3.1.2 Unit files

When you compile a unit or program that needs other units, the compiler will look for compiled versions of these units in the following way:

  1. It will look in the current directory.
  2. It will look in the directory where the source file resides.
  3. It will look in the directory where the compiler binary is.
  4. It will look in all the directories specified in the unit search path.

You can add a directory to the unit search path with the (-Fu (see page 107)) option. Every occurrence of one of these options will insert a directory to the unit search path. i.e. the last path on the command line will be searched first.

The compiler adds several paths to the unit search path:

  1. The contents of the environment variable XXUNITS, where XX must be replaced with one of the supported targets: GO32V2, LINUX,WIN32, OS2, BEOS, FREEBSD, SUNOS, DARWIN (the actual list depends on the available targets).
  2. The standard unit directory. This directory is determined from the FPCDIR environment variable. If this variable is not set, then it is defaulted to the following:

    After this directory is determined, the following paths are added to the search path:

    2. FPCDIR/units/FPCTARGET/rtl

    Here target must be replaced by the name of the target you are compiling for: this is a combination of CPU and OS, so for instance


    or, when cross-compiling


The -Fu option accepts a single * wildcard, which will be replaced by all directories found on that location, but not the location itself. For example, given the directories


the command

fpc -Fu"*/units/i386-linux"

will have the same effect as

fpc -Furtl/units/i386-linux -Fufcl/units/i386-linux

since both the rtl and fcl directories contain further units/i386-linux subdirectories. The packages directory will not be added, since it doesn’t contain a units/i386-linux subdirectory.

The following command

fpc -Fu"units/i386-linux/*"

will match any directory below the units/i386-linux directory, but will not match the units/i386-linux directory itself, so you should add it manually if you want the compiler to look for files in this directory as well:

fpc -Fu"units/i386-linux" -Fu"units/i386-linux/*"

Note that (for optimization) the compiler will drop any non-existing paths from the search path, i.e. the existence of the path (after wildcard and environment variable expansion) will be tested.

You can see what paths the compiler will search by giving the compiler the -vu option.

Note that unit file paths specified in a config file will be added at the end, while paths specified on the command-line are added at the beginning.

Imagine the following command-line:

fpc -n -Fu/home @cfg  -Fu/usr foo.pp

Where the file cfg has the following contents:


This will result in the following search path

Using unit path: /home/  
Using unit path: /usr/  
Using unit path: /etc/  
Using unit path: /data/FPC/installed/3.1.1/

Reverting the order of the files on the command-line :

fpc -n -Fu/usr @cfg  -Fu/home foo.pp

Results in

Using unit path: /usr/  
Using unit path: /home/  
Using unit path: /etc/  
Using unit path: /data/FPC/installed/3.1.1/

Moving the position of @cfg will not change the path:

fpc -n @cfg -Fu/home  -Fu/usr foo.pp

Results in

Using unit path: /home/  
Using unit path: /usr/  
Using unit path: /etc/  
Using unit path: /data/FPC/installed/3.1.1/

On systems where filenames are case sensitive (such as unix and linux), the compiler will :

  1. Search for the original file name, i.e. preserves case.
  2. Search for the filename all lowercased.
  3. Search for the filename all uppercased.

This is necessary, since Pascal is case-independent, and the statements Uses Unit1; or uses unit1; should have the same effect.

It will do this first with the extension .ppu (the compiled unit), .pp and then with the extension .pas.

For instance, suppose that the file foo.pp needs the unit bar. Then the command

fpc -Fu.. -Fuunits foo.pp

will tell the compiler to look for the unit bar in the following places:

  1. In the current directory.
  2. In the directory where the compiler binary is (not under linux).
  3. In the parent directory of the current directory.
  4. In the subdirectory units of the current directory
  5. In the standard unit directory.

Also, unit names that are longer than 8 characters will first be looked for with their full length. If the unit is not found with this name, the name will be truncated to 8 characters, and the compiler will look again in the same directories, but with the truncated name.

If the compiler finds the unit it needs, it will look for the source file of this unit in the same directory where it found the unit. If it finds the source of the unit, then it will compare the file times. If the source file was modified more recent than the unit file, the compiler will attempt to recompile the unit with this source file.

If the compiler doesn’t find a compiled version of the unit, or when the -B option is specified, then the compiler will look in the same manner for the unit source file, and attempt to recompile it.

It is recommended to set the unit search path in the configuration file fpc.cfg. If you do this, you don’t need to specify the unit search path on the command line every time you want to compile something.