The latest MingW runtime and headers defines 'ssize_t'. So this patch is needed:
--- openvpn-2.0_beta2/config-win32.h.in Thu May 13 23:56:23 2004 +++ config-win32.h.in Wed May 19 16:54:07 2004 @@ -42,8 +42,12 @@ #define random rand #define srandom srand -typedef unsigned int in_addr_t; +typedef unsigned long in_addr_t; + +#ifndef _SSIZE_T_ +#define _SSIZE_T_ typedef unsigned int ssize_t; +#endif /* Append a label to program startup title */ //#define DEBUG_LABEL "DEBUG1" ------ And an IPv4-address should not be an 'int'; does OpenVPN work on Win64 btw? --gv