The _environ variable is a pointer to an array of pointers to the multibyte-character strings that constitute the process environment. _environ is declared in STDLIB.H as
extern char **_environ;
In a program that uses the main function, _environ is initialized at program startup according to settings taken from the operating-system environment. The environment consists of one or more entries of the form
getenv and _putenv use the _environ variable to access and modify the environment table. When _putenv is called to add or delete environment settings, the environment table changes size. Its location in memory may also change, depending on the program's memory requirements. The value of _environ is automatically adjusted accordingly.
The _wenviron variable, declared in STDLIB.H as extern wchar_t **_wenviron;, is a wide-character version of _environ. In a program that uses the wmain function, _wenviron is initialized at program startup according to settings taken from the operating-system environment.
In a program that uses main, _wenviron is initially NULL, because the environment is composed of multibyte-character strings. On the first call to _wgetenv or _wputenv, a corresponding wide-character string environment is created and is pointed to by _wenviron.
Similarly, in a program that uses wmain, _environ is initially NULL because the environment is composed of wide-character strings. On the first call to _getenv or _putenv, a corresponding wide-character string environment is created and is pointed to by _environ.
When two copies of the environment (MBCS and Unicode) exist simultaneously in a program, the run-time system must maintain both copies, resulting in slower execution time. For example, whenever you call _putenv, a call to _wputenv is also executed automatically, so that the two environment strings correspond.
Caution In rare instances, when the run-time system is maintaining both a Unicode version and a multibyte version of the environment, these two environment versions may not correspond exactly. This is because, although any unique multibyte-character string maps to a unique Unicode string, the mapping from a unique Unicode string to a multibyte-character string is not necessarily unique. Therefore, two distinct Unicode strings may map to the same multibyte string.
Polling _environ in a UNICODE context is meaningless when /MD or /MDd linkage is used. For the CRT DLL, the type (wide or multibyte) of the program is unknown. Only the multibyte type is created since that is the most likely scenario.
The following pseudocode illustrates how this can happen.
int i, j; i = _wputenv( "env_var_x=string1" ); // results in the implicit call: // putenv ("env_var_z=string1") j = _wputenv( "env_var_y=string2" ); // also results in implicit call: // putenv("env_var_z=string2")
In the notation used for this example, the character strings are not C string literals; rather, they are placeholders that represent Unicode environment string literals in the _wputenv call and multibyte environment strings in the putenv call. The character-placeholders 'x' and 'y' in the two distinct Unicode environment strings do not map uniquely to characters in the current MBCS. Instead, both map to some MBCS character 'z' that is the default result of the attempt to convert the strings.
Thus in the multibyte environment the value of "env_var_z" after the first implicit call to putenv would be "string1", but this value would be overwritten on the second implicit call to putenv, when the value of "env_var_z" is set to "string2". The Unicode environment (in _wenviron) and the multibyte environment (in _environ) would therefore differ following this series of calls.