All C code is valid in C++.
Not to be pedantic, but this is not strictly true. Most C code, yes. All, no. There are a few behaviors in C that are not supported in C++. An easy example is that there are new keywords, so in C++ you can no longer write
int old, new;
You also can't perform implicit conversions from void * to other pointers. This used to work in C, for example, but is an error in C++:
char *str = malloc(100);
Mikal