An easy example of this is coreutils' "ls", when linked against libselinux:
libselinux will/may call fopen("/proc/filesystems", "re") during its own
constructor, which may be called before our.
Another example is inkscape, which contains static C++ classes (such as
Inkscape::Util::UnitTable::UnitTable) which may themselves call (f)open.
C++ runtime seems to call such static constructors before our contructor,
leading to the same issue. GCC documentation (as of version 8.0.1) says:
  However, at present, the order in which constructors for C++ objects
  with static storage duration and functions decorated with attribute
  constructor are invoked is unspecified. In mixed declarations,
  attribute init_priority can be used to impose a specific ordering.