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. 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.

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.

#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;
#endif

    fSuccess = CreateProcess(szAppName, NULL, NULL, NULL, TRUE, dwFlags,
        (LPVOID) chNewEnv,   // new environment block
        NULL, &si, &pi); 
 
    if (! fSuccess) 
    {
        printf("CreateProcess failed (%d)\n", GetLastError());
        return FALSE;
    }
    WaitForSingleObject(pi.hProcess, INFINITE);
    return TRUE;
}

Altering the environment variables of a child process during process creation is the only way one process can directly change the environment variables of another process. A process can never directly change the environment variables of another process that is not a child of that process.

If you want the child process to inherit most of the parent's environment with only a few changes, retrieve the current values using GetEnvironmentVariable, save these values, create an updated block for the child process to inherit, create the child process, and then restore the saved values using SetEnvironmentVariable, as shown in the following example.

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

#include <windows.h>
#include <tchar.h>
#include <stdio.h>

#define BUFSIZE 4096
#define VARNAME TEXT("MyVariable")

int _tmain()
{
    DWORD dwRet, dwErr;
    LPTSTR pszOldVal; 
    TCHAR szAppName[]=TEXT("ex3.exe");
    DWORD dwFlags=0;
    STARTUPINFO si;
    PROCESS_INFORMATION pi;
    BOOL fExist, fSuccess; 
 
    // Retrieves the current value of the variable if it exists.
    // Sets the variable to a new value, creates a child process,
    // then uses SetEnvironmentVariable to restore the original
    // value or delete it if it did not exist previously. 
 
    pszOldVal = (LPTSTR) malloc(BUFSIZE*sizeof(TCHAR));
    if(NULL == pszOldVal)
    {
        printf("Out of memory\n");
        return FALSE;
    }

    dwRet = GetEnvironmentVariable(VARNAME, pszOldVal, BUFSIZE);

    if(0 == dwRet)
    {
        dwErr = GetLastError();
        if( ERROR_ENVVAR_NOT_FOUND == dwErr )
        {
            printf("Environment variable does not exist.\n");
            fExist=FALSE;
        }
    }
    else if(BUFSIZE < dwRet)
    {
        pszOldVal = (LPTSTR) realloc(pszOldVal, dwRet*sizeof(TCHAR));   
        if(NULL == pszOldVal)
        {
            printf("Out of memory\n");
            return FALSE;
        }
        dwRet = GetEnvironmentVariable(VARNAME, pszOldVal, dwRet);
        if(!dwRet)
        {
            printf("GetEnvironmentVariable failed (%d)\n", GetLastError());
            return FALSE;
        }
        else fExist=TRUE;
    }
    else fExist=TRUE;

    // Set a value for the child process to inherit. 
 
    if (! SetEnvironmentVariable(VARNAME, TEXT("Test"))) 
    {
        printf("SetEnvironmentVariable failed (%d)\n", GetLastError()); 
        return FALSE;
    }
 
    // Create a child process. 

    SecureZeroMemory(&si, sizeof(STARTUPINFO));
    si.cb = sizeof(STARTUPINFO);
 
#ifdef UNICODE
    dwFlags = CREATE_UNICODE_ENVIRONMENT;
#endif

    fSuccess = CreateProcess(szAppName, NULL, NULL, NULL, TRUE, dwFlags, 
        NULL,     // inherit parent's environment 
        NULL, &si, &pi); 
    if (! fSuccess) 
    {
        printf("CreateProcess failed (%d)\n", GetLastError()); 
    }
    WaitForSingleObject(pi.hProcess, INFINITE);

    // Restore the original environment variable. 
 
    if(fExist)
    {
        if (! SetEnvironmentVariable(VARNAME, pszOldVal)) 
        {
            printf("SetEnvironmentVariable failed (%d)\n", GetLastError()); 
            return FALSE;
        }
    }
    else SetEnvironmentVariable(VARNAME, NULL);

    free(pszOldVal);
    
    return fSuccess;
}

The following example retrieves the process's environment block using GetEnvironmentStrings and prints the contents to the console.

#include <windows.h>
#include <tchar.h>
#include <stdio.h>

int _tmain()
{
    LPTSTR lpszVariable; 
    LPTCH lpvEnv; 
 
    // Get a pointer to the environment block. 
 
    lpvEnv = GetEnvironmentStrings();

    // If the returned pointer is NULL, exit.
    if (lpvEnv == NULL)
    {
        printf("GetEnvironmentStrings failed (%d)\n", GetLastError()); 
        return 0;
    }
 
    // Variable strings are separated by NULL byte, and the block is 
    // terminated by a NULL byte. 

    lpszVariable = (LPTSTR) lpvEnv;

    while (*lpszVariable)
    {
        _tprintf(TEXT("%s\n"), lpszVariable);
        lpszVariable += lstrlen(lpszVariable) + 1;
    }
    FreeEnvironmentStrings(lpvEnv);
    return 1;
}