mkdir("/") returns EINVAL and not EEXIST

[EvenOAndersen]

mkdir("/") returns EINVAL and not EEXIST

is this expected, or a bug?

The following program shows the problem:

`include <sys/stat.h>
#include <errno.h>
#include <assert.h>
#include <stdio.h>

int mkdir(const char *pathname, mode_t mode);

int main()
{
const int status0 = mkdir("/tmp", 0755);
const int errcode0 = errno;
fprintf(stdout, "err=%d\n", errcode0);
assert(status0 == -1);
assert(errcode0 == EEXIST);

const int status1 = mkdir("/", 0755);
const int errcode1 = errno;
fprintf(stdout, "err=%d\n", errcode1);
assert(status1 == -1);
assert(errcode1 == EINVAL);        // <-- this succeeds
assert(errcode1 == EEXIST);         // <-- this aborts

}
`