BS> There is no standard way to set an environment variable, although
 BS> there are  several more-or-less portable, albeit non-standard
 BS> extensions. 
It depends what one means by "non-standard".  Just because something isn't in 
ISO IEC 9899:1989 (The ISO C Standard) doesn't make it non-standard.
putenv() was in draft 3 of the first revision to ISO IEC 9445-1:1990, the 
Portable Operating System API (C language) standard.  Whether it made it into 
the final document I don't know.
*However*, putenv() is *certainly* defined by some non-ISO standards, such as 
UNIX95 and UNIX98 from The Open Group (putenv() can be found on page 672 of 
the Single UNIX Specification, version 2).  Since Jamie Kowinsky, the 
original poster, was targetting two UNIX-like platforms, HP-UX and Linux, he 
can perfectly happily use putenv().  
He is even, in practice, guaranteed portability to non-UNIX and non-POSIX 
platforms, like DOS, OS/2, and Windows NT, since just about all of the C/C++ 
compilers for those platforms implement the putenv() function (and, indeed, 
many other POSIX functions, such as read() and close()).
 ¯ JdeBP ®
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