MultithreadingAndCriticalsections: what changed

Revision 0 to 1. Removed lines have a minus sign; added lines have a plus sign.

Title: MultithreadingAndCriticalsections

New		Line
1	+	## Code Snippet demonstrating Thread programming
2	+
3	+	``` pascal
4	+	{$mode objfpc}{$H+}
5	+	program Project1;
6	+
7	+	uses
8	+	{$ifdef unix}cthreads,
9	+	{$endif}Classes,
10	+	SysUtils;
11	+
12	+	var
13	+	CriticalSection: TRTLCriticalSection;
14	+	Counter: integer;
15	+	UseCriticalSection: boolean;
16	+
17	+	function Supposedly: string;
18	+	begin
19	+	if UseCriticalSection then
20	+	Result:=''
21	+	else Result:='supposedly ';
22	+	end;
23	+
24	+	type
25	+	{ TMyThread }
26	+
27	+	TMyThread = class(TThread)
28	+	private
29	+	FAFinished: boolean;
30	+	FID: integer;
31	+	public
32	+	constructor Create(CreateSuspended: Boolean; ID: integer; const StackSize: SizeUInt=DefaultStackSize);
33	+	procedure Execute; override;
34	+	property AFinished: boolean read FAFinished write FAFinished;
35	+	end;
36	+
37	+	constructor TMyThread.Create(CreateSuspended: Boolean; ID: integer; const StackSize: SizeUInt);
38	+	begin
39	+	inherited Create(CreateSuspended, StackSize);
40	+	FID:=ID;
41	+	end;
42	+
43	+	procedure TMyThread.Execute;
44	+	var
45	+	j: integer;
46	+	begin
47	+	FAFinished := False;
48	+	if UseCriticalSection then
49	+	EnterCriticalSection(CriticalSection);
50	+	try
51	+	for j := 1 to 1000000 do
52	+	Inc(Counter);
53	+	WriteLn(Format('Thread %d has %sfinished making 1000000 increments to Counter',[FID, Supposedly]));
54	+	finally
55	+	if UseCriticalSection then
56	+	LeaveCriticalSection(CriticalSection);
57	+	end;
58	+	FAFinished := True;
59	+	end;
60	+
61	+	function HasCriticalSection: string;
62	+	begin
63	+	if UseCriticalSection then
64	+	Result:=''
65	+	else Result:='NOT ';
66	+	end;
67	+
68	+	procedure ExerciseThreads(aUseCriticalSection: boolean);
69	+	var
70	+	Threads: array[1..20] of TMyThread;
71	+	AllFinished: boolean;
72	+	i: integer;
73	+	begin
74	+	Counter := 0;
75	+	UseCriticalSection:=aUseCriticalSection;
76	+	WriteLn(Format('A critical section is %sbeing used; Counter starts at %d',[HasCriticalSection, Counter]));
77	+	// start 20 threads
78	+	for i := Low(Threads) to High(Threads) do
79	+	Threads[i] := TMyThread.Create(False, i);
80	+	// wait till all threads finished
81	+	repeat
82	+	AllFinished := True;
83	+	for i := Low(Threads) to High(Threads) do
84	+	if not Threads[i].AFinished then
85	+	AllFinished := False;
86	+	until AllFinished;
87	+	// free the threads
88	+	for i := Low(Threads) to High(Threads) do
89	+	Threads[i].Free;
90	+	WriteLn('Final value of Counter=' + IntToStr(Counter));
91	+	WriteLn;
92	+	end;
93	+
94	+	var
95	+	b: boolean;
96	+	begin
97	+	// create the CriticalSection
98	+	InitCriticalSection(CriticalSection);
99	+
100	+	for b in Boolean do
101	+	ExerciseThreads(b);
102	+
103	+	// free the CriticalSection
104	+	DoneCriticalSection(CriticalSection);
105	+
106	+	{$IfDef WINDOWS}
107	+	Write(' Press [Enter] to finish');
108	+	ReadLn;
109	+	{$EndIf}
110	+	end.
111	+	```
112	+
113	+	---
114	+	From the Free Pascal wiki, page "MultithreadingAndCriticalsections", as it stood on 2021-05-06 after a change by Arent, out of the wiki's dump of 2024-05-26. Copyright stays with the author who wrote it, according to the old wiki's [[Licensing]] page. If you have concerns about this material, [message the admin](/wiki-contact).

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