1. How to use the the font in the application? Any code example?
2. How to install the font for linux?
3. How to make the installed font as the system default font?
4. How font/text is rendered in linux? Any inforamation about the font system in linux?

Thanks.

Maybe start from here: http://fontconfig.org/fontconfig-user.html .

Moved to Linux-Newbie (not a programming question)

xh*;
You will have to give us a lot more information: eg what version of Linux, what applications you are using, etc. And please tell us the context---ie what overall problem are you trying to solve?

I want to change the font used in my application.
for example, change another font or change the font size, etc.

What application?

For example, in a word processor, you simply click in the menu to change the font.

You really need to give us more information.

I am programming my own application. and in my own application, I want to change the font. Is this clear?!
For example, In QT, I can change the font like this:

QFont font( "Times" );
font.setPointSize( 24 );
font.setWeight( QFont::Bold );
font.setUnderline( TRUE );
xxx->setFont( font );

How to do this in linux?

and I think I have give enough information.

I think of Qt as a graphical toolkit to be used with a programming language such as C. By contrast, Linux is an operating system. Thus--saying "If I do XYZ in Qt, then how do I do it in Linux" does not make sense.

So--are you writing an application to run on Linux?

Are you programming in C? (or maybe C++, Python, or some other language?)

I am writing in C++ for linux.
If you do not know, do reply any more. I think it is quite clear, just you do not understand.

and why move this programming question to this newbie section. it is not suitable.

Excuse me--you will not get good help from anyone here if you take that attitude.

First, your original post gave NO CLUE that you were writing an application. Now that you have told us that, this thread will go back to Programming.

Second, you need to provide complete information. Allow me to guess what you are doing:
You are writing an application in C++ using the Qt graphics toolkit. If that's correct, then it seems you already have the necessary commands. If you are NOT using Qt, then what graphics toolkit ARE you using?

Put your energy into clear communication, and not into little retorts such as the one quoted above.

AND--How about acknowledging the help offered from Sergei??

Linux itself has no concept of fonts — it doesn't even have a GUI!

The GUI is provided by a separate userspace program, the X Window System (aka "X"). So you're really asking about X fonts, not "linux fonts", which don't even exist.

There are also console fonts.

But I'm almost certain the OP is talking about a GUI.

Hint for the OP: if people are trying to read your mind like this, then you weren't clear enough.

Another notion to understand is desktop theme and how applications select one - IMO this is the key to fontset per applications.