I think GnuTLS is probably the second most popular TLS library, after openssl. Though actually, maybe, firefox's NSS is now that I think about it a second more

I think GnuTLS is probably the second most popular TLS library, after openssl. Though actually, maybe, firefox's NSS is now that I think about it a second more. Firefox is at least some fraction of C-library-based ssl traffic out there. I'll also mentions s2n and rustls-ffi for completeness as C libraries, though the former isn't widely used, and the latter is very experimental still. https://github.com/aws/s2n-tls https://github.com/aws/s2n-tls and https://github.com/rustls/rustls-ffi https://github.com/rustls/rustls-ffi respectively.