open(“/tmp”, O_RDWR | O_TMPFILE, 0) fails with Permission denied

Question

I'm trying to open a tempfile using

fd = open("/tmp", O_RDWR | O_TMPFILE, 0);
if (fd == -1) {
    perror("open()");
    exit(1);
}

and I am getting "open(): Permission denied".

I have permissions in /tmp:

drwxrwxrwt  13 root root 1.2K Apr 23 13:55 /tmp/

and I believe my kernel is new enough for O_TMPFILE:

Linux frosties 3.14-1-amd64 #1 SMP Debian 3.14.2-1 (2014-04-28) x86_64 GNU/Linux

What am I missing?

Answer 1

The call failed because you have specified mode 0 , which means no permissions. See this LWN article for a discussion of this behavior of the Linux kernel, and an apparent bugfix.