OPENSSL_Applink.3ssl man page
OPENSSL_Applink — glue between OpenSSL BIO and Win32 compiler run-time
__declspec(dllexport) void **OPENSSL_Applink();
OPENSSL_Applink is application-side interface which provides a glue between OpenSSL BIO layer and Win32 compiler run-time environment. Even though it appears at application side, it's essentially OpenSSL private interface. For this reason application developers are not expected to implement it, but to compile provided module with compiler of their choice and link it into the target application. The referred module is available as <openssl>/ms/applink.c.