The problem where is that Visual Studio 8 includes new security features to help write more secure code.  One of these features is parameter validation.  Many of the CRT functions, including lseek, assert on illegal parameter values in debug builds.  They also call parameter validation callback routines that can be registered.  We solve this problem by defaulting the error value to -1 and then only calling lseek if the fd != -1.

my_seek.c:
  Only call lseek if the fd is not -1 on Windows