Author: Richard Winston

How do you set an environment variable that will apply outside the process that set 
the variable or those spawned by it?

Answer:

On Windows 2000, if you open the control panel and double click on the system icon, 
the system properties dialog box will open.  On the "Advanced" tab, you can click 
the "Environment Variables" tab to see a list of the user and system environment 
variables. The procedures and functions below allow you to read and write those 
variables. 

It is worth mentioning that you can also use "GetEnvironmentVariable" and 
"SetEnvironmentVariable" to read and write environment variables.  However, if you 
set and environment variable with "SetEnvironmentVariable", the value you set 
applies only to the process that called "SetEnvironmentVariable" or are spawned by 
it. 

The first two procedures read and write environment variables for the current user. 
1   
2   function GetUserEnvironmentVariable(const name: string): string;
3   var
4     rv: DWORD;
5   begin
6     with TRegistry.Create do
7     try
8       RootKey := HKEY_CURRENT_USER;
9       OpenKey('Environment', False);
10      result := ReadString(name);
11      SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, 0, LParam
12        (PChar('Environment')), SMTO_ABORTIFHUNG, 5000, rv);
13    finally
14      Free
15    end
16  end;
17  
18  procedure SetUserEnvironmentVariable(const name, value: string);
19  var
20    rv: DWORD;
21  begin
22    with TRegistry.Create do
23    try
24      RootKey := HKEY_CURRENT_USER;
25      OpenKey('Environment', False);
26      WriteString(name, value);
27      SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, 0, LParam
28        (PChar('Environment')), SMTO_ABORTIFHUNG, 5000, rv);
29    finally
30      Free
31    end
32  end;


The next two procedures read and write environment variables for the system and thus
  affect all users.
33  
34  function GetSystemEnvironmentVariable(const name: string): string;
35  var
36    rv: DWORD;
37  begin
38    with TRegistry.Create do
39    try
40      RootKey := HKEY_LOCAL_MACHINE;
41      OpenKey('SYSTEM\CurrentControlSet\Control\Session ' +
42        'Manager\Environment', False);
43      result := ReadString(name);
44      SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, 0, LParam
45        (PChar('Environment')), SMTO_ABORTIFHUNG, 5000, rv);
46    finally
47      Free
48    end
49  end;
50  
51  // Modified from
52  // http://www.delphiabc.com/TipNo.asp?ID=117
53  // The original article did not include the space in
54  // "Session Manager" which caused the procedure to fail.
55  
56  procedure SetSystemEnvironmentVariable(const name, value: string);
57  var
58    rv: DWORD;
59  begin
60    with TRegistry.Create do
61    try
62      RootKey := HKEY_LOCAL_MACHINE;
63      OpenKey('SYSTEM\CurrentControlSet\Control\Session ' +
64        'Manager\Environment', False);
65      WriteString(name, value);
66      SendMessageTimeout(HWND_BROADCAST, WM_SETTINGCHANGE, 0, LParam
67        (PChar('Environment')), SMTO_ABORTIFHUNG, 5000, rv);
68    finally
69      Free
70    end
71  end;