Windows10 1803更新后CreateFile创建Mailslot报ERROR_BAD_NETPATH(53)

Problem Overview #

Starting with the Windows 10 May 2018 Update (version 1803), calling CreateFile("\\<computername>\mailslot\<slotname>", ...) consistently returns error code 53 (ERROR_BAD_NETPATH). This happens regardless of whether the computer name is valid (even the local machine) or invalid. The only working syntax is explicitly using the local path: \\.\mailslot\<slotname>, which functions correctly.

Prior to Windows 10 1803, this remote/network mailslot path syntax worked across all Windows versions. Initial testing pointed to KB4103721, but further confirmation showed the root cause is the 1803 feature update itself.

Test Code Reproduction #

Client Code (Fails with Remote/Computer Name Paths) #

This client works when run without arguments (uses . as the computer name) but fails when specifying any explicit computer name:

#include <windows.h>
#include <stdio.h>
#include <tchar.h>
LPTSTR SlotName = TEXT("\\\\%hs\\mailslot\\sample_mailslot");
BOOL WriteSlot(HANDLE hSlot, LPTSTR lpszMessage)
{
    BOOL fResult;
    DWORD cbWritten;
    fResult = WriteFile(hSlot, lpszMessage, (DWORD) (lstrlen(lpszMessage)+1)*sizeof(TCHAR), &cbWritten, (LPOVERLAPPED) NULL);
    if (!fResult)
    {
        // this failure is valid if computername is not valid
        printf("WriteFile failed with %d.\n", GetLastError());
        return FALSE;
    }
    printf("Slot written to successfully.\n");
    return TRUE;
}
int main(int nArgs,char * arg[])
{
    HANDLE hFile;
    TCHAR szSlot[256];
    _stprintf (szSlot,SlotName,nArgs > 1 ? arg[1] : ".");
    _tprintf(TEXT("Writing to slot %s\n"),szSlot);
    hFile = CreateFile(szSlot, GENERIC_WRITE, FILE_SHARE_READ, (LPSECURITY_ATTRIBUTES) NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, (HANDLE) NULL);
    if (hFile == INVALID_HANDLE_VALUE)
    {
        // this is the failure I'm trying to debug
        printf("CreateFile failed with %d.\n", GetLastError());
        return FALSE;
    }
    WriteSlot(hFile, TEXT("Message one for mailslot."));
    WriteSlot(hFile, TEXT("Message two for mailslot."));
    Sleep(5000);
    WriteSlot(hFile, TEXT("Message three for mailslot."));
    CloseHandle(hFile);
    return TRUE;
}

Server Code (Works with Local Path) #

This server reads and displays messages sent to the local mailslot:

#include <windows.h>
#include <tchar.h>
#include <stdio.h>
#include <strsafe.h>
HANDLE hSlot;
LPTSTR SlotName = TEXT("\\\\.\\mailslot\\sample_mailslot");
BOOL ReadSlot()
{
    DWORD cbMessage, cMessage, cbRead;
    BOOL fResult;
    LPTSTR lpszBuffer;
    TCHAR achID[80];
    DWORD cAllMessages;
    HANDLE hEvent;
    OVERLAPPED ov;
    cbMessage = cMessage = cbRead = 0;
    hEvent = CreateEvent(NULL, FALSE, FALSE, TEXT("ExampleSlot"));
    if( NULL == hEvent )
        return FALSE;
    ov.Offset = 0;
    ov.OffsetHigh = 0;
    ov.hEvent = hEvent;
    fResult = GetMailslotInfo( hSlot, // mailslot handle
        (LPDWORD) NULL, // no maximum message size
        &cbMessage, // size of next message
        &cMessage, // number of messages
        (LPDWORD) NULL); // no read time-out
    if (!fResult)
    {
        printf("GetMailslotInfo failed with %d.\n", GetLastError());
        return FALSE;
    }
    if (cbMessage == MAILSLOT_NO_MESSAGE)
    {
        printf("Waiting for a message...\n");
        return TRUE;
    }
    cAllMessages = cMessage;
    while (cMessage != 0) // retrieve all messages
    {
        // Create a message-number string.
        StringCchPrintf((LPTSTR) achID, 80, TEXT("\nMessage #%d of %d\n"), cAllMessages - cMessage + 1, cAllMessages);
        // Allocate memory for the message.
        lpszBuffer = (LPTSTR) GlobalAlloc(GPTR, lstrlen((LPTSTR) achID)*sizeof(TCHAR) + cbMessage);
        if( NULL == lpszBuffer )
            return FALSE;
        lpszBuffer[0] = '\0';
        fResult = ReadFile(hSlot, lpszBuffer, cbMessage, &cbRead, &ov);
        if (!fResult)
        {
            printf("ReadFile failed with %d.\n", GetLastError());
            GlobalFree((HGLOBAL) lpszBuffer);
            return FALSE;
        }
        // Concatenate the message and the message-number string.
        StringCbCat(lpszBuffer, lstrlen((LPTSTR) achID)*sizeof(TCHAR)+cbMessage, (LPTSTR) achID);
        // Display the message.
        _tprintf(TEXT("Contents of the mailslot: %s\n"), lpszBuffer);
        GlobalFree((HGLOBAL) lpszBuffer);
        fResult = GetMailslotInfo(hSlot, // mailslot handle
            (LPDWORD) NULL, // no maximum message size
            &cbMessage, // size of next message
            &cMessage, // number of messages
            (LPDWORD) NULL); // no read time-out
        if (!fResult)
        {
            printf("GetMailslotInfo failed (%d)\n", GetLastError());
            return FALSE;
        }
    }
    CloseHandle(hEvent);
    return TRUE;
}
BOOL WINAPI MakeSlot(LPTSTR lpszSlotName)
{
    hSlot = CreateMailslot(lpszSlotName,
        0, // no maximum message size
        MAILSLOT_WAIT_FOREVER, // no time-out for operations
        (LPSECURITY_ATTRIBUTES) NULL); // default security
    if (hSlot == INVALID_HANDLE_VALUE)
    {
        printf("CreateMailslot failed with %d.\n", GetLastError());
        return FALSE;
    }
    return TRUE;
}
int main()
{
    if (!MakeSlot(SlotName))
        return FALSE;
    while(TRUE)
    {
        ReadSlot();
        Sleep(1000);
    }
    CloseHandle(hSlot);
    return TRUE;
}

Key Observations #

• Local path works: Using \\.\mailslot\<slotname> always succeeds in opening the mailslot for writing.
• Remote/computer name paths fail: Any path using \\<computername>\mailslot\<slotname> (even with the local machine's name) returns ERROR_BAD_NETPATH 53 on Windows 10 1803+.
• Regression from prior versions: This behavior is a breaking change introduced in the 1803 feature update.

内容的提问来源于stack exchange，提问作者truefish