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A program is either an executable program, that is, the complete and runnable application, or it is that portion of a Pascal Source code file or files that can be compiled and is not declared to be a unit or library. This is sometimes referred to as the main program.

main program

program is a reserved word that introduces a classical program source code file:

program hiWorld(input, output, stderr);


Meanwhile FPC discards the program header, i.e. the first line. The output file name is determined by the source code file's name. However, the program name does become a reserved identifier. In the example above, e.g. attempting to define a constant named hiWorld would trigger a duplicate identifier compile-time error.

The file descriptor list is completely ignored. The text variables input, output and stderr are always opened and their names can not be changed. (cf.: SysInitStdIO is always called in rtl/linux/system.pp)

Thus with FPC the following complete source code example compiles identically as does the previous example.


program structure

A program file has to follow a certain structure. There can be at most one uses-clause and it has to be at the top of the program right after the (depending on used compiler possibly optional) program header.

program hiWorld(input, output, stderr);

	// sysutils for applicationName
	// gettext for internationalization routines
	sysutils, gettext;

	helloWorld = 'Hi!';

	translateResourcestrings(applicationName() + '');

What comes thereafter, the exact order and number of various sections after the uses-clause until final the compound statement beginend. is free of choice. However, there are some plausible considerations.

  • A type-section comes prior any section that can use types, e.g. var-sections or routine declarations.
  • Since goto is known as the devil's tool, a label-section, if any, is as close as possible to the block it is supposed to declare labels for.
  • Generally you go from general into specifics: For example a var-section comes in front of a threadvar-section. A const-section comes before a resourcestring-section.
  • resourcestring-sections can be either static or global, that means they should appear relatively soon after the uses-clause.
  • Direct usage of global variables in routines (or even the mere possibility) is considered as bad style. Instead, declare/define your routines prior any var-(like)-section. (beware: play it safe and set {$writeableConst off})
  • Global compiler directives, especially such that allow or restrict what can be written (e.g. {$goto on} allows the use of goto) or implicitly add unit dependencies like {$mode objfpc} should appear soon after the program header.

Taking all considerations into account the rough program structure should look like this (except for label and {$goto on} which are only mentioned for the sake of completeness):

program sectionDemo(input, output, stderr);

// global compiler directives ----------------------------
{$mode objfpc}
{$goto on}


	answer = 42;

	helloWorld = 'Hello world!';

	primaryColor = (red, green, blue);

procedure doSomething(const color: primaryColor);

// M A I N -----------------------------------------------
	i: longint;

	z: longbool;


[The example consciously ignores the possibility of “typed constants”, sticking rather to traditional concepts than unnecessarily confusing beginners.]

see also