Produce steps:
1. Attached file is the C source code;
2. Build the source file(I built it on Ubuntu 17.04, Intel® HD Graphics
(KabyLake 3x8 GT2)), "gcc -o copySubImageFloatRGB copySubImageFloatRGB.c -lX11
-lepoxy";
3. run "./copySubImageFloatRGB", will print message as follow:
        Visual 0xb9 selected
        Render: Mesa DRI Intel(R) HD Graphics 630 (Kaby Lake GT2)
        Version: 3.0 Mesa 17.1.4

       233 glCheckFramebufferStatus error no = 0x8cdd.

The error is result from that the FBO is not completeness when the internal
format of attached image is RGB32F. In another word, The internal format RGB32F
is not color-renderable. 
According to GL 3.0 Spec description, Section 4.4 page 291 and Table 3.16 Page
184, The internal format RGB32F is color-renderable

In additional, I have run this test application on Nvidia graphic, it work
successfully