written by xqtr of another droid bbs [andr01d.zapto.org:9999]

BBSes, today are running in multiple platforms... windows, linux, rpi, os2,
c64, macos etc. Writing mods/apps/doors for all of these systems it may
be possible, but it's not easy. Someone could use a programming language
that is supported by many of these systems and compile his program to
each one. It may sound logical and easy, but it's not.

Each system differs in small or even big things, that the programmer must
have in mind to successfully compile his program. This text will only
show some of the most common mistakes and its not, by any mean, an in
depth analysis for cross compilation.

Conditional Compilation
The most important tool in your arsenal for cross compiling programs. You
wont cross compile a thing, with out writing code which is unique for
each system. For sure in a cc program, the 80% of code is usable in all
systems, but you will have to write a 20%, which will be unique. To do
this, we have to use conditional compiling.

We write code for each system and let the compiler know with some
flags/definitions. For example in FreePascal we can write something like
this:
  {$IfDef Windows}
    ..code here..
  {$EndIf}
  {$IfDef Linux}
    ..code here..
  {$EndIf}
When the compiler reach the first IfDef, will check if is compiling in
a Windows system. If so, then it will compile the code between that
IfDef block. If the system is not Windows, it will bypass this block and
go to the next one, which tell to the compiler to check if it's compiling
in a Linux machine etc.

So, we can write code, for each and every system we care about.
Conditional compilation is a big chapter and each programming language
implements it in a different way, so you have to learn about it.


Path Delimiters
The most common mistake a programmer does, is to use the exact path
delimiter that his system uses, inside the code. For example in Linux the
path delim. is '/' but in Windows is '\'. If we use the exact character
in the code, then the program will not compile in both systems. Instead
we should use a variable which will represent the path delim. char.
depending the system we compile the program. For example in FreePascal
such a variable is PathDelim from the SysUtils unit. So instead of writing:
                 mypath := currentfolder + '\' + filename;
we should write:
              mypath := currentfolder + pathdelim + filename;
The first will compile only under Windows, but the second will compile
under all systems that FPC has a compiler.


Know the OS!
You wont be able to cross compile if you don't have some knowledge of the
system OS you are going to compile for. Each OS has its own
characteristics, libraries, tools, structure etc. So you have to know
about them. When you will learn them, you will be able to decide if you
will use compiler directives/definitions or write code that is compatible
with all systems or both. Its not advisable to always use compiler
directives and in some cases you can't write the code with out using
them. But to know this and make the wise decision, you must have some
knowledge of the systems you are going to compile for. Some OSes use big
or little endian format, the alignment for records, differs from system
to system or the default signedness etc.


Uppercase or Lowercase, Not Both!
Basically this is the same as above, but because its a very common mistake
it deserves to be mentioned separately. When you use files in a program and
also use their file names inside your code, make sure that the case is 
the same. Don't include files with upper case filenames and refer to them,
in your code, with lower case names. This will cause too much trouble in 
systems like Linux/MacOS/RPi. Prefer to use lower case filenames and stick
to that guide, so you avoid trouble.


Write once use many...
When you write your code, make it in a way that its going to be easy to
reuse it again and again. Make your own Units/Libraries of code that can
cross compile and use them, instead of re-writing the same code over and
over. Just make it simple and not over do it.


Use standards
Use standard tools, libraries, components etc. as you can. Don't
customize your code with extra stuff that are not very usable. The case
will be, that a custom applet/component/lib that is used in one
system is not used in another. But if you use standard libraries from
your programming language, you will have less problems cross compiling
the code.


Go slow and steady, to go faster...
Don't try to write a program for a system and then port it to another.
By doing so, you wont manage to simultaneously publish your code for all
systems, but each port will have a delay. Also while porting the code to
another system, perhaps you may "brake" the code. You will write code for
one port that it may not be compatible with another and vice versa.
Instead, write the code for a procedure/function and make sure that it
compiles in all systems you want. Then proceed with another and so on.
This way, you will be sure that your code inside your functions is
working in all systems and the debugging process will be easier.



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