Securily changing ownership of a file

chown() operates on a file name, and can be vulnerable to a race condition.  Instead

open() the file in read only mode
fstat() the open file descriptor
lstat() the file name
This may be overkill, depending on the situation
Make sure both stat calls point to the file you expect
Make sure this is the file you wanted (check uid/etc if needed)
fchown() the file descriptor.