This is not a C question since this only depends on how your
shell handles things. But, that out of the way, you simply
can't do that (at least with no shell I ever heard of). Your
C program is a process started by your shell and such a child-
process can never change anything in its "parent" process.
The OP didn't seem to be asking about propagating the setting to the
parent process (assuming that "parent process" is meaningful). As far
as I can tell, the POSIX setenv() function (not standard C) would
satisfy the requirement he stated. Whether it would satisfy his
actual requirement is another question.
<OT>In Unix-like systems, there are various ways for a process to set
one of its own environment variables in response to information from a
child process. You demonstrated one of them in part of your article
that I ruthlessly snipped; other ways are possible.
comp.unix.programmer would be a better place to discuss it -- after
checking any FAQ lists, since this kind of question comes up
frequently.</OT>