My 2 CLP.
On linux/glibc you do not have to do this, there is a posix_spawn_file_actions_addclosefrom_np action which is implemented using  close_range(2) which will give you the fastest way to this job without hassle.

for the not posix_spawn case, interate over /proc/self/fd and set the CLOEXEC flag using fcntl.. if close_Range is not available or returns returns different than 0