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Windows Environment Strings - Felix John COLIBRI.
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- abstract : reading and writing Windows environment variables
- key words : Environment, GetEnvironmentStrings, GetEnvironmentVariable,
SetEnvironmentVariable
- software used : Windows XP, Delphi 6
- hardware used : Pentium 1.400Mhz, 256 M memory, 140 G hard disc
- scope : Delphi 1 to 8 for Windows
- level : Delphi developer
- plan :
1 - Introduction
For a simple cgi server, we had to read and create environment strings. This
simple unit shows how to get values from the environment, and create new
environment blocks.
2 - The Environment block
Each Windows Process uses its own environment block. When a child process is
created, the parent can specify which environment block should be used. The
syntax of CreateProcess is (Windows.Pas):
function CreateProcess(lpApplicationName: PChar;
lpCommandLine: PChar;
lpProcessAttributes, lpThreadAttributes: PSecurityAttributes;
bInheritHandles: BOOL;
dwCreationFlags: DWORD;
lpEnvironment: Pointer;
lpCurrentDirectory: PChar;
const lpStartupInfo: TStartupInfo;
var lpProcessInformation: TProcessInformation): BOOL; stdcall;
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and lpEnvironment is a pointer to a block with the following syntax:
- a zero terminated list of key / values
- each key / value is a zero terminated string with key=value strings
So to create an environment block for a new process, we simply build such a
block, and hand a pointer to this block over to the CreateProcess function.
Within a Process, we can retrieve the complete block, fetch single values, or
add values. The basic primitives are GetEnvironmentStrings,
GetEnvironmentVariable and SetEnvironmentVariable.
3 - The Delphi unit
Our Delphi unit simply offers the following INTERFACE:
function f_write_environment_key_value(p_key, p_value: String): Boolean;
function f_get_environment_value(p_key: pChar): String;
function f_get_all_environment_string: String;
type c_environment= Class
m_size: Integer;
m_oa_environment: array of char;
constructor create_environment(p_c_strings: tStrings);
end; // c_environment
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4 - The test project
This is the simple test project, with:
- a tButton to fetch and display all the environment strings
- a tButton to extract the value corresponding to the key written in
Edit1.Text
- a tButton using the c_environment CLASS in order to build in
c_environment.m_oa_environment the null terminated block
The code of the tButton events is:
procedure TForm1.display_enviromnent_Click(Sender: TObject);
begin
display(f_get_all_environment_string);
end; // display_enviromnent_Click
procedure TForm1.read_environment_key_Click(Sender: TObject);
begin
display(f_get_environment_value(pChar(edit1.Text)));
end; // read_environment_key_Click
procedure TForm1.build_environment_block_Click(Sender: TObject);
begin
with c_environment.create_environment(Memo2.Lines) do
begin
display(f_display_hex(@ m_oa_environment[0], m_size,
8, k_hex_default));
Free;
end; // with c_environment
end; // build_environment_block_Click
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Here is a snapshot of the project:
Note that
- f_get_environment_value uses a pChar as parameter, since most of the time
we employ a literal string, which is automatically converted to a pChar by
the Delphi compiler. Another version with a String parameter could be used
as well
- the pointer handed over to the CreateProcess function is @
c_environment.m_oa_environment[0]. We used a Class since both the block
and its size are required by the function call
5 - Download the Sources
Here are the source code files:
Those .ZIP files contain:
- the main program (.DPR, .DOF, .RES), the main form (.PAS, .DFM), and any
other auxiliary form
- any .TXT for parameters
- all units (.PAS) for units
Those .ZIP
- are self-contained: you will not need any other product (unless expressly
mentioned).
- can be used from any folder (the pathes are RELATIVE)
- will not modify your PC in any way beyond the path where you placed the .ZIP
(no registry changes, no path creation etc).
To use the .ZIP:
- create or select any folder of your choice
- unzip the downloaded file
- using Delphi, compile and execute
To remove the .ZIP simply delete the folder.
As usual:
- please tell us at fcolibri@felix-colibri.com if you had some problem downloading the file, or
found some bug. Resulting corrections will be helpful for other readers
- we welcome any comment, criticism, enhancement. Just send an e-mail to
fcolibri@felix-colibri.com.
6 - The Author
Felix John COLIBRI works at the Pascal
Institute. He programs in Pascal since 1979, and is mainly active in the area
of software development and training. His web site features
tutorials, technical papers about programming with downloadable source code,
and the description and calendar of forthcoming Delphi, Interbase and UML
training sessions.
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