Linus Benedict Torvalds: Father of Linux OS

Linus was born in Helsinki, Finland, the son of journalists Anna and Nils Torvalds, and the grandson of poet Ole Torvalds. His family belongs to the Swedish-speaking minority (roughly 5%) of Finland's population. Torvalds was named after Linus Pauling, the American Nobel Prize-winning chemist, although he claims he was named after Linus in the Peanuts comic strip. In the book Rebel Code: Linux and the Open Source Revolution, Torvalds is quoted as saying, "I think I was named equally for Linus the peanut-cartoon character," noting that this makes him "a mix of 50 percent Nobel-prize-winning chemist and blanket-carrying cartoon character." Both of his parents were campus radicals at the University of Helsinki in the 1960s.Torvalds attended the University of Helsinki from 1988 to 1996, graduating with a master's degree in computer science. His M.Sc. thesis was titled Linux: A Portable Operating System. From 1997 to 1999 he was involved in 86open helping choose the standard binary format for Linux and Unix.His interest in computers began with a Commodore VIC-20. After the VIC-20 he purchased a Sinclair QL which he modified extensively, especially its operating system. He programmed an assembler and a text editor for the QL, as well as a few games. He is known to have written a Pac-Man clone named Cool Man. In 1990 he purchased an Intel 80386-based IBM PC and spent a few weeks playing the game Prince of Persia before receiving his Minix copy which in turn enabled him to begin his work on Linux.About 2% of the current Linux kernel is written by Torvalds himself[citation needed]. Since Linux has had thousands of contributors, such a percentage is very respectable. Torvalds remains the ultimate authority (aka Benevolent Dictator for Life) on what new code is incorporated into the Linux kernel.Torvalds owns the "Linux" trademark, and monitors use of it chiefly through the non-profit organization Linux International. Linux's wide and passionate userbase makes trademark abuse difficult as misuse is rapidly detected.

Why do a lot of #defines in the Linux kernel use do { ... } while(0)?

There are a couple of reasons:

• Empty statements give a warning from the compiler so this is why you see #define FOO do { } while(0).
• It gives you a basic block in which to declare local variables.
• It allows you to use more complex macros in conditional code. Imagine a macro of several lines of code like:

#define FOO(x) \
printf("arg is %s\n", x); \
do_something_useful(x);
Now imagine using it like:

if (blah == 2)
FOO(blah);

This interprets to:

if (blah == 2)
printf("arg is %s\n", blah);
do_something_useful(blah);;

As you can see, the if then only encompasses the printf(), and the do_something_useful() call is unconditional (not within the scope of the if), like you wanted it. So, by using a block like do { ... } while(0), you would get this:

if (blah == 2)
do {
printf("arg is %s\n", blah);
do_something_useful(blah);
} while (0);
Which is exactly what you want.

If you want a block statement so you can have several lines of code and declare local variables. But then the natural thing would be to just use for example:

#define exch(x,y) { int tmp; tmp=x; x=y; y=tmp; }
However that wouldn't work in some cases. The following code is meant to be an if-statement with two branches:

if (x > y)
exch(x,y); // Branch 1
else
do_something(); // Branch 2

But it would be interpreted as an if-statement with only one branch:

if (x > y) { // Single-branch if-statement!!!
int tmp; // The one and only branch consists
tmp = x; // of the block.
x = y;
y = tmp;
}
; // empty statement
else // ERROR!!! "parse error before else"
do_something();

The problem is the semi-colon (;) coming directly after the block. The solution for this is to sandwich the block between do and while (0). Then we have a single statement with the capabilities of a block, but not considered as being a block statement by the compiler. Our if-statement now becomes:

if (x > y)
do {
int tmp;
tmp = x;
x = y;
y = tmp;
} while(0);
else
do_something();

AJAX and XML Response

If you want to return XML as a response to AJAX request, there are few things which needs to be taken care of:

1) Set the content type of the response as xml:
response.setContentType("application/xml");
Otherwise function xmlHttpRequest.responseXML would return a null object and the XML you are trying to return as response would be returned as String object rather than a XML object and you can access that by xmlHttpRequest.responseText.

2) Construct a well-formed XML which you are going to return in response, make the XML as standalone :
returnXML = "<?xml version='1.0' encoding='UTF-8' standalone='yes'?>
<root>
<allxmlelements></allxmlelements>
</root>";
Important: If the XML constructed above is not well formed you won't be able get the correct response back when you try to access the returned XML in JavaScript using:
var rspXml = xmlHttpRequest.responseXML;

Q: How to check if the XML you are returning in AJAX response is correct/well-formed or not?
A: Best way to check whether the returned XML is well formed or not is to type the URL being passed in the AJAX request directly into the browser. If the response XML is well formed then browser would display the complete XML or else it would display error in the XML. (This is the best and easiest way to check the response.)


Example: in an STRUTS application when the AJAX request is processed in an Action class you can return the XML using code below;
response.setContentType("application/xml");
response.getWriter().write(returnXML);
return (null);

Note: In a STRUTS application for an AJAX request to complete successfully, the execute method of the Action class being called should return "null" instead of an ActionForward.

This article has now moved to my new BLOG.