
for example:
void func()
{
/* do something */
}
p = func;
You can assign a different value to p as long as that value is the
address of a function with the same signature.
Now we can pass p to another function, caller(), which will call
the function to which p points:
void caller( void(*ptr)() )
{
ptr(); /* call the function to which points */
}
int main()
{
p = func;
callback(p); /* pass address of func() */
}
If you assign a different value to p, caller() will invoke a
different callback function, without having to know the name of the
actual function being called.
Let's say you need to sort a large file containing 1.6 million
records with 252 characters each (the file's size exceeds 400
megabytes). The file is encoded in EBCDIC (the character coding used
on IBM mainframes instead of ASCII) and has to remain in this format.
You can't use the "sort" command because it's inapplicable to EBCDIC
files. Thus, there's no escape from writing a program to sort the
file. Although writing a program that reads the entire file into a
buffer and then performs in-memory sorting is a trivial task, this
option is ruled out due to the file's size. Therefore, a different
technique must be used. Instead of reading complete records into
memory, the trick is to use a handle to represent each record. A
typical handle is int, which occupies four bytes per record instead of
252.
Now suppose you have exactly 1,600,000 records. First, allocate an
array of 1,600,000 integers -- let's call it arr -- and fill it with
ascending numbers: 0, 1, ....1,599,999. Each array element will
represent a record. Next, use qsort() to sort the array. Qsort() takes
four arguments: the address of the array's first element, the total
number of elements in the array, the size of an element, and most
important -- a pointer to a user-defined callback function that
compares two elements and returns 0 if the elements are equal, a
positive value if the first elements should come after the second, or
a negative value otherwise.
Here's the second trick: don't compare the two integers received
from qsort() in your callback function. Instead, read the records with
the corresponding indices, i.e., for the integers 100 and 101, your
function will retrieve the 100th and 101st records from the file
(using fseek() and fread()), compare these records, and return 0 if
they are equal, a positive value if record 100 should come after
record 101, or a negative value otherwise. When qsort() ends, arr[0]
will contain the index of the record that should come first in a
sorted file, arr[1] will contain the index of the subsequent record,
etc. Note that up until now, the file has remained intact; we
manipulated only the array. To actually sort the file, open it again
and create another empty file. Write a loop that traverses the entire
array and, on each iteration, read arr[n], retrieve the nth record
from the nonsorted file, and copy that record to the new, empty file.
Your task is accomplished after you've reached the last element in
arr.
*Note: The last code sample in the previous newsletter had a
misspelled function name. The function name 'callback' should be
changed to 'caller'. Here's the correct version:
int main()
{
p = func;
caller(p); /* pass address of func() */
}
Currently, there are at least four Web browsers that run on Linux.
This column is a quick overview of those browsers and their major
advantages and weaknesses.
- Netscape -
Netscape is included with most Linux distributions. The latest
version, Communicator 4.7, supports the latest standards, including
HTML 4.0, XML, and CSS. However, its resource-gluttony and instability
are two noticeable weaknesses. For acceptable performance, you need at
least 64Mb of RAM and a Pentium 233 or higher. Netscape still crashes
occasionally, although recent versions have shown continuous
improvements in this respect. The latest version is available from
http://www.netscape.com.
Pros: fully-featured, supports the latest standards, free, includes
a mail client.
Cons: bulky, resource hungry, still unstable.
- Mozilla -
The Mozilla project has been working, over the past couple of
years, on an Open Source Web browser. Originally, Mozilla relied on
Netscape's legacy browsing engine, but later it was abandoned in favor
of an entirely new engine written from scratch. Thus, Mozilla i
independent of Netscape. M13, the latest release, is still an alpha
version, yet fully-featured. Future releases, however, will add
missing functionality. The latest binaries and source files are
available from
http://www.mozilla.org.
Pros: free, open source, graphical, written from scratch; includes
a mail client.
Cons: bulky, still unstable, not fully-featured.
- Opera -
Opera offers a small, fast, and efficient browser that performs
satisfactorily, even on old PCs with vintage processors and little
memory. The latest release, Opera for Linux 4.0a, is still an alpha
version. While it offers an interesting alternative to the more
cumbersome mainstream browsers, it's not free: a registered copy costs
$35. Still, Opera's creators claim that the product offers
incontestable qualities in terms of compactness and speed. You can
download a 30-day trial version from
http://www.opera.com
and decide whether it's worth the price.
Pros: small, fast, can run on old PCs.
Cons: costs money, still a preview version.
- Lynx -
Lynx, a text-based Web browser, is included with most Linux
distributions. What's the use of an age-old browser that displays only
text without graphics? For some folks, this is a plus -- many sites
are replete with ads, banners, and animation that take forever to
download and consume precious bandwidth and connection time. Lynx
filters out non-textual contents such as applets, images, animation,
and audio files, thereby significantly decreasing download time.
Pros: free, small, fast, and secure.
Cons: displays only text, command line operated.
Although the most convenient method of transferring a binary file
is to send it as an email attachment, there are certain conditions for
which you may need to use an alternative technique. To send and
receive an attachment, both the sender and the recipient's mail
clients must support the MIME protocol. However, legacy mail utilities
such as Unix "mail" don't support this protocol. Furthermore, for
security reasons, some gateways deliberately block binary file
attachments, allowing only plain-text messages to be sent through
them. Thus, even if both the sender and handler can handle a binary
file, it might still be impossible to send it as an attachment.
In situations like these, you can send the binary file by email
using uuencode translation. The uuencode utility creates an
ASCII-encoded version of the original binary file. The resultant ASCII
file is about one third larger than the original. You can view a
uuencode file in vi, emacs, or any similar text editor. Because a
uuencode file contains plain text, you can send it to a mail client
that can't handle attachments. The recipient, in turn, will decode
this file to reconstitute the original binary file. For this purpose,
the uuencode and uudecode commands are used.
Here's an example of this process: Suppose you want to send a
compressed file archive called myapp.tar.gz to
bustopher.jones@cats.com
and you know that Bustopher doesn't have a mail client that supports
attachments. First, uuencode myapp.tar.gz like this: (See the code in
EXAMPLE 1 at the following url)
Let's parse this command carefully: the first parameter is the file
being uuencoded. The second parameter is the name the reconstituted
file will have when the recipient uudecodes it (you can use any name
you like). Finally, the greater-than sign writes the output into a
file called myapp.uue. Note that, by default, uuencode files have a
.uue extension.
Once you have created the uuencode file, you can send it to
Bustopher like this: (See the code in EXAMPLE 2 at the following url)
This command will reconstitute the file myapp.tar.gz from myapp.uue
and store it in the current directory. Remember that uuencode does not
encrypt files; it merely converts them to ASCII so they can be
transferred as plain text. If you wish to protect your emails from
eavesdroppers, you should also use a dedicated encryption utility.
My previous column discussed filename auto-completion. In this
column, I will show two more useful techniques that can save you a lot
of keystrokes -- namely filename wildcards and command history.
- Wildcards -
The bash command shell accepts two characters as filename wildcards --
an asterisk and a question mark. You can use these wildcards when you
need to perform operations on a group of files, e.g., copying,
compressing or deleting a directory. The ? character matches any
single character in a filename. Suppose the current directory contains
the following files:
inventory.tmp
inventory.bmp
inventory.dat
You wish to delete inventory.tmp and inventory.bmp at once, while
retaining inventory.dat. Type the following command:
$ rm inventory.?mp
Consequently, the shell command will delete the files inventory.tmp
and inventory.bmp. If you wish to delete any file called "inventory"
that has a three letter extension, use the following command:
$ rm inventory.???
The wildcard * is less restrictive than ?. It matches any number of
characters (including zero). For instance, if the current directory
contains the following files:
vir.exe
virus.exe
dialer.exe
shapes.plx
clip.mov
then, to delete the files vir.exe and virus.exe, type the following
command:
$ rm vir*.exe
To remove all the files in the current directory, use the following
command:
$ rm *.*
- Command History -
The bash command shell remembers the commands you've typed recently.
You can retrieve these commands by pressing the up and down keys.
Pressing the up key retrieves the most recent command you typed;
repeat this step to navigate the contents of earlier commands. Command
history retrieval is useful when you want to repeat the same operation
several times. You may also edit the contents of the previously typed
commands using the right and left keys. For example, to uudecode the
following files in a sequence:
picture1.uue
picture2.uue
First type the following command:
uudecode picture1.uue
The uudecode utility will decode the file. Next, press the up key.
The previous command will appear again. Move with the right key until
the cursor is located on the number 1, type "2", and the command will
change to:
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