Notes on POSIX
There are few instances where OpenSSL requires a POSIX C library, at least version 1-2008, or compatible enough functionality.
There are exceptions, though, for platforms that do not have a POSIX library, or where there are quirks that need working around. A notable platform is Windows, where POSIX functionality may be available, but where the function names are prefixed with an underscore, and where some POSIX types are not present (such as ssize_t).
Platforms that do have a POSIX library may still not have them accessible unless the following macros are defined:
This is, for example, the case when building with gcc or clang and using the flag -ansi.