Why ssize_t in Visual Studio 2010 is defined as unsigned?

Would this be a good solution?

#if defined(_MSC_VER)
#include <BaseTsd.h>
typedef SSIZE_T ssize_t;
#endif

ssize_t is not standard C, it is a typedef from Posix. That you found it in a code analysis header for VS2010 probably has something to do with the origin, most code analysis tooling started on Unix. It is removed again in VS2012 and up.

That it is present in the BaseTsd.h SDK file in all caps certainly is not a mistake, Windows supported a Posix subsystem. These typedefs insulate the operating system from compiler implementation details, the basic reason that Windows managed to survive architecture changes, moving from 16 to 32 to 64-bit.

So the real problem is that you are trying to compile a Posix program on Windows but without using Posix headers. Trivial to solve, just add your own typedef before the #includes.