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This chapter is from the book

Environment Blocks and Strings

Figure 6-1 includes the process environment block. The environment block contains a sequence of strings of the form

Name = Value
DWORD GetEnvironmentVariable (
   LPCTSTR lpName,
   LPTSTR lpValue,
   DWORD cchValue)

BOOL SetEnvironmentVariable (
   LPCTSTR lpName,
   LPCTSTR lpValue)

Each environment string, being a string, is NULL-terminated, and the entire block of strings is itself NULL-terminated. PATH is one example of a commonly used environment variable.

To pass the parent's environment to a child process, set lpEnvironment to NULL in the CreateProcess call. Any process, in turn, can interrogate or modify its environment variables or add new environment variables to the block.

The two functions used to get and set variables are as follows:

lpName is the variable name. On setting a value, the variable is added to the block if it does not exist and if the value is not NULL. If, on the other hand, the value is NULL, the variable is removed from the block. The "=" character cannot appear in an environment variable name, since it's used as a separator.

There are additional requirements. Most importantly, the environment block strings must be sorted alphabetically by name (case-insensitive, Unicode order). See MSDN for more details.

GetEnvironmentVariable returns the length of the value string, or 0 on failure. If the lpValue buffer is not long enough, as indicated by cchValue, then the return value is the number of characters actually required to hold the complete string. Recall that GetCurrentDirectory (Chapter 2) uses a similar mechanism.

Process Security

Normally, CreateProcess gives PROCESS_ALL_ACCESS rights. There are, however, several specific rights, including PROCESS_QUERY_INFORMATION, CREATE_PROCESS, PROCESS_TERMINATE, PROCESS_SET_INFORMATION, DUPLICATE_HANDLE, and CREATE_THREAD. In particular, it can be useful to limit PROCESS_TERMINATE rights to the parent process given the frequently mentioned dangers of terminating a running process. Chapter 15 describes security attributes for processes and other objects.

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