How to get information about free memory from /dev/shm

How to get information about free memory from /dev/shm

I need a way in C or C++ to get the free memory available from /dev/shm .
Note that on my ARM architecure on Linux, unfortunately, ipcs reports a
wrong max. available memory information, but df -h correctly gives me the
current available memory from tmpfs.
The problem is that I am trying to allocate shared memory via
boost::interprocess::shared_memory_object::truncate , but this function
does not throw when the memory is not available. This problem is not
apparently in boost::interprocess, but comes from the underlying
ftruncate() which does not return the appropriate error when there is no
memory available ( https://svn.boost.org/trac/boost/ticket/4374 ), so
boost cannot throw anything.