Common mistakes


Contents


Ignoring this manual.

Most problems are addressed in this manual. If you aren't sure about some parts of Allegro check particular section of manual. The FAQ section can also be very useful.


main() not returning int.

On platforms that need it, Allegro uses END_OF_MAIN to mangle your main() function and supply its own that is required by the platform. Allegro assumes that main() returns an integer, as required by various C standards. If you change the return type of your main() to something else Allegro's main() will get confused and return some nonsense value which some system can recognize as an error and crash your program.


Semicolon at END_OF_MAIN.

         int main(void)
         {
            allegro_init();
            /* more stuff goes here */
            ...
            return 0;
         }
         END_OF_MAIN(); /* wrong */

The semicolon is not only unnecessary after END_OF_MAIN(), but it can also cause some compilers to issue a warning.


Getting bitmap's size.

Many people don't know how to get the dimensions of a bitmap. This can be done by accessing the `w' and `h' fields of the BITMAP structure: 
         BITMAP *image;
         ...
         allegro_message("Bitmap size: %d x %d\n",
                         image->w, image->h);


Creating bitmaps before loading.

         BITMAP *image = create_bitmap(width, height);
         image = load_bitmap("image.bmp", pal);

When loading a bitmap, Allegro will automatically create a bitmap big enough to store it. In the above code the address returned by create_bitmap() is overwritten by the second assignment statement, to the return value of the call to load_bitmap(). Since the address of the first (unnecessary) bitmap has been lost, there is no way to destroy it so there is a memory leak.


Loading a bitmap/font/sound inside a global object constructor.

Almost all Allegro functions require Allegro to be initialized first, before they can be used. Since global object constructors are called before main() (from where allegro_init() would be called) this condition is violated. You need to postpone calls to Allegro functions to after initializing Allegro.


Calling set_color_depth without resetting graphic mode.

set_color_depth() tells Allegro which color depth to use the next time a graphic mode is set or bitmap is created or loaded. It doesn't change the color depth of the current graphic mode or existing bitmaps. You need to be sure that all your bitmaps and/or graphic mode are in the same color depth or Allegro will be forced to do slow color conversions between them.


Destroying global objects like `screen'.

Unlike other bitmaps `screen' is created by calling set_gfx_mode() and must not be destroyed by calling destroy_bitmap(). The proper way to destroy `screen' is calling set_gfx_mode(GFX_TEXT, 0, 0, 0, 0).