17 Language support library [support]

17.15 C headers [support.c.headers]

17.15.1 General [support.c.headers.general]

For compatibility with the C standard library, the C++ standard library provides the C headers shown in Table 41 .

The intended use of these headers is for interoperability only.

It is possible that C++ source files need to include one of these headers in order to be valid ISO C. Source files that are not intended to also be valid ISO C should not use any of the C headers.

[Note 1:

The C headers either have no effect, such as <stdbool.h> and <stdalign.h>, or otherwise the corresponding header of the form <cname> provides the same facilities and assuredly defines them in namespace std.

— end note]

[Example 1:

The following source file is both valid C++ and valid ISO C. Viewed as C++, it declares a function with C language linkage; viewed as C it simply declares a function (and provides a prototype).

#include <stdbool.h> // for bool in C, no effect in C++ #include <stddef.h> // for size_t #ifdef __cplusplus // see [cpp.predefined] extern "C" // see [dcl.link] #endif void f(bool b[], size_t n); — end example]

Table 41: C headers [tab:c.headers]

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

17.15.2 Header <complex.h> synopsis [complex.h.syn]

The header <complex.h> behaves as if it simply includes the header <complex>.

[Note 1:

Names introduced by <complex> in namespace std are not placed into the global namespace scope by <complex.h>.

— end note]

17.15.3 Header <iso646.h> synopsis [iso646.h.syn]

The C++ header <iso646.h> is empty.

[Note 1:

and, and_eq, bitand, bitor, compl, not_eq, not, or, or_eq, xor, and xor_eq are keywords in C++ ([lex.key]).

— end note]

17.15.4 Header <stdalign.h> synopsis [stdalign.h.syn]

The contents of the C++ header <stdalign.h> are the same as the C standard library header <stdalign.h>, with the following changes: The header <stdalign.h> does not define a macro named alignas.

17.15.5 Header <stdbool.h> synopsis [stdbool.h.syn]

The contents of the C++ header <stdbool.h> are the same as the C standard library header <stdbool.h>, with the following changes: The header <stdbool.h> does not define macros named bool, true, or false.

17.15.6 Header <tgmath.h> synopsis [tgmath.h.syn]

The header <tgmath.h> behaves as if it simply includes the headers <cmath> and <complex>.

[Note 1:

The overloads provided in C by type-generic macros are already provided in <complex> and <cmath> by “sufficient” additional overloads.

— end note]

[Note 2:

Names introduced by <cmath> or <complex> in namespace std are not placed into the global namespace scope by <tgmath.h>.

— end note]

17.15.7 Other C headers [support.c.headers.other]

Every C header other than <complex.h>, <iso646.h>, <stdalign.h>,
<stdatomic.h>, <stdbool.h>, and <tgmath.h>, 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, except for the functions described in [sf.cmath], the declaration of std::byte ([cstddef.syn]), and the functions and function templates described in [support.types.byteops].

It is unspecified whether these names are first declared or defined within namespace scope ([basic.scope.namespace]) of the namespace std and are then injected into the global namespace scope by explicit using-declarations ([namespace.udecl]).

[Example 1:

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]