Easy for you, hard for me

By default, a child process inherits the environment variables of its parent process. However, you can specify a different environment for the child process by creating a new environment block and passing a pointer to it as a parameter to the CreateProcess function.

The GetEnvironmentStrings function returns a pointer to the environment block of the calling process. This should be treated as a read-only block; do not modify it directly. Instead, use the SetEnvironmentVariable function to change an environment variable. When you are finished with the environment block obtained from GetEnvironmentStrings, call the FreeEnvironmentStrings function to free the block.

The GetEnvironmentVariable function determines whether a specified variable is defined in the environment of the calling process, and, if so, what its value is.

For more information, there is the examples of Changing Environment Variables.
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Changing Environment Variables

Each process has an environment block associated with it. The environment block consists of a null-terminated block of null-terminated strings (meaning there are two null bytes at the end of the block), where each string is in the form:

name=value
 


All strings in the environment block must be sorted alphabetically by name. Because the equal sign is a separator, it must not be used in the name of an environment variable.

By default, a child process inherits a copy of the environment block of the parent process. The following example demonstrates how to create a new environment block to pass to a child process.

LPTSTR lpszCurrentVariable;
BOOL fSuccess;
 
// Copy environment strings into an environment block.
 
lpszCurrentVariable = tchNewEnv;
if (lstrcpy(lpszCurrentVariable, "OperatingSystem=Windows") == NULL)
   ErrorExit("lstrcpy failed");
 
lpszCurrentVariable += lstrlen(lpszCurrentVariable) + 1;
if (lstrcpy(lpszCurrentVariable, "API=Win32") == NULL)
   ErrorExit("lstrcpy failed");
 
// Terminate the block with a NULL byte.
 
lpszCurrentVariable += lstrlen(lpszCurrentVariable) + 1;
*lpszCurrentVariable = '\0';
 
// Create the child process, specifying a new environment block.
 
fSuccess = CreateProcess(NULL, "childenv", NULL, NULL, TRUE, 0,
   (LPVOID) tchNewEnv,        // new environment block
   NULL, &siStartInfo, &piProcInfo);
 
if (! fSuccess)
    ErrorExit("CreateProcess failed");
 


If you want the child process to inherit most of the parent’s environment with only a few changes, save the current values, make changes for the child process to inherit, create the child process, and then restore the saved values, as shown following.

LPTSTR lpszOldValue;
TCHAR tchBuf[BUFSIZE];
BOOL fSuccess;
 
// lpszOldValue gets current value of "varname", or NULL if "varname"
// environment variable does not exist. Set "varname" to new value,
// create child process, then use SetEnvironmentVariable to restore
// original value of "varname". If lpszOldValue is NULL, the "varname"
// variable will be deleted.
 
lpszOldValue = ((GetEnvironmentVariable("varname",
    tchBuf, BUFSIZE) > 0) ? tchBuf : NULL);
 
// Set a value for the child process to inherit.
 
if (! SetEnvironmentVariable("varname", "newvalue"))
   ErrorExit("SetEnvironmentVariable failed");
 
// Create a child process.
 
fSuccess = CreateProcess(NULL, "childenv", NULL, NULL, TRUE, 0,
   NULL,     // inherit parent's environment
   NULL, &siStartInfo, &piProcInfo);
if (! fSuccess)
   ErrorExit("CreateProcess failed");
 
// Restore the parent's environment.
 
if (! SetEnvironmentVariable("varname", lpszOldValue))
   ErrorExit("SetEnvironmentVariable failed");
 


The following example, taken from a console process, prints the contents of the process’s environment block.

LPTSTR lpszVariable;
LPVOID lpvEnv;
 
// Get a pointer to the environment block.
 
lpvEnv = GetEnvironmentStrings();
 
// Variable strings are separated by NULL byte, and the block is
// terminated by a NULL byte.
 
for (lpszVariable = (LPTSTR) lpvEnv; *lpszVariable; lpszVariable++)
{
   while (*lpszVariable)
      putchar(*lpszVariable++);
   putchar('\n');
}
 

Tiutin, I think you misunderstood my question a little bit.
As you mentioned in your answer, GetEnvironmenStrings returns the environment block of the calling application. But this block is the environment block of the calling application when it started!

That means, when during the lifetime of your program, the user changes the system environment, you will never notice that; but I need to notice it...

I'm not familiar with anything in the control panel that affects the environment settings.  The environment is DOs stuff and the control panel is windows stuff.  When the user changes the windows settings with the control panel all applications receive a message (WM_SETTINGSCHANGED or something like that)  But this has no relation to changing the DOS environment settings.

Correction: it may work with Win95.  Documentation say it's OK but I'm suspicious.

Anyway, The environment in NT is created from a composition of two registry entries.

One holds the "global" environment:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Session Manager\Environment

The other holds the "per user" environment:
HKEY_CURRENT_USER\Environment

Most of the entries in the environment that a process gets are taken from one of those keys.  However, certain values (like "path") are a composite of their values in both keys.

An application may send an WM_SETTINGCHANGE message (a.k.a WM_WININICHANGE) when it changes entries in the registry but, if I understand the docs correctly, it is not done automatically.

Also, keep in mind that when you change the environment from a console window, only that process environment is changed.

Also see KnowledgeBase article Q104011

Thanks. That's exactly, what I needed.
Merry Christmas and a happy new year!
  Mirko