Returning Values to the MPR

The Windows Networking functions return WN_SUCCESS on success, or they return a unique nonzero value if the function encounters an error. In addition, they return extended error information using WNetSetLastError and SetLastError.

To support the above behavior, network provider functions should not call SetLastError before returning. This is because the MPR calls SetLastError for the functions in the Network Provider API after they return. If the network providers call SetLastError directly, they will be making redundant function calls. Network provider functions should simply return an error code. The error codes are specified in the function description or Return Values. Also, network provider functions may return any System Error Codes, such as insufficient memory. The only exception is NPGetCaps, which should return a mask indicating the functions supported by the network provider.

If a network provider function needs to return extended error information, it should call WNetSetLastError. This function is provided by the Windows operating system for use by network providers. When the provider calls WNetSetLastError, it can set a string containing additional information about the error. This information is stored on a per-thread basis. This is analogous to SetLastError for Windows applications. The Windows operating system calls WNetSetLastError to check for a string stored using WNetSetLastError and, if found, returns the extended error information to the calling application that initiated the network request.