A new data type, SOCKET, has been defined. This is needed because a Windows Sockets application cannot assume that socket descriptors are equivalent to file descriptors as they are in UNIX. Furthermore, in UNIX, all handles, including socket handles, are small, non-negative integers, and some applications make assumptions that this will be true. Windows Sockets handles have no restrictions, other than that the value INVALID_SOCKET is not a valid socket. Socket handles may take any value in the range 0 to INVALID_SOCKET-1.
Because the SOCKET type is unsigned, compiling existing source code from, for example, a UNIX environment may lead to compiler warnings about signed/unsigned data type mismatches.
This means, for example, that checking for errors when the socket and accept routines return should not be done by comparing the return value with -1, or seeing if the value is negative (both common, and legal, approaches in BSD). Instead, an application should use the manifest constant INVALID_SOCKET as defined in WINSOCK2.H. For example:
TYPICAL BSD STYLE:
s = socket(...);
if (s == -1) /* or s < 0 */
{...}
PREFERRED STYLE:
s = socket(...);
if (s == INVALID_SOCKET)
{...}