The putenv() function is not required to be reentrant, and the one in libc4, libc5 and glibc2.0 is not, but the glibc2.1 version is.
Description for libc4, libc5, glibc: If the argument string is of the form name, and does not contain an `=' character, then the variable name is removed from the environment. If putenv() has to allocate a new array environ, and the previous array was also allocated by putenv(), then it will be freed. In no case will the old storage associated to the environment variable itself be freed.
The libc4 and libc5 and glibc 2.1.2 versions conform 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 2.0-2.1.1 differs: a copy of the string is used. On the one hand this causes a memory leak, and on the other hand it violates SUSv2. This has been fixed in glibc2.1.2.
The BSD4.4 version, like glibc 2.0, uses a copy.