Here's a quick summary of things that may cause problems when moving your code from one platform to another (you can find a more detailed version of this in the docs section of the Allegro website).
The Windows and Unix versions require you to write END_OF_MAIN() after your main() function, which is used to magically turn an ANSI C style main() into a Windows style WinMain(), and so that the Unix code can grab a copy of your argv parameter.
On many platforms Allegro runs very slowly if you rely on it in order to automatically lock bitmaps when drawing onto them. For good performance, you need to call acquire_bitmap() and release_bitmap() yourself, and try to keep the amount of locking to a minimum.
The Windows version may lose the contents of video memory if the user switches away from your program, so you need to deal with that.
None of the currently supported platforms require input polling, but it is possible that some future ones might, so if you want to ensure 100% portability of your program, you should call poll_mouse() and poll_keyboard() in all the relevant places.
Allegro defines a number of standard macros that can be used to check various attributes of the current platform:
Text string containing the name of the current platform.
Defined if you are building for a relevant system. Often several of these will apply, eg. DOS+Watcom, or Windows+GCC+MinGW32.
Defined if you are building for a processor of the relevant type.
Defined if the screen is a single large surface that is then partitioned into multiple video sub-bitmaps (eg. DOS), rather than each video bitmap being a totally unique entity (eg. Windows).
Defined if when you are not in a graphics mode, there is a text mode console that you can printf() to, and from which the user could potentially redirect stdout to capture it even while you are in a graphics mode. If this define is absent, you are running in an environment like Windows that has no stdout at all.
Defined if Allegro uses a magic main, i.e takes over the main() entry point and turns it into a secondary entry point suited to its needs.
Non-zero if long filenames are supported, or zero if you are limited to 8.3 format (in the djgpp version, this is a variable depending on the runtime environment).
Defined to whatever represents a 64-bit "long long" integer for the current compiler, or not defined if that isn't supported.
Defined to a path separator character other than a forward slash for platforms that use one (eg. a backslash under DOS and Windows), or defined to a forward slash if there is no other separator character.
Defined to the filename device separator character (a colon for DOS and Windows), or to zero if there are no explicit devices in paths (Unix).
Allegro also defines a number of standard macros that can be used to insulate you from some of the differences between systems:
If you define this prior to including Allegro headers, Allegro will be set up for building a console application rather than the default GUI program on some platforms (especially Windows).
Use this in place of the regular "inline" function modifier keyword, and your code will work correctly on any of the supported compilers.
Use this to declare zero-sized arrays in terminal position inside structures, like in the BITMAP structure. These arrays are effectively equivalent to the flexible array members of ISO C99.
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