I constantly see recommendations to #include <cstdio> instead of using stdio.h and the same for the other C headers. What most posters fail to mention is that this should put all the symbols into namespace std and NOT into the global namespace. This you have to write std::printf(...). Simply writing printf alone will not work The same applies to the headers cassert ciso646 csetjmp cstdio ctime cctype climits csignal cstdlib cwchar cerrno clocale cstdarg cstring cwctype cfloat cmath cstddef.

This is clearly defined by ISO 14882 in footnote 160:
The ".h" headers dump all their names into the global namespace, whereas the newer forms keep their names in namespace
std. Therefore, the newer forms are the preferred forms for all uses except for C++ programs which are intended to be strictly compatible
with C.
Most compilers completely fail to implement this correctly and put all the names into both namespaces when using the new headers. As this is incorrect behaviour, it is quite likely to change in the future which means that code you write now may break on newer compilers as they get closer to the standard.

As an aside note that many compilers fail to give even a warning (and even on the highest warning level) when you write void main(), even though the standard clearly dictates that main MUST return int.

Nathan Myers, a member of the C++ standards committee wrote this paper on header strategy here: http://www.cantrip.org/cheaders.html