#include <stdlib.h> int putenv(char *string);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
|putenv()||Thread safety||MT-Unsafe const:env|
Since version 2.1.2, the glibc implementation conforms to SUSv2: the pointer string given to putenv() is used. In particular, this string becomes part of the environment; changing it later will change the environment. (Thus, it is an error is to call putenv() with an automatic variable as the argument, then return from the calling function while string is still part of the environment.) However, glibc versions 2.0 to 2.1.1 differ: a copy of the string is used. On the one hand this causes a memory leak, and on the other hand it violates SUSv2.
The 4.4BSD version, like glibc 2.0, uses a copy.