Changing Environment Variables

ex1

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

#include <iostream>

#define BUFSIZE 4096

int _tmain()
{
    TCHAR chNewEnv[BUFSIZE];
    LPTSTR lpszCurrentVariable;
    DWORD dwFlags = 0;
    TCHAR szAppName[] = TEXT("E:\\TEST\\wincode\\ex1\\Debug\\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());
        DWORD err = GetLastError();
        std::cout << err << "\n";
        return FALSE;
    }
    WaitForSingleObject(pi.hProcess, INFINITE);
    system("pause");
    return TRUE;
}

　　ex2

#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");
    TCHAR szAppName[] = TEXT("E:\\TEST\\wincode\\ex1\\Debug\\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;
}

　　ex3

#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;
}