Why can't I "freopen" a "tmpfile"? - c

Question

I tried to compile and run C code that has the following lines:
FILE *preproc_producer = NULL;
preproc_producer = tmpfile();
// preproc_producer is not NULL here
preproc_producer = freopen(NULL, "r+", preproc_producer);
// preproc_producer is NULL here
However, when running the code, preproc_producer ends up NULL, and error code is Stale NFS file handle
What is the issue with the above code?
What is the purpose of the freopen call here? I commented out the freopen line and the rest of the program seems to be working.
I'm using GCC 4.7.2, running Ubuntu 64 12.04 inside a Docker 0.6.7 Linux container. The above code seems to work outside the Docker container.
Update: strace dump:
stat("/tmp", {st_mode=S_IFDIR|S_ISVTX|0777, st_size=4096, ...}) = 0
gettimeofday({1385247432, 199732}, NULL) = 0
getpid() = 127
open("/tmp/tmpf9l14HD", O_RDWR|O_CREAT|O_EXCL, 0600) = 3
unlink("/tmp/tmpf9l14HD") = 0
fcntl(3, F_GETFL) = 0x8002 (flags O_RDWR|O_LARGEFILE)
brk(0) = 0xc94000
brk(0xcb5000) = 0xcb5000
fstat(3, {st_mode=S_IFREG|0600, st_size=0, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f7afb9d0000
lseek(3, 0, SEEK_CUR) = 0
lstat("/proc/self/fd/3", {st_mode=S_IFLNK|0700, st_size=64, ...}) = 0
munmap(0x7f7afb9d0000, 4096) = 0
open("/proc/self/fd/3", O_RDWR) = -1 ESTALE (Stale NFS file handle)

Answer 1

From the C99 standard:
The freopen function opens the file whose name is the string pointed to by filename
and associates the stream pointed to by stream with it. The mode argument is used just
as in the fopen function.
If filename is a null pointer, the freopen function attempts to change the mode of
the stream to that specified by mode, as if the name of the file currently associated with
the stream had been used. It is implementation-defined which changes of mode are
permitted (if any), and under what circumstances.
So, probably who wrote this code meant to change the temporary file open mode from w+b to r+ (which mostly boils down to change the stream to text mode). Unfortunately, it seems that in your implementation it's not possible to change the open mode of a temporary file in that way.
I suppose that it may come from the fact that closing a temporary file also deletes it, but it may also be that glibc implementation of freopen doesn't support mode changes in freopen (the manpage doesn't even mention the possibility to pass NULL as first argument).