Difference between revisions of "Program"

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{{Program}}
 
{{Program}}
  
A '''program''' is either an [[Executable program|executable program]], that is, the complete and runnable [[Application|application]], or it is that portion of a [[Pascal]] [[Source code]] [[File|file]] or files that can be compiled and is not declared to be a [[Unit|unit]]. This is sometimes referred to as the [[main]] program.
+
A '''program''' is either an [[Executable program|executable program]], that is, the complete and runnable [[Application|application]], or it is that portion of a [[Pascal]] [[Source code]] [[File|file]] or files that can be compiled and is not declared to be a [[Unit|unit]] or [[Library|library]].
 +
This is sometimes referred to as the main program.
  
[[Category:Pascal]]
+
== main program ==
=Example=
+
<syntaxhighlight lang="pascal" enclose="none">program</syntaxhighlight> is a [[Reserved word|reserved word]] that introduces a classical program source code file:
 +
<syntaxhighlight lang="pascal" highlight="1">
 +
program hiWorld(input, output, stderr);
  
<syntaxhighlight lang="Pascal">
 
program project1;  {$R *.res}
 
 
begin
 
begin
  writeLn('demo of program');
+
writeLn('Hi!');
 
end.
 
end.
 
</syntaxhighlight>
 
</syntaxhighlight>
 +
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 <syntaxhighlight lang="pascal" enclose="none">hiWorld</syntaxhighlight> would trigger a duplicate identifier compile-time error.
 +
 +
The file descriptor list is completely ignored.
 +
The [[Text|<syntaxhighlight lang="pascal" enclose="none">text</syntaxhighlight>]] [[Variable|variables]] {{Doc|package=RTL|unit=system|identifier=input|text=<syntaxhighlight lang="pascal" enclose="none">input</syntaxhighlight>}}, {{Doc|package=RTL|unit=system|identifier=output|text=<syntaxhighlight lang="pascal" enclose="none">output</syntaxhighlight>}} and {{Doc|package=RTL|unit=system|identifier=stderr|text=<syntaxhighlight lang="pascal" enclose="none">stderr</syntaxhighlight>}} are always opened and their names can not be changed. (cf.: <syntaxhighlight lang="pascal" enclose="none">SysInitStdIO</syntaxhighlight> is always called in [https://svn.freepascal.org/cgi-bin/viewvc.cgi/tags/release_3_0_4/rtl/linux/system.pp?view=markup#l367 <tt>rtl/linux/system.pp</tt>])
 +
 +
Thus with FPC the following complete source code example compiles identically as does the previous example.
 +
<syntaxhighlight lang="pascal">
 +
begin
 +
writeLn('Hi!');
 +
end.
 +
</syntaxhighlight>
 +
 +
== program structure ==
 +
A <syntaxhighlight lang="pascal" enclose="none">program</syntaxhighlight> file has to follow a certain structure.
 +
There can be at most one [[Uses|<syntaxhighlight lang="pascal" enclose="none">uses</syntaxhighlight>-clause]] and it has to be at the top of the program right after the (depending on used compiler possibly optional) program header.
 +
<syntaxhighlight lang="pascal" highlight="3-6">
 +
program hiWorld(input, output, stderr);
 +
 +
uses
 +
// sysutils for applicationName
 +
// gettext for internationalization routines
 +
sysutils, gettext;
 +
 +
resourcestring
 +
helloWorld = 'Hi!';
 +
 +
begin
 +
translateResourcestrings(applicationName() + '.%s.mo');
 +
writeLn(helloWorld);
 +
end.
 +
</syntaxhighlight>
 +
What comes thereafter, the exact order and number of various sections after the <syntaxhighlight lang="pascal" enclose="none">uses</syntaxhighlight>-clause until final the compound statement [[Begin|<syntaxhighlight lang="pascal" enclose="none">begin</syntaxhighlight>]]…[[End|<syntaxhighlight lang="pascal" enclose="none">end.</syntaxhighlight>]] is free of choice.
 +
However, there are some plausible considerations.
 +
* A [[Type|<syntaxhighlight lang="pascal" enclose="none">type</syntaxhighlight>-section]] comes prior any section that can use types, e.g. [[Var|<syntaxhighlight lang="pascal" enclose="none">var</syntaxhighlight>-sections]] or routine declarations.
 +
* Since [[Goto|<syntaxhighlight lang="pascal" enclose="none">goto</syntaxhighlight>]] is known as the devil's tool, a [[Label|<syntaxhighlight lang="pascal" enclose="none">label</syntaxhighlight>-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 <syntaxhighlight lang="pascal" enclose="none">var</syntaxhighlight>-section comes in front of a [[Threadvar|<syntaxhighlight lang="pascal" enclose="none">threadvar</syntaxhighlight>-section]]. A [[Const|<syntaxhighlight lang="pascal" enclose="none">const</syntaxhighlight>-section]] comes before a [[Resourcestring|<syntaxhighlight lang="pascal" enclose="none">resourcestring</syntaxhighlight>-section]].
 +
* <syntaxhighlight lang="pascal" enclose="none">resourcestring</syntaxhighlight>-sections can be either static or global, that means they should appear relatively soon after the <syntaxhighlight lang="pascal" enclose="none">uses</syntaxhighlight>-clause.
 +
* Direct usage of [[Global variables|global variables]] in routines (or even the mere possibility) is considered as bad style. Instead, declare/define your routines prior any <syntaxhighlight lang="pascal" enclose="none">var</syntaxhighlight>-(like)-section. (beware: play it safe and set <syntaxhighlight lang="pascal" enclose="none">{$writeableConst off}</syntaxhighlight>)
 +
* Global compiler directives, especially such that allow or restrict what can be written (e.g. <syntaxhighlight lang="pascal" enclose="none">{$goto on}</syntaxhighlight> allows the use of <syntaxhighlight lang="pascal" enclose="none">goto</syntaxhighlight>) or implicitly add unit dependencies like [[Mode ObjFPC|<syntaxhighlight lang="pascal" enclose="none">{$mode objfpc}</syntaxhighlight>]] should appear soon after the program header.
 +
Taking all considerations into account the rough program structure should look like this (except for <syntaxhighlight lang="pascal" enclose="none">label</syntaxhighlight> and [[sGoto|<syntaxhighlight lang="pascal" enclose="none">{$goto on}</syntaxhighlight>]] which are only mentioned for the sake of completeness):
 +
<syntaxhighlight lang="pascal">
 +
program sectionDemo(input, output, stderr);
 +
 +
// global compiler directives ----------------------------
 +
{$mode objfpc}
 +
{$goto on}
 +
 +
uses
 +
sysutils;
 +
 +
const
 +
answer = 42;
 +
 +
resourcestring
 +
helloWorld = 'Hello world!';
 +
 +
type
 +
primaryColor = (red, green, blue);
 +
 +
procedure doSomething(const color: primaryColor);
 +
begin
 +
end;
 +
 +
// M A I N -----------------------------------------------
 +
var
 +
i: longint;
 +
 +
threadvar
 +
z: longbool;
 +
 +
label
 +
42;
 +
begin
 +
end.
 +
</syntaxhighlight>
 +
<small>[The example consciously ignores the possibility of “typed constants”, sticking rather to traditional concepts than unnecessarily confusing beginners.]</small>
 +
 +
== see also ==
 +
* [[Unit|unit]]
 +
* [[Library|library]]

Revision as of 11:39, 3 May 2018

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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);

begin
	writeLn('Hi!');
end.

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.

begin
	writeLn('Hi!');
end.

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);

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

resourcestring
	helloWorld = 'Hi!';

begin
	translateResourcestrings(applicationName() + '.%s.mo');
	writeLn(helloWorld);
end.

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}

uses
	sysutils;

const
	answer = 42;

resourcestring
	helloWorld = 'Hello world!';

type
	primaryColor = (red, green, blue);

procedure doSomething(const color: primaryColor);
begin
end;

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

threadvar
	z: longbool;

label
	42;
begin
end.

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

see also