Header File 

stdlib.h 

Syntax 

extern char ** _environ; 

extern wchar_t ** _wenviron 

Description 

_environ is an array of pointers to strings; it is used to access and alter the operating system environment variables. Each string is of the form: 

envvar = varvalue 

where envvar is the name of an environment variable (such as PATH), and varvalue is the string value to which envvar is set (such as C:\Utils;C:\MyPrograms). The string varvalue can be empty. 

When a program begins execution, the operating system environment settings are passed directly to the program. Note that env, the third argument to main, is equal to the initial setting of _environ. 

The _environ array can be accessed by getenv; however, the putenv function is the only routine that should be used to add, change or delete the _environ array entries. This is because modification can resize and relocate the process environment array, but _environ is automatically adjusted so that it always points to the array. 

Portability