Problems with native Unix domain sockets on Win 10/2019

Michael McMahon
Wed Sep 23 11:25:51 GMT 2020


I searched for related issues but haven't found anything.

I am having some trouble with Windows native Unix domain sockets
(a recent feature in Windows 10 and 2019 server) and Cygwin.
I think I possibly know the cause since I had to investigate a similar
looking issue on another platform built on Windows.

The problem is that cygwin commands don't seem to recognise native Unix
domain sockets correctly. For example, the socket "foo.sock" should
have the same ownership and similar permissions to other files
in the example below:

$ ls -lrt
total 2181303

-rw-r--r--  1 mimcmah      None             1259   Sep 23 10:22 test.c
-rwxr-xr-x  1 mimcmah      None             3680   Sep 23 10:22 test.obj
-rwxr-xr-x  1 mimcmah      None             121344 Sep 23 10:22 test.exe
-rw-r-----  1 Unknown+User Unknown+Group         0 Sep 23 10:23 foo.sock
-rw-r--r--  1 mimcmah      None             144356 Sep 23 10:27 check.ot

A bigger problem is that foo.sock can't be deleted with the cygwin "rm"

$ rm -f foo.sock
rm: cannot remove 'foo.sock': Permission denied

$ chmod 777 foo.sock
chmod: changing permissions of 'foo.sock': Permission denied

$ cmd /c del foo.sock

But, native Windows commands are okay, as the third example shows.

I think the problem may relate to the way native Unix domain sockets are
implemented in Windows and the resulting special handling required.
They are implemented as NTFS reparse points and when opening them
with CreateFile, you need to specify the FILE_FLAG_OPEN_REPARSE_POINT
flag. Otherwise, you get an ERROR_CANT_ACCESS_FILE. There are other
complications unfortunately, which I'd be happy to discuss further.

But, to reproduce it, you can compile the attached code snippet
which creates foo.sock in the current directory. Obviously, this
only works on recent versions of Windows 10 and 2019 server.

All the best,
Michael McMahon

-------------- next part --------------

#include <windows.h>
#include <winsock2.h>
#include <WS2tcpip.h>
#include <stddef.h>
#include <stdio.h>
#include <afunix.h>


int __cdecl main(int argc, char *argv[])
    WSADATA wsadata;
    struct sockaddr_un addr;
    socklen_t len;
    int z = AF_UNIX;
    SOCKET s, s0;

    if (WSAStartup(MAKEWORD(2,2), &wsadata) != 0) {
        printf("STartup failed\n");
        return 0;
    s0 = socket(AF_UNIX, SOCK_STREAM, 0);
    memset(&addr, 0, sizeof(addr));

    addr.sun_family = AF_UNIX;
    //strcpy(addr.sun_path, argv[1]);
    strcpy(addr.sun_path, "foo.sock");

    z = bind(s0, (const struct sockaddr *) &addr, strlen(addr.sun_path) + sizeof (addr.sun_family));
    if (z != 0) {
        printf("bind failed %ld\n", WSAGetLastError());
    len = sizeof(addr);
    z = getsockname(s0, (struct sockaddr *)&addr, &len);
    if (z != 0) {
        printf("getsockname failed %ld\n", WSAGetLastError());
    } else {
        printf("getsockname works\n");
        printf("fam = %d, len = %d\n", addr.sun_family, len);
        int clen = len - offsetof(struct sockaddr_un, sun_path);
        printf("offsetof clen = %d\n", clen);
        printf("strlen = %zd\n", strlen(addr.sun_path));
        printf("name = %s\n", addr.sun_path);

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