Multiplatform Programming Guide

This page is the start of a tutorial with regard to writing multiplatform applications on Lazarus. It will cover both the necessary precautions to ensure that a program can be easely ported and the porting process for an already existing program. I invite others to help improve the article.

Introduction to Multiplatform Programming

How many boxes do you need?

To answer this question, you should first determine who your potential users are and how your program will be used. For example, if you're developing desktop software you may decide that it only makes sense to develop for Windows and OS X. OS X is the most popular Unix in the world and is surging on the desktop, whereas Linux has mostly stalled there. Remember the 100:10:1 rule, which says that in any large organization or group of users, it's not unusual to find Windows, Mac and Linux desktop computers in roughly these ratios. That is, for every 100 Windows machines, there will be perhaps 10 Macs and maybe 1 Linux machine used to run desktop software. Are the handful of users running OS X and Linux worth the extra effort that will be required to develop, package, distribute and support separate versions for them? Obviously if the project requires this, then the answer will be yes. Or if the amount of extra effort will be small, then the answer will probably be yes as well. If you want to support them out of principle, then the answer may also be yes as long as the extra effort doesn't get in the way of finishing the software.

If you're developing software that will run on a Web server, then perhaps only Windows and Linux make sense as your target platforms.

If you're developing for only one platform, then perhaps Free Pascal and Lazarus are not the best tools for the job, unless you simply want to preserve the possibility of future versions of your software for other platforms. For example, if you're only developing Windows desktop software, then Delphi may be a better choice.

Once you've mastered cross-platform development, you can usually just focus on the problem the software is designed to solve and do most of your development on whatever platform your have available or feel most comfortable with. That is, once you've addressed any cross-platform issues in your design, you can largely ignore the other platforms, much as you would when developing for a single platform. However, at some point you'll need to test deploying and running your program on the other platforms and it will be helpful to have unrestricted access to machines running the all target operating systems. If you don't want multiple physical boxes, you can look into configuring a dual-boot Windows-Linux box or running Windows and Linux on your Mac under an emulator like Virtual PC or iEmulator.

Porting process between Windows and Linux

Windows API Functions

Many Windows programs use the Windows API extensively. On crossplatform applications those functions cannot appear, or must be enclosed by a condition compile (e.g. {$IFDEF Win32} ).

Fortunately many Windows API functions - the most often used - are implemented in a multiplatform way in the unit LCLIntf. This can be a solution for programs which rely heavily on the Windows API, although the best solution is to substitute these calls with true crossplatform components from the LCL. You can substitute calls to GDI painting functions with the TCanvas object, for example.

File system differences

A basic concern when porting application between Linux and Windows is the file system. To start with Windows filenames are not case sensitive, while they are on Unix platforms. This can be the cause of annoying bugs, so any portable application should use consistently filenames.

On Linux there is no "application directory"

One concern when porting applications between Linux and Windows is the file system. Many programmers are used to call ExtractFilePath(ParamStr(0)) or Application.ExeName to get the location of the executable, and then search for the necessary files for the program execution (Images, XML files, database files, etc) based on the location of the executable. This is incorrect in Linux. The string on ParamStr(0) may not only not contain the directory of the executable, as it also varies between different shell programs (sh, bash, etc).

Even if Application.ExeName could in fact know the directory where the file under execution is, that file could be a symbolic link, so you would get the directory of the link instead.

So what should we do? On Linux you should use two different places to store configurations and resource files:

• Resource files (i.e. images, help files)

A fixed privileged location for the executable and resource files which will not change.

This location can be something like: /usr/share/app_name or /opt/app_name

Most programs will be executed without root privileges and the fixed directory for a given application usually only available for the root user to write, so don't write on this directory. Only read information from it.

• Configuration files

You can use the GetAppConfigDir function from SysUtils unit to get a suitable place to store configuration files on different system. The function has one parameter, called Global. If it is True then the directory returned is a global directory, i.e. valid for all users on the system. If the parameter Global is false, then the directory is specific for the user who is executing the program. On systems that do not support multi-user environments, these two directories may be the same.

There is also the GetAppConfigFile witch will return an appropriate name for an application configuration file.

Here is an example of the output of those functions on different systems:

program project1;

{$mode objfpc}{$H+}

uses
  SysUtils;

begin
  WriteLn(GetAppConfigDir(True));
  WriteLn(GetAppConfigDir(False));
  WriteLn(GetAppConfigFile(True));
  WriteLn(GetAppConfigFile(False));
end.

The output on a GNU/Linux system:

/etc
/home/felipe/project1
/etc/project1.cfg
/home/felipe/.project1

You can notice that glocal configuration files are stored on the /etc directory and local configurations are stored on a hidden folder on the user's home directory. Directories whose name begin with a dot (.) are hidden on Linux. You can create a directory on the location returned by GetAppConfigDir and then store configuration files there.

The output on Windows XP:

C:\Programas\teste
C:\Documents and Settings\felipe\Configurações Locais\Dados de aplicativos\project2
C:\Programas\teste\project2.cfg
C:\Documents and Settings\felipe\Configurações Locais\Dados de aplicativos\project2\project2.cfg

Notice that the function uses the directory where the application is to store global configurations on Windows.

Note: The use of UPX interferes with the use of the GetAppConfigDir and GetAppConfigFile functions.

See Also

• http://www.midnightbeach.com/jon/pubs/Kylix.html A guide for Windows programmers starting with Kylix. Many of concepts / code snippets apply to Lazarus.