# How to use the PI constant in C++

I want to use the PI constant and trigonometric functions in some C++ program. I get the trigonometric functions with `include <math.h>`. However, there doesn’t seem to be a definition for PI in this header file.

How can I get PI without defining it manually?

On some (especially older) platforms (see the comments below) you might need to

```#define _USE_MATH_DEFINES
```

and then include the necessary header file:

```#include <math.h>
```

and the value of pi can be accessed via:

```M_PI
```

In my `math.h` (2014) it is defined as:

```# define M_PI           3.14159265358979323846  /* pi */
```

but check your `math.h` for more. An extract from the “old” `math.h` (in 2009):

```/* Define _USE_MATH_DEFINES before including math.h to expose these macro
* definitions for common math constants.  These are placed under an #ifdef
* since these commonly-defined names are not part of the C/C++ standards.
*/
```

However:

1. on newer platforms (at least on my 64 bit Ubuntu 14.04) I do not need to define the `_USE_MATH_DEFINES`

2. On (recent) Linux platforms there are `long double` values too provided as a GNU Extension:

```# define M_PIl          3.141592653589793238462643383279502884L /* pi */
```