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But you seem to be assuming that >> calling fchmod on a socket descriptor should affect the permissions on the >> socket file (assuming the socket is bound). Is that documented anywhere? POSIX >> says that the behavior of fchmod on a socket descriptor is unspecified > > The socket file descriptor for a bound UNIX sockets refers to an object in a filesystem > (it's practically a file), which the bind() system call creates. The access to the socket > is controlled by the permission bits, when someone actually tries to connect to it, > so permissions should be working for these objects (otherwise, there's no other way!) > > And fchmod() for a bound Unix socket works on Linux and many other Unix flavors, actually. That's not what I'm seeing when I run your test program on Linux: $ ./sun fstat mode = 140666 stat mode = 140777 $ ls -l .socket srwxrwxrwx. 1 kbrown kbrown 0 Jul 2 17:47 .socket= So calling fchmod on the socket descriptor did not change the permissions of the file to which the socket was bound. And on freeBSD, calling fchmod on a socket descriptor is apparently an error: https://www.freebsd.org/cgi/man.cgi?query=fchmod&sektion=2&n=1 Ken