Lazarus/FPC Libraries

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This page describes the possibilities how to create libraries with Lazarus/FPC and how to use them in projects and packages.

Related Topics


Static linking: FPC compiles and links as default a static executable. That means it tells the linker to put all .o files of the project and all packages into one big executable. Advantage: no external dependencies. Disadvantage: No code is shared between different programs on the same computer. And you can not load/unload a plugin.

Dynamic libraries: The idea of dynamic libraries is to share code between programs, saving the memory of the code, reducing the startup time for often used libraries and allowing plugins. The disadvantages of dynamic libraries are: they are slower for seldom used libs, their structure and internals are more complicated (this is mainly a problem for the compiler), their initialization is different (see below) and sharing code requires a version system to only mix compatible code.

Operating Systems

Dynamic libraries:

Operating System Dynamic library Static library
FreeBSD .so .a
MacOSX .dylib ?
Linux .so .a
Windows .dll .lib




A dynamic library filename has always the form 'lib'+packagename+'.so'+version. For example: and

Linux searches a library in the paths of the environment variable LD_LIBRARY_PATH, then in /lib, then /usr/lib and finally the paths of /etc/

To share memory (GetMem/FreeMem, strings) with other libraries (not written in FPC) under Linux you should use the unit cmem. This unit must be added as the very first unit in the uses section of the project main source file (typically .lpr), so that its initialization section is called before any other unit can allocate memory.


Windows searches a library in the current directory, the system directory and the environment variable PATH.

ppumove, .ppu, .ppl

FPC normally creates for every unit a .ppu and .o file. The .ppu file contains every important information of the .pas/.pp file (types, required filenames like the .o file), while the .o file contains the assembler code and the mangled names understood by the current system.

The ppumove tool included with every FPC installation, converts one or several .ppu and .o files into a dynamic library. It does this by calling the linker to gather all .o files into a .so (windows: .dll) file and removes the .o filename references from the .ppu file. These new .ppu files are normally called .ppl files.

For example:

You have the output directory of a package (where the .ppu files are):

 ppumove -o packagename -e ppl *.ppu

This will convert all .ppu files into .ppl files and creates a (windows: packagename.dll). Note that under Linux the prefix 'lib' is always prepended.

This new library can already be used by other programming languages like C. Or by FPC programs by using the external modifiers. But the initialization/finalization sections must be called automatically. This includes the initialization/finalization of the heap manager. This means no strings or GetMem. Of course FPC programmers are spoiled and they can get more.

Loadlibrary - loading a dynamic library

Loading a dynamic library is simple with the dlopen Loadlibrary function of the unit dl unit dynlibs.

Since 1.9.4, dynlibs provides a portable alternative to unit dl. Note that pretty much any use of unit dl that can't be substituted by using dynlibs is typically already unportable amongst Unices. This alone makes it advisable to use unit dynlibs as much as possible.

The main problem is to get the filename, which depends on the version and the operating system. Since 2.2.2, a constant "sharedsuffix" is declared in unit dynlibs to ease this. It maps to the relevant extension (dll/so/dylib) without a point as prefix.


Every unit can contain an initialization section. The order of the initialization sections depend on the uses sections of each unit.

How to initialize a dynamic library: ToDo


Every unit can contain an finalization section. The order is the reverse order of the initialization sections.

Versions, Distribution

Libraries tend to grow and change over time. Adding new features is no problem, but removing a public method or changing its parameters makes the library incompatible. That means either an installed library (.so, .dll, .dylib) is replaced by a compatible one or a new library must be added to the system. That's why every library contains a version.

To load a dynamic library (dlopen of unit dl) the correct filename must be known. Under Linux this means, you have to know the version number.

ToDo: proposal how the IDE should create version numbers

See Also

Translations / i18n / localizations for programs