Question

I see this line from the c++ reference for cstdio:

Every element of the library is defined within the std namespace.
but I tried the code:

std::printf("hello world"); 

printf("hello world");

is it true that C++ headers puts the names in both the std and the global namespace?

Answer 1

Including cstdio imports the symbol names in std namespace and possibly in Global namespace.
Including stdio.h imports the symbol names in Global namespace and possibly in std namespace.

The same applies for all c-styled headers.

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Reference:
C++11 standard

Annex D (normative) Compatibility features [depr] states:

D.6 C standard library headers

1 For compatibility with the C standard library and the C Unicode TR, the C++ standard library provides the 25 C headers, as shown in Table 151.

Which include:

<assert.h> <float.h> <math.h> <stddef.h> <tgmath.h>
<complex.h> <inttypes.h> <setjmp.h> <stdio.h> <time.h>
<ctype.h> <iso646.h> <signal.h> <stdint.h> <uchar.h>
<errno.h> <limits.h> <stdarg.h> <stdlib.h> <wchar.h>
<fenv.h> <locale.h> <stdbool.h> <string.h> <wctype.h>

Further on,

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. —end example ]