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分类: C/C++

2009-07-25 13:40:44

Linux 2.2 Framebuffer Device Programming Tutorial
This is a quick document I produced while figuring out how to use the framebuf
fer device to produce graphics on the Linux console. I don't claim to be an ex
pert in this area, but when I looked for a general guide to doing this, I foun
d absolutely nothing, except the header file

Therefore, this is meant to be a jump-start guide for those who want to begin
using graphics in the linux console. Hopefully someone out there will write a
decent graphical browser, so I don't have to start up X every time I want to s
urf!

As always, no warranty, express or implied...etc. Give me a break, I took my f
irst look at this device less than 24 hours ago! Any corrections, money, gifts
, etc. (I wish!), just contact me
You may freely distribute this document, as long as you do not change it in an
y way, and credit me as author. In particular, I encourage Linux distributions
to include this in their packages, as I feel there isn't half as much program
ming information in most distributions as there ought to be. Copyright James D
abell, May 1999.
This document was last updated June 11th, 1999. (Corrections from Joerg Beyer
& Ian Thompson-Bell)

Part One:

First of all, configure your system for the framebuffer console device. It may
come in handy. Note that the device is only available for graphics cards that
implement VESA 2.0. Fortunately, virtually all recent cards out there do this
.

When you have got high-res textmodes, then you can start experimenting. You wi
ll have a device /dev/fb0 that you can look at like any normal file. To take a
screenshot, all you have to do is

cat /dev/fb0 > ~/sshot

And you will have a pretty big file with the contents of your graphics card's
memory inside. Now, if you clear the screen, and type

cat ~/sshot > /dev/fb0

You should have a display that looks exactly like before. Of course, the secon
d you start typing the display reverts to normal.

Part Two:

So now, we can move on to using the device in a C program. Take a look at the
following code:


代码:
#include
#include
#include
int main()
{
        int fbfd = 0;

        fbfd = open("/dev/fb0", O_RDWR);
        if (!fbfd) {
                printf("Error: cannot open framebuffer device.\n");
                exit(1);
        }
        printf("The framebuffer device was opened successfully.\n");

        close(fbfd);
        return 0;
}
All in all, a pretty simple program. We open the framebuffer device file using
the low-level open(), because the buffered, higher level fopen() is unsuitabl
e for our needs.
Next, we test the file descriptor to make sure that we were successful, and pr
int a message out that tells the user what happened.
Note that if you #include , then you will be able to find out exactly
why the open() call fails. Check the man page for open() for more information
on this
Finally, we clean up by closing the file descriptor and returning.

Part Three:

Using the newly created file descriptor, we can perform operations on the fram
ebuffer with the ioctl() function. This function is used to talk to special de
vice files in ways that are particular to that device. In this case, we can us
e it to obtain information on the video card. We can also use the file descrip
tor to map the file into memory, and use a pointer to access it, which is more
efficient and easier on us. This is done using the mmap() function. If you ha
ven't used this before, here is some sample code:


代码:
#include
#include
#include
#include
#include

int main()
{
        int fbfd = 0;
        struct fb_var_screeninfo vinfo;
        struct fb_fix_screeninfo finfo;
        long int screensize = 0;
        char *fbp = 0;
       
        fbfd = open("/dev/fb0", O_RDWR);
        if (!fbfd) {
                printf("Error: cannot open framebuffer device.\n");
                exit(1);
        }
        printf("The framebuffer device was opened successfully.\n");

        if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)) {
                printf("Error reading fixed information.\n");
                exit(2);
        }

        if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {
                printf("Error reading variable information.\n");
                exit(3);
        }

        screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;
        fbp = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED,
fbfd, 0);
        if ((int)fbp == -1) {
                printf("Error: failed to map framebuffer device to memory.\n")
;
                exit(4);
        }
        printf("The framebuffer device was mapped to memory successfully.\n");


        munmap(fbp, screensize);
        close(fbfd);
        return 0;
}
As you can see, we have to #include an extra header file to deal with mmap().
We use the information returned by the ioctl()s to figure out how much memory
to map. The members of fb_var_screeninfo that are used are xres, yres, and bit
s_per_pixel.
Note that there is also what is known as the virtual screen size, which can be
utilised for scrolling, etc, but that is beyond the scope of this document. H
owever, if you are planning on using scrolling, you will probably want to use
xres_virtual and yres_virtual to calculate how much memory to map.
Finally, remember to munmap() the memory you have mapped for use with the fram
ebuffer.

Part Four:

In this section we finally get to plot a pixel on the screen. First of all, we
need to know in what format we should put the data. As it is the most common,
and also the only type I have access to, I will be talking about the type of
framebuffer that utilises packed pixels. All that is necessary to put a pixel
on the screen, is to put bytes corresponding to the colours blue, green, red a
nd transparency, in that order, at the correct location in memory, starting at
0 for x = 0, y = 0, and increasing by four bytes for every x, and y * the len
gth of the line in bytes for every y. Standard graphics stuff; people used to
the good ol' days back in mode 13h programming in DOS will catch on quickly. A
nyway, here's the code:


代码:
#include
#include
#include
#include
#include

int main()
{
        int fbfd = 0;
        struct fb_var_screeninfo vinfo;
        struct fb_fix_screeninfo finfo;
        long int screensize = 0;
        char *fbp = 0;
        int x = 0, y = 0;
        long int location = 0;

        /* Open the file for reading and writing */
        fbfd = open("/dev/fb0", O_RDWR);
        if (!fbfd) {
                printf("Error: cannot open framebuffer device.\n");
                exit(1);
        }
        printf("The framebuffer device was opened successfully.\n");

        /* Get fixed screen information */
        if (ioctl(fbfd, FBIOGET_FSCREENINFO, &finfo)) {
                printf("Error reading fixed information.\n");
                exit(2);
        }

        /* Get variable screen information */
        if (ioctl(fbfd, FBIOGET_VSCREENINFO, &vinfo)) {
                printf("Error reading variable information.\n");
                exit(3);
        }

        /* Figure out the size of the screen in bytes */
        screensize = vinfo.xres * vinfo.yres * vinfo.bits_per_pixel / 8;
       
        /* Map the device to memory */
        fbp = (char *)mmap(0, screensize, PROT_READ | PROT_WRITE, MAP_SHARED,

                fbfd, 0);     
        if ((int)fbp == -1) { printf("Error: failed to map
        framebuffer device to memory.\n"); exit(4);
        }
        printf("The framebuffer device was mapped to memory successfully.\n");


        x = 100; y = 100;      /* Where we are going to put the pixel */

        /* Figure out where in memory to put the pixel */
    location = (x+vinfo.xoffset) * (vinfo.bits_per_pixel/8) +
           (y+vinfo.yoffset) * finfo.line_length;

        *(fbp + location) = 100;    /* Some blue */
        *(fbp + location + 1) = 15;    /* A little green */
        *(fbp + location + 2) = 200;    /* A lot of red */
        *(fbp + location + 3) = 0;      /* No transparency */

        munmap(fbp, screensize);
        close(fbfd);
        return 0;
}
Now that you know how to plot a pixel, it becomes a trivial matter to write fu
nctions to draw lines, boxes, windows, etc. Hopefully, by now you are well on
your way to writing that web browser for me - consider it payment for this tut
orial ;).


Talking of payment, I get paid every time somebody buys a book this way. Sorry
, but I'm a poor student, and let's face it, if you are reading this page, the
n you will probably be interested in them, if you don't have them already. I h
ave personally read each of these, and can say with all honesty that they are
great books.

Beginning Linux Programming:
This book really has it all for getting to grips with Linux programming. If yo
u can already program in C, and want to know how to use some of the more speci
fic Linux stuff, then get this book. It covers all sorts, from shell programmi
ng, through socket programming in C, to X programming using tcl/tk.

C: The Complete Reference:
This is the only book you should consider getting if you want to learn C. This
was all I needed to get programming in C, and since it also doubles as a refe
rence book, you will keep coming back to it again and again. It's by Herbert S
childt, one of the best authors on the planet when it comes to C. Forget the "
learn C in 14 days" cons, get this. Now.

Genetic Algorithms in C++:
This is all you need to understand the theory behind, and implement Genetic Al
gorithms. Genetic Algorithms model themselves on nature, by "evolving" solutio
ns based upon how well they perform in an environment (your program). It sound
s complex, but I assure you that it is *really* easy to get into.
One thing though. All of the sample code is targetted at MS Windows, so if you
are the kind of person who relies on the code in a book to get through it, an
d you are developing for Linux, then this book may not be for you. Personally,
I didn't even need to look at the code before implementing my own stuff, the
text is more than sufficient.

Internet Programming With Python
This book is co-authored by Guido Van Rossum, the guy who wrote the Python lan
guage. It also comes with a CD containing Python for many different platforms,
as well as sample code. It covers CGI programs written in Python, and also a
couple of subjects not usually mentioned in books of this kind, namely generat
ing HTML (in a *very* nice way) and extending the language. It also covers the
basics in a way that makes this book also good for learning the language from
scratch.
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