12.1 Programs

The program header is provided for backwards compatibility, and is ignored by the compiler. The uses clause serves to identify all units that are needed by the program. The system unit doesn’t have to be in this list, since it is always loaded by the compiler. The order in which the units appear is significant, it determines in which order they are initialized. Units are initialized in the same order as they appear in the uses clause. Identifiers are searched in the opposite order, i.e. when the compiler searches for an identifier, then it looks first in the last unit in the uses clause, then the last but one, and so on. This is important in case two units declare different types with the same identifier. When the compiler looks for unit files, it adds the extension .ppu to the name of the unit. On linux and in operating systems where filenames are case sensitive when looking for a unit, the following mechanism is used:

1. The unit is first looked for in the original case.
2. The unit is looked for in all-lowercase letters.
3. The unit is looked for in all-uppercase letters.

Additionally, If a unit name is longer than 8 characters, the compiler will first look for a unit name with this length, and then it will truncate the name to 8 characters and look for it again. For compatibility reasons, this is also true on platforms that support long file names.

Note that the above search is performed in each directory in the search path.