Currently the man man page doesn't mention any header to include for 'environ':
> environ — user environment
> extern char **environ;
Shouldn't there be a header where it is declared?
No, it "must be declared by the user if it is to be used directly," according to https://pubs.opengroup.org/onlinepubs/9699919799/functions/environ.html. Perhaps the man page should make that more clear.