The Allegro FAQ
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Contents

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Trivial problems

   What is Allegro?
See readme.txt. 

   Where can I get it?
From any SimTel mirror (http://alleg.sourceforge.net/mirrors.html), in the directory gnu/djgpp/v2tk/, or from the Allegro homepage, http://alleg.sourceforge.net/. 

   How do I use it?
See allegro.txt. 
The giftware licence
The Allegro licence is absolutely clear unless you are a lawyer. However, it's written quite informally so this mini-FAQ tries to clarify some things. 

   Does the licence mean that Allegro is public domain?
No. Every Allegro contributor holds the copyright for the code he or she wrote. This means he or she grants you the right to use it for any purpose you want. But the fact that the contributor holds the copyright means that you e.g. cannot claim that you wrote the code. 

   Can I use Allegro for commercial purposes?
Yes. 

   Is Allegro compatible with licence XXX (e.g. GPL)?
Because the Allegro licence has no restrictions on use you can combine it with any licence you want. 

   Is Allegro open source (OSI) compatible?
See the previous question. Since Allegro is compatible with about any license, it also is OSI compatible. It clearly is compatible with all the points in the OSD (open source definition). You can search the OSI mailing list for "Allegro" to see that OSI shares this view. (OSI officials still were reluctant to consider it for approval though and suggested to use an existing license like MIT instead.) 

   How are modifications of the library licensed?
You can modify Allegro and distribute the modified Allegro under any licence you want, but the original unmodified parts will still be licenced under the Allegro licence. If you offer your modifications to be included in the official Allegro distribution (e.g. by sending them to the Allegro developers mailing list) you automatically agree with licencing the modification under the Allegro licence. 

   I took your code. Will you sue me? Your library destroyed my PC. Who do I sue?
We are programmers not lawyers. This FAQ represents our current understanding of copyright law (or copyright laws in different countries). But given the license, which allows everything, and the disclaimer, which says we take no responsibility for anything, we really don't see how there could be any problems. 
DJGPP problems

   What is this make program that I'm supposed to run?
Make is part of the standard djgpp distribution, in the file v2gnu/mak*b.zip (whatever the latest version number is). You can get this from http://www.delorie.com/djgpp/. 

   Make doesn't work properly.
Are you sure that you have installed djgpp correctly? Check the instructions in the djgpp readme.1st file, and in particular confirm that you have set the DJGPP environment variable and added the djgpp\bin directory to your path. 

   My djgpp installation is fine, but make still doesn't work properly.
Are you sure that you are using the GNU make program, and not some other version like the Borland one? If in doubt, run make -v and check that the GNU copyright message is displayed. 

   When I run make, it asks me for some parameters. What do I type?
You shouldn't be running make from the Win95 explorer! Bring up a DOS box, change to your Allegro directory, and type make at the command prompt. 

   When I run make, it says "makefile has modification time in the future".
Are you sure that your system time and date are set correctly? Alternatively you may have downloaded a fresh file made by someone in a more eastern timezone. In the latter case you can either wait a few hours, temporarily set your clock forward, or use a utility such as touch to change the timestamp of the files. 

   When I run make, it says "virtual memory exhausted".

As the error message suggests, you need to provide more memory for the compiler to use. The go32-v2 program will tell you how much is currently available. If you are running under DOS, try to free up more disk space for use as a swapfile. When using win95, increase the DPMI memory limit in the properties for your DOS session to 65535 (you'll have to type this in by hand, because the pulldown list doesn't go above 16384). 

   When I run make, it says "conflicting types for va_list".

As the error message suggests, there is a conflict between newer versions of gcc and older versions of the libc. You must upgrade your djdevxxx.zip package to the latest update (timestamp 11 August 2002) of DJGPP-2.03 or above. 

   When I try to compile a program using Allegro, I get lots of errors like:
C:\TEMP\ccdaaaaa(.text+0x9):x.c: undefined reference to `allegro_init'
C:\TEMP\ccdaaaaa(.text+0xe):x.c: undefined reference to `install_keyboard'

You haven't read the docs, have you? :-) You need to link your program with the library file, liballeg.a. First, make sure you have installed everything properly (running make install should do this for you). Second, if you are compiling from the command prompt or with a makefile, add -lalleg to the end of your gcc command line, or if you are using Rhide, go to the Options/Libraries menu, type alleg into the first empty field, and make sure the box next to it is checked. 

   I give up: I can't compile this! Can you send me a precompiled version?

No, sorry. For starters, liballeg.a is about 450k, but you'd probably also want various utilities like the grabber, sound setup, etc. And what about all the example programs? If we included compiled versions of everything, a binary distribution would be over 7 megs: way too big to be practical! More seriously though, there just isn't any reason why you can't build it yourself. A compiler is a deterministic process: given a particular input file and command line, it will always produce the same output. If this isn't working, you either have the wrong input files (ie. your copy of Allegro is broken in some way), or the wrong command line (it is hard to see how that could happen, since all you have to do is type make...) or your compiler is broken, ie. you didn't install djgpp properly. You need to find and fix the problem, not just sweep it under the carpet by getting someone else to compile Allegro for you... 

   Compile errors scroll off the screen too quickly for me to read them!

GNU tools write their error messages to the error stream, stderr. Unfortunately command.com is too stupid to know how to redirect this, but fortunately DJ was smart enough to work around that, so you can use his redir program to capture the output messages, for example 

         redir -eo make > logfile.txt

      

DOS problems

   Does the DOS version of Allegro work under Windows NT?

Your mileage may vary. Some people have reported problems, while others say that it works fine. Use the Windows version of Allegro if you want to make Windows programs. If you want to run DOS programs, use DOS! 

   Does Allegro work under OpenDOS?

Yes, but with some caveats. If you are using the OpenDOS version of EMM386, you must disable their DPMI implementation (specify DPMI=OFFon the EMM386.EXE device line in your config.sys). You should also make sure the PIC=OFF flag is set, but this is the default so it won't be a problem unless you have specifically enabled it. 

   How can I extend the 256k limit on Mode-X video memory? I have two megs in my card.

You can't. The limit is imposed by the VGA hardware, not by Allegro. To access more than 256k of video memory you need to use an SVGA mode, which means either switching to a higher resolution or getting a copy of the SciTech Display Doctor, which provides several low resolution SVGA modes. 

   Does DOS Allegro work with the SB-Live soundcard?

It does for some people, but not for others. The problem is that Creative Labs refuse to release any specs, so we don't know how to write a driver for it. Complain to them, or buy a different card from a more reasonable manufacturer. 

   Why does DOS Allegro play sounds so much quieter than in Windows?

This might be because you have the volume set very low: try changing this in the setup program. Also, Allegro is mixing several sounds into a single output buffer, unlike the Windows sound player that only plays one sample at a time, so each individual sound can only get a smaller percentage of the total output volume. This is just the price you pay for multiple output channels. If you don't like it, use the setup program to tweak the number of channels: this can be any power of two less than or equal to 64, and the smaller you make it, the louder the sound will be. Alternatively, use set_volume_per_voice(), described in the docs. This will enable you to adjust the overall volume of Allegro's digital sound output. 

   Why doesn't it work with my video card?

Try using a FreeBE/AF driver (http://www.talula.demon.co.uk/freebe/), or the commercial SciTech Display Doctor (http://www.scitechsoft.com). If it still doesn't work, post a description of your problem to the Allegro mailing list, along with a copy of the output from running the afinfo and vesainfo programs. 

   Why not use some sort of dynamic linking system?

The VBE/AF interface already provides this for the video drivers: see the FreeBE/AF project on http://www.talula.demon.co.uk/freebe/. For more general things like the sound, VESA, and mode-X routines, this would be very difficult to do because the drivers depend on a lot of helper functions from the core lib. The djgpp DXE format is nowhere near flexible enough to support this, and we don't want to make Allegro dependent on any other dynamic linking packages. 

   I get an "out of environment space" error when I run fix.bat or vcvars32.bat!

Well duh, you need to increase the size of your environment then :-) You can do this by changing the settings for your DOS box (click the system menu and select "properties"), or at startup in your config.sys file (eg. shell=c:\command.com /e:8192 /p). 
Windows problems

   I get an "out of environment space" error when I run fix.bat or vcvars32.bat!

Well duh, you need to increase the size of your environment then :-) You can do this by changing the settings for your DOS box (click the system menu and select "properties"), or at startup in your config.sys file (eg. shell=c:\command.com /e:8192 /p). 

   When I run make, it says "missing separator".

Make sure that you don't have a semi-colon appended to your MSVCDIR variable if you are using MSVC, to your MINGDIR variable if you are using MinGW or to your BCC32DIR variable if you are using BCC. Also run make -v from the command line and make sure you are using GNU make and not Borland make or Microsoft make. 

   I get lots of compile errors about things like "LPDIRECTDRAWSURFACE2"!

You need to tell your compiler how to find the DirectX include files and libraries: put the DirectX SDK /include and /lib directories in the compiler/linker path. Alternatively, if you don't want to modify any configuration stuff, you can simply copy the files from the DirectX SDK /include and /lib directories to the corresponding ones for your compiler. 

   I get lots of compile errors about things like "DDEDM_STANDARDVGAMODES"!

You need to upgrade to a more recent version of the DirectX SDK, at least version 5, which you can get from the Microsoft Developer site. 

   When I run make, it says "windres: no resources".

If an anti-virus software (Norton or McAfee for example) is running in the background on your computer, try to disable it temporarily. 

   I get an error about a missing WinMain() or main() function!

You need to write END_OF_MAIN() just after your main() function. Allegro uses this, along with some preprocessor magic, to turn a normal main() function into a Windows-style WinMain() entry point. 

   I get an error about an "unresolved external symbol _main"!

Make sure you're building a Windows GUI Application, and not a Windows Console Application. This is a setting when you create the project in MSVC, Dev-C++ or Borland C++ Builder. Alternatively, this is specified on the command line by the -subsystem:windows option for the MSVC linker, by the -Wl,--subsystem,windows option for the MinGW compiler or the -tW option for the Borland C++ compiler. Either that, or define the preprocessor symbol ALLEGRO_USE_CONSOLE prior to including Allegro headers if you really need the console for your program. 

   I get lots of compile errors about conflicting types for "struct BITMAP"!

If your program directly includes Win32 API header files, see the "Windows specifics" section of the documentation. 

If your program only includes header files of the C++ standard library, you're very likely trying to compile it with gcc 3.x for MinGW; if so, define the symbol __GTHREAD_HIDE_WIN32API on the compiler command line: 

         g++ -D__GTHREAD_HIDE_WIN32API program.cpp -lalleg

      

   In windowed mode artifacts are left on the desktop when the window is moved or when another window is placed on top of it.

Disable direct updating for the DirectX windowed driver by using the dedicated configuration variable. See the 'Configuration routines' section in the docs and the allegro.cfg template file for more detailed informations. 

   In windowed mode the screen stays black unless the window is placed under another window on the desktop.

Disable direct updating for the DirectX windowed driver by using the dedicated configuration variable. See the 'Configuration routines' section in the docs and the allegro.cfg template file for more detailed informations. 
Unix problems

   I get an error about "_mangled_main_address" being undefined!

You need to write END_OF_MAIN() just after your main() function. Allegro uses this, along with some preprocessor magic, to get a copy of your argv[] parameters (it needs those for various internal things). 

   I get an error about a duplicate definition of the "_main" symbol!

You are probably on a Darwin/MacOS X system. If so, make sure the symbol USE_CONSOLE is not defined in your program: it is a deprecated symbol that must be replaced by ALLEGRO_USE_CONSOLE. Also note that the magic main can't be disabled on such a system: you simply can't define the symbol ALLEGRO_NO_MAGIC_MAIN in a program linked against Allegro. 

   Compile errors scroll off the screen way too quickly for me to read them!

You need to redirect stderr into a file, so you can view them later. The method for doing this depends on your shell: if you are using a Bourne-style shell like bash, try make 2> logfile.txt

   When I try to run Allegro programs, I get "error while loading shared libraries: liballeg.so.4.1: cannot open shared object file: No such file or directory".

You need to teach the dynamic linker where to find the Allegro shared library. See docs/build/unix.txt, near the end of the 'Installing Allegro' section. 

   When I try to use DGA2, I get "resolution not supported", but the X server does actually support that resolution !

There are two possible reasons: 1) DGA2 may support different resolutions/color depths than X, run the gfxinfo program to know what combinations you can use, 2) You may have a buggy DGA2 implementation, see next question. 

   When I try to use DGA2, the screen goes black and the PC completely halts!

You are probably using a XFree86 server with a very buggy DGA2 implementation, such as 4.0.3 (shipped with Red Hat 7.1 for example). Upgrading to 4.1.0 will probably solve your problem. You can obtain it from ftp://ftp.xfree86.org/pub/XFree86/4.1.0/binaries/ selecting the directory suited to your platform and following the instructions you find in the Install file. 
General problems

   I'm trying to compile the grabber, but it doesn't work!

The grabber needs to be linked with the code in datedit.c. But you shouldn't have to worry about doing this by hand: just run make and that will build everything for you. 

   When I compile Allegro, make fails with something like `Error: operands given don't match any known 386 instruction' or `Error: no such instruction `maskmovq %mm3, %mm1''.

You need to update your version of GNU binutils. See readme.txt to find out what the minimum required version you need is. 

   Can I use Allegro with my <foobar> compiler?

Not unless <foobar> is mentioned in readme.txt as one of the supported platforms. You could port it, but that is usually a lot of work. If <foobar> is a 16 bit DOS compiler like Borland C, you might as well just forget the idea :-) 

   What is this "Allegro WIP" that I see people talking about?

WIP stands for "work in progress", and refers to any changes that are more recent than the last official release. WIP versions of the library can be obtained as patches from the Allegro website (http://alleg.sourceforge.net/), and are usually quite stable, although obviously not so well tested as a final release version. 

   I got the WIP patch, but I can't apply it.

Do you have a copy of patch.exe? If not, go and get it from the same place that you downloaded the rest of djgpp: this tool is a standard part of the compiler distribution. Similarly, you can get the Mingw compiled version from http://sourceforge.net/projects/gnuwin32/. 

If you do have the patch program but it isn't working properly, make sure that you are installing the patch over the top of an unmodified copy of whatever Allegro version it is intended to update (this will usually be the most recent official release from before the WIP was made, but check the text file that comes with the WIP to be sure). 

   When I run the demo game, it says it can't find demo.dat.

That file is distributed separately in the WIP versions. It is here: http://alleg.sourceforge.net/wip.html. (alldata.zip) 

   How can I display several different images at the same time? When I show the second one it messes up the palette for the first.

This is just the way that the video hardware works: there can only be one palette in use at any given moment. Either convert your images so they all use the same palette, or switch to a truecolor graphics mode. 

   How can I convert my graphics to all use the same 256 color palette?

See the Allegro.cc homepage (http://www.allegro.cc/) for some utilities, for example FixPal and Smacker. 

   My colors always come out wrong. And how can I make a greyscale palette with 256 different shades?

The VGA hardware only uses 6 bits for each color component, which means the red, green, and blue values in the palette structure range from 0 to 63, not all the way up to 255. That gives you a potential 2^18 = 262144 different colors, or 64 shades of grey. If you need more than this you could try using VESA function 0x4F08 to select an 8 bit wide DAC, but Allegro doesn't support this directly and I don't know how reliable it will be across different hardware. 

   Why do I have a funny color border at the edges of my monitor?

When you are in a 256 color mode, the VGA card displays color #0 around the border of the display area (in truecolor modes it displays black). Your funny color will go away if you change the palette so that color #0 is black. 

   How can I fade the screen in a truecolor graphics mode?

With great difficulty :-) There is no such easy trick as just altering the palette, so you will have to repeatedly redraw the image in a lighter or darker form. You could draw black translucent rectangles over the screen to darken it down, or use draw_lit_sprite() to tint a bitmap while copying it to the screen, but be warned that these operations are expensive and will require a fast PC! 

Also, have a look at http://www.allegro.cc/ for add-on packages (notably FBlend v0.5) that attempt to make this operation as fast as possible. 

   I tried using fade_in() and fade_out() with truecolor modes but nothing happened. What am I doing wrong?

fade_in() and fade_out() only work in 8-bit paletted modes. See the previous question for details. 

   How can I generate FLI or FLC format animations?

See the Allegro.cc homepage (http://www.allegro.cc/) for some utilities, for example DTA and Smacker. Also, you can use ASE (Allegro Sprite Editor, http://ase.sourceforge.net/), which allows drawing/editing each frame of the animation and save it as a FLI. Or you can use The GIMP (http://www.gimp.org/) along with it's GFli plugin, using layers as frames, and saving your work as file.fli. The GIMP allows you to edit existent FLI files too. 

   How can I make parts of my sprite transparent in truecolor modes? I don't understand what the docs mean by "bright pink"...

In your favourite paint program, get hold of the RGB sliders and drag the red and blue ones up as far as they go (usually to 255, but this will depend on what units your software uses), and the green one right down to zero. The result is a special shade of Magic Pink, or as some people prefer to call it, magenta. 

   I can't get the 3D polygon functions to work!

Remember that the vertex positions are stored in fixed point format, so you must use the itofix() macro or shift your coordinates 16 bits to the left. 

   I can't get the rotate_sprite() function to work!

Remember that the angle of rotation is stored in fixed point format, so you must use the itofix() macro or shift your coordinates 16 bits to the left. For example, rotate_sprite(bmp, spr, x, y, itofix(32))will rotate the graphic by 45 degrees. 

   I can't get the d_bitmap_proc() or d_icon_proc() GUI object to work!

You are probably trying to initialise the dialog structure with a pointer to your bitmap, right? That won't work because the dialog is created at compile time, but the bitmap is only loaded at runtime, so the compiler doesn't yet know where it will be located. You need to fill in the dialog structure with a NULL pointer, and then copy the real bitmap pointer into the dp field as part of your program init code, after you've loaded the bitmap into memory. 

   Should I use regular bitmaps, RLE sprites, or compiled sprites?

It depends on exactly what you are doing. If your images are totally opaque, there is no advantage to using an RLE sprite, and it will probably be faster to use a regular bitmap with the blit() function. If your graphics contain masked areas, an RLE sprite may be both smaller and faster than the draw_sprite() function, depending on your CPU and your bitmaps. Compiled sprites are in general quite a bit faster than both the others for masked images, and slightly faster for opaque graphics, but this is far more variable. They are at their best with small sprites, on older machines and in mode-X, and may actually be slower than blit() when using SVGA modes on a pentium (the large size of a compiled sprite is very bad for the cache performance). 

   How can I make my game run at the same speed on any computer?

You need to make sure the game logic gets updated at a regular rate, but skip the screen refresh every now and then if the computer is too slow to keep up. This can be done by installing a timer handler that will increment a global variable at your game logic speed, eg: 

      volatile int speed_counter = 0;

      void increment_speed_counter()
      {
         speed_counter++;
      }

      END_OF_FUNCTION(increment_speed_counter)

      void play_the_game()
      {
         LOCK_VARIABLE(speed_counter);
         LOCK_FUNCTION(increment_speed_counter);

         install_int_ex(increment_speed_counter, BPS_TO_TIMER(60));

         while (!game_over) {
            while (speed_counter > 0) {
               update_game_logic();
               speed_counter--;
            }

            update_display();
         }
      }

   How can I take a screenshot of my Allegro program?

Add a call to save_bitmap() somewhere in your code. See the save_bitmap() documentation for a discussion of one common pitfall when doing this, and some example code. 

   How can I generate a random number?

Call srand(time(NULL)) at the beginning of your program, and then use rand()%limit to obtain a pseudo-random number between 0 and limit-1. 

   Why not make a "lite" version of Allegro? I don't need any of the sound, maths, or GUI routines...

There is no need. The linker will only include the parts of the library that you actually use, so if you don't call any of, say, the texture mapping or FLIC playing functions, they will be left out of your executable. This doesn't work perfectly because a lot of the Allegro code uses tables of function pointers that cause some unnecessary routines to be linked in, so the majority of the graphics functions will be included in every executable, but I have tried to keep this to a minimum. See allegro.txt for information about more precise ways to remove some of the graphics and sound drivers. 

   Will you ever add support for 3D accelerator hardware?

No. This sort of hardware support would be most useful as part of a proper 3D API, which Allegro is not, and will never be. If you want to do some work on this, the MESA library (a free implementation of OpenGL) is IMHO the place to start. 

However, if you are interested in using OpenGL for graphics and Allegro for everything else, you can try the various add-ons libraries linked from http://www.allegro.cc/ such as AllegroGL. 

   Why not add a MOD playing function?

Several very good ones already exist, for instance the JGMOD or DUMB packages. See the audio library extensions section on the Allegro.cc website (http://www.allegro.cc/). You are not allowed to suggest that one of these libraries be merged into Allegro, because this topic has already been done to death on the mailing list and we are tired of it. 

   Why not add networking support?

There are several networking packages currently in development or floating around on the net, though, and in our opinion this sort of code is more useful as an external library than it would be as part of Allegro. 

   Why can't Allegro read GIF files?

Unisys has a patent on the LZW compression algorithm that is used by the GIF format. We want everything in Allegro to be freely usable without any restrictions whatsoever, which means we can't include any code that is subject to licensing or the payment of royalties. 

   My program crashes all the time. Could this be a bug in Allegro?

Perhaps. Try to isolate the smallest fragment of code that is able to reproduce the problem, and we'll have a look at it. If you can send us a 10 line program, we will fix it. 100 lines, and we can probably fix it. 1000 lines, and we don't have a chance :-) 

   Can I use Allegro in my <whatever> commercial application?

Sure. See the giftware terms in readme.txt. We don't mind what you do with it, and there are no problems with commercial usage. 

   When will <foobar> be finished? I can't wait...

Whenever it is done! A little encouragement is always welcome, but we don't have any completion deadlines and we're not going to make one up for you :-) As soon as it is finished, it will be released. 

   Where can I get some fonts to use with Allegro?

The grabber can import directly from GRX or BIOS format .fnt files, or you can draw them onto a .pcx image using any paint program. See http://www.talula.demon.co.uk/ttf2pcx/ for a utility that will convert Windows TrueType fonts into this .pcx format. 

   Where can I find a set of instrument samples for the DIGMID driver?

See the Allegro homepage (http://alleg.sourceforge.net/) for some links. You can use Gravis patches (.pat format), or SoundFont 2.0 (.sf2) files, but the latter must be converted into a patches.dat file with the pat2dat utility. 

   How can I convert the documentation into Windows Help format?

You need to download the makertf conversion utility (ftp://ftp.coast.net/Coast/win3/winhelp/mkrtf104.zip), and the Windows Help compiler (ftp://ftp.microsoft.com/Softlib/MSLFILES/HC505.EXE). Make a temporary directory, copy the allegro.txi file from the allegro/docs dir, and run the commands makertf --no-warn allegro.txi -o allegro.rtf -J allegro.hpj followed by hcp allegro.hpj. The second command will give a lot of warnings, but they can safely be ignored. 

   How can I print out the documentation?

The allegro.rtf file can be read directly into Microsoft Word and printed from there, but you should right-click and update the table of contents and index fields to fill them with the correct data first. Alternatively you can install the TeX package and use the tex and dvips programs to convert allegro.txi into Postscript format. 

   Where can I find example source code, add-on packages, and tutorials?

Check the Allegro.cc homepage, http://www.allegro.cc/. If you have anything to add to this, please post the URL! 

   Why is it considered good coding practice to define PI as a constant, rather than just using the value 3.141592 in my code?

It simplifies the maintenance of your program, in case the value of PI ever needs to be changed. Also it will make your program more portable to other compilers that use different values of PI. 

   Why doesn't vsync() seem to work?

A number of graphics cards have buggy or incomplete VESA implementations, and often the vsync() function is not implemented. For examples on flicker-free drawing, look at the code for the demo game, which uses a variety of methods to draw itself. 

   Why doesn't busy waiting on the key array work? For example, "while (!key[KEY_ENTER]);" doesn't work.

If the code works without optimisations, then it could be the compiler's fault. You can try beating the compiler into submission, for example: 

      while (!key[KEY_ENTER])
         rest(0);

For this case, however, it would be better to use readkey() instead. Or consider upgrading or downgrading your compiler. 

   I get lots of errors when I try to compile my C++ Allegro program.

You are probably declaring the use of a namespace before including Allegro headers. For example: 

      #include <iostream>
      using namespace std;

      #include <allegro.h>

Move the `using' declaration after the `include' directives referring to Allegro headers: 

      #include <iostream>
      #include <allegro.h>

      using namespace std;

   I'm still confused. Where can I get help?

See http://alleg.sourceforge.net/maillist.html.