/Zc:cmath (Enable libc math)

The /Zc:cmath compiler option replaces the runtime implementation of math functions in the <math.h> C runtime header and <cmath> C++ standard library header with LLVM's libc. This implementation provides correctly rounded results in all rounding modes.

You need this option for compile-time evaluation of the C++ standard library math functions that become constexpr in C++23 or later.

This option is experimental.

Syntax

/Zc:cmath

Remarks

You need the /Zc:cmath option, along with /std:c++23, for compile-time evaluation of the math functions that become constexpr in C++23 by proposal P0533R9: constexpr for <cmath> and <cstdlib> and LWG Issue 3834: Missing constexpr for std::intmax_t math functions in <cinttypes>.

The /Zc:cmath option is available starting in MSVC Build Tools version 14.52. It's off by default.

Enabling this option might affect the performance and accuracy of math functions declared in <math.h> and <cmath>.

When you use /Zc:cmath, the C++ toolset automatically links against libvcmath-mt.lib or libvcmath-md.lib when you call math functions from a standard library header. These libraries are statically linked into the resulting binary.

Note

System include directives use angle brackets, so #include <math.h> is recognized but #include "math.h" isn't.

This option replaces system include directives that include <math.h> and <stdlib.h> with directives that include standard library replacement headers. The option affects <cmath> and <cstdlib> because they include <math.h> and <stdlib.h>. The replacement headers differ from their corresponding UCRT headers in the following ways:

• Math function definitions are noexcept, which conflicts with declarations that aren't noexcept. See C2382. Potential fixes are to remove unnecessary forward declarations or to add noexcept to forward declarations.

• Functions and macro definitions that aren't part of the C++ standard aren't provided. This exclusion includes the Math Constants, such as M_PI. The <numbers> standard library header provides conformant definitions of corresponding variables. To continue using these math constants, directly include <corecrt_math_defines.h>. _USE_MATH_DEFINES has no effect when you use /Zc:cmath.

Background

By default, the Universal C Runtime (UCRT) implements the math functions declared in <math.h> and <cmath>. Windows maintains and deploys the UCRT, so math function results can vary by Windows version.

When you enable /Zc:cmath, LLVM's libc implements these math functions in statically linked libraries named libvcmath-mt.lib and libvcmath-md.lib. The C++ build tools deploy these libraries, not the Windows operating system. Because these functions return correctly rounded results, their results are stable across operating system versions and hardware architectures.

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:cmath to the Additional Options: pane.

See also

/Zc (Conformance)
/std (Specify language standard version)
Math Constants