I don't know about POSIX, but C doesn't mandate an implementation. The C Standard says about getenv(): Quote: The getenv function searches an environment lis

I don't know about POSIX, but C doesn't mandate an implementation. The C Standard says about getenv(): Quote: The getenv function searches an environment list, provided by the host environment, for a string that matches the string pointed to by name. The set of environment names and the method for altering the environment list are implementation-defined. The implementation shall behave as if no library function calls the getenv function. Returns The getenv function returns a pointer to a string associated with the matched list member. The string pointed to shall not be modified by the program, but may be overwritten by a subsequent call to the getenv function. If the specified name cannot be found, a null pointer is returned. End quote. The takeaway from me is that yes, environments do define a key/value store of some sort, but how they're implemented isn't stated. It can be an array of strings in "key=value" format stored in RAM, but it could just as well be a hash table stored in ROM.