When using C headers in C++, should we use functions from std:: or the global namespace?

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From the C++11 Standard (emphasis mine):

D.5 C standard library headers [depr.c.headers]

  1. For compatibility with the C standard library …
  2. Every C header, each of which has a name of the form name.h, behaves as if each name placed in the standard
    library namespace by the corresponding cname header is placed within the global namespace scope. It is unspecified whether these names are first declared or defined within namespace scope (3.3.6) of the namespace std and are then injected into the global namespace scope by explicit using-declarations (7.3.3).
  3. Example: The header <cstdlib> assuredly provides its declarations and definitions within the namespace
    std    . It may also provide these names within the global namespace. The header <stdlib.h> assuredly provides the same declarations and definitions within the global namespace, much as in the C Standard. It
    may also provide these names within the namespace std.

Using the «name.h» headers is deprecated, they have been identified as candidates for removal from future revisions.

So, I would suggest to include the «cname» headers and to use the declarations and definitions from the std namespace.

If you have to use the «name.h» headers for some reasons (it’s deprecated, see above), I would suggest to use the declarations and definitions from the global namespace.

In other words: prefer

#include <cstdio>

int main() {
    std::printf("Hello world\n");
}

over

#include <stdio.h>

int main() {
    printf("Hello world\n");
}

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