OpenSSL and Windows Socket
A program I'm working on gives many compilation errors like this: c:\Program Files\...\PlatformSDK\Include\WinSock2.h(108) : error C20011: 'fd_set' : 'struct' type redefinition c:\Program Files\...\PlatformSDK\Include\WinSock.h(54) : see declaration of 'fd_set' This occurs when both WinSock.h and WinSock2.h are included and WinSock.h is included before WinSock2.h . Since WinSock2.h guards against WinSock.h by defining _WINSOCKAPI_ , including WinSock2.h before WinSock.h is not a problem. windows.h also includes WinSock.h , thus windows.h must come after WinSock2.h . In my code, all the platform dependent stuff goes into platform.h , in which WinSock2.h is included before windows.h , so what is the problem? It turns out that my code uses OpenSSL and openssl/rand.h includes windows.h . Putting platform.h in front of openssl/rand.h solves the problem.