Changing Environment Variables - Win32 Programmer's Reference

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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');

}

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