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, Unix and MacOS X versions require you to write END_OF_MAIN() after your main() function. This is used to magically turn an ISO C style main() into a Windows style WinMain(), or by the Unix code to grab a copy of your argv parameter, or by the MacOS X code to shell the user main() inside a Cocoa application.
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.
On Unix the shared files by Allegro (like `language.dat') may require a special use due to the nature of distributing the resources in separate paths instead of putting everything in the same directory. Check the beginning of your platform's specific chapter to learn more about this.
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+MinGW.
Note that ALLEGRO_LINUX is a misnomer. It will only be defined if Linux console support is enabled. It is not a reliable way to check if the program is being built on a Linux system.
Defined if you are building for a processor of the relevant type.
Defined if the library is internally multi-threaded on this system.
Defined if the compiler supports constructor/destructor functions.
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 can be customized at compile time to a certain extent with the following macros:
If you define this prior to including Allegro headers, Allegro won't touch the main() entry point. This effectively removes the requirement on a program to be linked against the Allegro library when it includes the allegro.h header file. Note that the configuration and file routines are not guaranteed to work on Unix systems when this symbol is defined. Moreover, on Darwin/MacOS X systems, this symbol simply prevents the program from being linked against the Allegro library! This highly non portable feature is primarily intended to be used under Windows.
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).
If you define this prior to including Allegro headers, Allegro will not automatically include some standard headers (eg <stddef.h>) its own headers depend upon.
If you define this prior to including Allegro headers, Allegro will omit the definition of the KEY_* constants, which may clash with other headers.
The fixed point functions used to be named with an "f" prefix instead of "fix", eg. fixsqrt() used to be fsqrt(), but were renamed due to conflicts with some libc implementations. So backwards compatibility aliases are provided as static inline functions which map the old names to the new names, eg. fsqrt() calls fixsqrt(). If you define this symbol prior to including Allegro headers, the aliases will be turned off.
If you define this symbol prior to including Allegro headers in a C++ source file, the 'fix' class will not be made available. This mitigates problems with the 'fix' class's overloading getting in the way.
The `curses' API also defines functions called vline() and hline(). To avoid a linker conflict when both libraries are used, we have internally renamed our functions and added inline function aliases which remap vline() and hline(). This should not be noticeable to most users.
If you define ALLEGRO_NO_VHLINE_ALIAS prior to including Allegro headers, Allegro will not define the vline() and hline() aliases, e.g. so you can include curses.h and allegro.h in the same module.
If you define this prior to including Allegro headers, Allegro will not define the clear() backwards compatibility alias to clear_bitmap().
If you define this prior to including Allegro headers, Allegro will not include the backward compatibility layer. It is undefined by default so old programs can still be compiled with the minimum amount of issues, but you should define this symbol if you intend to maintain your code up to date with the latest versions of Allegro. It automatically turns off all backwards compatibility aliases.
Allegro also defines a number of standard macros that can be used to insulate you from some of the differences between systems:
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 a function with a volatile return value.
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.
Use this in place of the regular "const" object modifier keyword, and your code will work correctly on any of the supported compilers.
On platforms that require it, this macro does a simple shift transformation of the libc rand() function, in order to improve the perceived randomness of the output series in the lower 16 bits. Where not required, it directly translates into a rand() call.