Each process has an environment block associated with it.The environment block consists of a null-terminated block

of null-teminated strings (meaning there are two 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.

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.

#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; }

 

Example 2

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 the 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; }

Example 3

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; }

 

转自:https://blog.csdn.net/xiaoyafang123/article/details/53514866