is there something that will compile in C but not in C++?

Question

it is a interview question

Answer 1

From Page 26 of C++ Complete Reference Third Edition:

Because C/C++ allows separate modules of a large program to be separately compiled
and linked together, there must be some way of telling all the files about the global
variables required by the program. Although C technically allows you to define a
global variable more than once, it is not good practice (and may cause problems when
linking). More importantly, in C++, you may define a global variable only once.

Answer 2

Sure, anything that has been changed in C++ from C will affect the code.

Case in point:
int p = malloc(sizeof p).

Standard malloc idiom in C. You normally use new in C++, but if you tried to do the above malloc in C++ you would get a compile error on incompatible types. You need an explicit cast in C++.

Answer 3

No, Nothing in C doesn't work under C++.
Any C Code works 100% under C++.

Answer 4

No, C++ is completely compatible with C. Linking is another issue, though.