USER AUTH Enviromental Variables

Environment Variables

7 out of 17 rated this helpful - Rate this topicEvery process has an environment block that contains a set of environment variables and their values. There are two types of environment variables: user environment variables (set for each user) and system environment variables (set for everyone).By default, a child process inherits the environment variables of its parent process. Programs started by the command processor inherit the command processor's environment variables. To specify a different environment for a child process, create a new environment block and pass a pointer to it as a parameter to the CreateProcessfunction.The command processor provides the set command to display its environment block or to create new environment variables. You can also view or modify the environment variables by selecting System from the Control Panel, selecting Advanced system settings, and clicking Environment Variables.Each environment block contains the environment variables in the following format:Var1=Value1\0Var2=Value2\0Var3=Value3\0...VarN=ValueN\0\0The name of an environment variable cannot include an equal sign (=).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.Calling SetEnvironmentVariable has no effect on the system environment variables. To programmatically add or modify system environment variables, add them to the HKEY_LOCAL_MACHINE\System\CurrentControlSet\Control\Session Manager\Environment registry key, then broadcast a WM_SETTINGCHANGE message with lParamset to the string "Environment". This allows applications, such as the shell, to pick up your updates.The maximum size of a user-defined environment variable is 32,767 characters. There is no technical limitation on the size of the environment block. However, there are practical limits depending on the mechanism used to access the block. For example, a batch file cannot set a variable that is longer than the maximum command line length.Windows Server 2003 and Windows XP:  The maximum size of the environment block for the process is 32,767 characters. Starting with Windows Vista and Windows Server 2008, there is no technical limitation on the size of the environment 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.To retrieve a copy of the environment block for a given user, use the CreateEnvironmentBlock function.To expand environment-variable strings, use the ExpandEnvironmentStrings function.

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Changing Environment VariablesUser Environment Variables 

PowerShell has a standard provider for environment variables named "env:".The environment variables can be listed byGet-ChildItem env:The value of a given environment variable can be achieved by$env:<variable name>Example:$env:TEMPA userdefined environment variable can be created with the assignment operator "=":$env:<variable name>=<value>Example:$env:myvar=42

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Changing Environment Variables

6 out of 20 rated this helpful - Rate this topicEach 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=valueAll strings in the environment block must be sorted alphabetically by name. The sort is case-insensitive, Unicode order, without regard to locale. Because the equal sign is a separator, it must not be used in the name of an environment variable.



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Example 1

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 using CreateProcess.This example uses the code in example three as the child process, Ex3.exe.

C++ #include <windows.h> #include <tchar.h> #include <stdio.h> #include <strsafe.h> #define BUFSIZE 4096 int _tmain() { TCHAR chNewEnv[BUFSIZE]; LPTSTR lpszCurrentVariable; DWORD dwFlags=0; TCHAR szAppName[]=TEXT("ex3.exe"); STARTUPINFO si; PROCESS_INFORMATION pi; BOOL fSuccess; // Copy environment strings into an environment block. lpszCurrentVariable = (LPTSTR) chNewEnv; if (FAILED(StringCchCopy(lpszCurrentVariable, BUFSIZE, TEXT("MySetting=A")))) { printf("String copy failed\n"); return FALSE; } lpszCurrentVariable += lstrlen(lpszCurrentVariable) + 1; if (FAILED(StringCchCopy(lpszCurrentVariable, BUFSIZE, TEXT("MyVersion=2")))) { printf("String copy failed\n"); return FALSE; } // Terminate the block with a NULL byte. lpszCurrentVariable += lstrlen(lpszCurrentVariable) + 1; *lpszCurrentVariable = (TCHAR)0; // Create the child process, specifying a new environment block. SecureZeroMemory(&si, sizeof(STARTUPINFO)); si.cb = sizeof(STARTUPINFO); #ifdef UNICODE dwFlags = CREATE_UNICODE_ENVIRONMENT;