Releasing units without source code

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It can be useful to release a FreePascal unit without publishing its source code:

  • The source code contains proprietary information.
  • In teaching, because you want to force students to use a unit by its interface (contract) only, and not by looking at its implementation.

FreePascal allows you to do so in the following way.

The provider of the unit (and owner of its source code) should:

  • Compile the unit separately; it is recommended to use the compiler option -Ur (Generate release unit files; see User's Manual for details)
  • Publish both the resulting *.ppu and *.o files. Also see Section 3.3 of the User's Manual (Compiling a unit).

The user of the provided unit should:

  • Compile the using program (the client), such that the compiler can find both the *.ppu and *.o files of the unit (e.g. through the compiler option -Fu).

Thus, there are two compiler contexts that matter:

  • The compiler installation of the provider
  • The compiler installation of the user (client)


  • The provider and user should use the same compiler version. Although backwards compatibility between compiled units is never broken on purpose, this regularly happens in order to support new features or to fix bugs.
  • The Target OS of the provided unit should match the target OS used for compiling the client program.
  • If the provided unit depends on another unit U, then the unit U of the client context needs to be compatible with the unit U in the provider context. For that purpose, the providing compiler embeds a checksum of the interface section of unit U in the *.ppu file of the provided unit. The client compiler checks the embedded checksum against the checksum of unit U in the client context. If the checksums differ, then the client compiler will attempt to recompile the provided unit, and this will fail because the source is missing. With the compiler option -vu you get more information on the handling of unit files, and you can spot a line stating Recompiling ..., checksum changed for ....
  • In particular, the System unit of the provider context should be compatible with the System unit of the client context, because every unit implicitly depends on the System unit. Therefore, it is recommended to use a stable release of the compiler to compile the provided unit.
  • There may be some other compiler options to consider (besides setting the Target OS):
    • -M (Mode)
    • -C (Checking), such as -Cr (range checking), -Ci (i/o checking), -Co (overflow checking), -Ct (stack checking)
    • -Sa (Include assert statements in compiled code)
    • -O (Optimization)
    • -gl (Generating lineinfo code)
  • For older versions of the FreePascal compiler, the name of the provided unit's source file should be in all lower case letters. For recent versions of the compiler, this is no longer an issue. (The User's Manual is not up to date on this topic, I believe. If you know more details, e.g. from which version on this changed, then please put it here.)

See also