You may recall a month and a half ago I posted Notes on the Cross-Platform Availability of Header Files and then promptly had to take most of it down because it was insufficiently researched. Well, the research is ongoing, but I’ve got a shiny new set of results, some high-level conclusions, and several ways Viewers Like You can help!

First, the high-level conclusions:

• Except perhaps in deeply-embedded environments, all of C89’s library is universally available.
• Code not intended to run on Windows can also assume most of C99 and much of POSIX. The less-ubiquitous headers from these categories are also the less-useful headers.
• Code that is intended to run on Windows should only use C89 headers and <stdint.h>. If MSVC 2008 support is required, not even <stdint.h> can be used. (Windows compilers do provide a small handful of POSIX headers, but they do not contain the expected set of declarations!)
• Many different Unix variants ship a similar set of nonstandard headers. We don’t yet know whether the contents of these headers are reliable cross-platform.
• There is a large set of obsolete headers that are still widespread but should not be used in new code. This is underdocumented.

The raw data is here: https://github.com/zackw/header-survey/

If you want to help, we need more inventories (especially for OSes further from the beaten path), and I’m also interested in finding a good way to crunch the raw data into something presentable. (I used to have a giant generated HTML table but I gave up on that, it was too big to be readable.) If you are an old beard, there are also places where I’m not entirely sure of my methodology – see the README in the source repo.