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description Learn more about: /Zc:externC (Use Standard C++ extern "C" rules)
title /Zc:externC (Use Standard C++ extern "C" rules)
ms.date 12/02/2021
f1_keywords
/Zc:externC
helpviewer_keywords
-Zc:externConstexpr compiler option (C++)
extern constexpr variables (C++)
ms.assetid 4da5e33a-2e4d-4ed2-8616-bd8f43265c27

/Zc:externC (Use Standard C++ extern "C" rules)

The /Zc:externC compiler option tells the compiler to conform to the C++ standard and enforce consistent parameter declarations for functions declared as extern "C".

Syntax

/Zc:externC
/Zc:externC-

Remarks

The /Zc:externC compiler option checks the definitions of functions declared by using extern "C".

The /Zc:externC option is available starting in Visual Studio 2019 version 16.3. It's off when the /permissive- option isn't set. In earlier versions of Visual Studio, and by default or if /Zc:externC- is specified, Visual Studio is permissive about matching declarations of extern "C" functions. The /permissive- option enables /Zc:externC, so it's on by default in projects that use /std:c++20 or /std:c++latest. The /Zc:externC option must come after a /permissive- option on the command line.

Mismatched extern "C" declarations can cause compiler errors C2116 and C2733. In C++ code, an error can occur if you declare an extern "C" function more than once and use different parameter types, even if the types have the same definitions. The /Zc:externC- option relaxes this check, and doesn't produce these errors.

To set this compiler option in Visual Studio

  1. Open the project's Property Pages dialog box. For details, see Set C++ compiler and build properties in Visual Studio.

  2. Select the Configuration Properties > C/C++ > Command Line property page.

  3. Add /Zc:externC or /Zc:externC- to the Additional options: pane.

See also

/Zc (Conformance)