M_PI flagged as undeclared identifier

m_pi undefined visual studio
perl m_pi
mingw m_pi
pi in c++
cmake _use_math_defines
cpp cmath pi
math.h pi

When I compile the code below, I got these error messages:

(Error  1   error C2065: 'M_PI' : undeclared identifier 
2   IntelliSense: identifier "M_PI" is undefined)

What is this?

#include <iostream>
#include <math.h>

using namespace std;

double my_sqrt1( double n );`enter code here`

int main() {
double k[5] = {-100, -10, -1, 10, 100};
int i;

for ( i = 0; i < 5; i++ ) {
    double val = M_PI * pow( 10.0, k[i] );
    cout << "n: "
         << val
         << "\tmysqrt: "
         << my_sqrt1(val)
         << "\tsqrt: "
         << sqrt(val)
         << endl;

return 0;

double my_sqrt1( double n ) {
int i;
double x = 1;

for ( i = 0; i < 10; i++ ) {
    x = ( x + n / x ) / 2;

return x;

It sounds like you're using MS stuff, according to their docs

Math Constants are not defined in Standard C/C++. To use them, you must first define _USE_MATH_DEFINES and then include cmath or math.h.

So you need something like

#include <cmath>

as a header.

using MEX file and constants - MATLAB Answers, now I'm getting an error): Error GMSK_viterbi.c: 66 undeclared identifier `PI' Then use M_PI for the value in your code instead of PI, or you could add this  Greetings friend, I don't know why I get all those error, can you help me with it? VS version I use is Visual Studio Community 2017, MSbuild version is 15.0; if you don't have time, please at least tell me what tool you use to build Deep

math.h does not define M_PI by default.

So go with this:

#ifndef M_PI
    #define M_PI 3.14159265358979323846

This will handle both cases either your header have M_PI defined or not.

VS Build error, 'M_PI': undeclared identifier, and other strange error , Greetings friend, I don't know why I get all those error, can you help me with it? VS version I use is Visual Studio Community 2017, MSbuild  This question already has an answer here: M_PI works with math.h but not with cmath in Visual Studio 4 answers

M_PI is supported by GCC too, but you've to do some work to get it

#undef __STRICT_ANSI__
#include <cmath>

or if you don't like to pollute your source file, then do

g++ -U__STRICT_ANSI__ <other options>

M_PI unresolved?? (iOS-only), /Users/knassar/dev/sotf/sotf/Classes/Geometry.swift:210:28: Use of unresolved identifier 'M_PI'. Running Xcode 7.3.1 (7D1014) & OSX 10.11.6 Beta (15G12a). #ifndef M_PI #define M_PI (3.14159265358979323846) #endif bitbionic added a commit to bitbionic/godot that referenced this issue Nov 5, 2017 Adding fix for godotengine#12688 by wrapping M_PI in an #ifndef check

As noted by shep above you need something like

#include <cmath>

However you also include iostream.

iostream includes a lot of stuff and one of those things eventually includes cmath. This means that by the time you include it in your file all the symbols have already been defined so it is effectively ignored when you include it and the #define _USE_MATH_DEFINES doesn't work

If you include cmath before iostream it should give you the higher precision constants like M_PI

#include <cmath>
#include <iostream>

11196 – _GNU_SOURCE vs. M_PI - GCC, After undefined the M_PI, no error be found. declared /usr/include/c++/4.1.2/​cstdlib:177: error: 'lldiv_t' does not name a type /usr/include/c++/4.1.2/cstdlib:181: error: Bug 35557 has been marked as a duplicate of this bug. c++ - M_PI works with math.h but not with cmath in Visual Studio . I am using Visual Studio 2010. I have read that in C++ it is better to use<cmath> rather than<math.h>. But in the program I am trying to write(Win32 console application, empty project) if I write:…

I used C99 in NetBeans with remote linux host with its build tools. Try adding #define _GNU_SOURCE and add the -lm during linking.

Compile Errors - MSDN, Compiling NormInv2.cpp. C:\Testing\NormInv2\NormInv2.cpp(23) : error C2065: 'M_PI' : undeclared identifier Marked as answer by Wesley Yao Monday, March 29, 2010 3:51 AM. Monday, March 22, 2010 6:07 PM. Reply. CopyFile flagged as undeclared identifier. Ask Question Asked 4 years, 8 months ago. Active 4 years, 8 months ago. Viewed 2k times 2. Created a Copy Function and when

M_PI works with math.h but not with cmath in Visual Studio, M_PI works with math.h but not with cmath in Visual Studio - c++. #include <​cmath> it fails with error C2065: 'M_PI' : undeclared identifier Is it normal? eigenViewMatrix = Map<Matrix4f>(viewMatrix, 4, 4); I guess there is a flag to add, but I  Problem. With these premises, if I aim to update for example icub-main/ctrlLib to the use of cmath along with _USE_MATH_DEFINES (removing thus the explicit definition of M_PI), I will then easily get the library compiled, but this change will cause very likely errors while compiling code depending on ctrlLib, unless I'll put the same directive add_definitions(-D_USE_MATH_DEFINES) in the

VS2017 'M_PI': undeclared identifier [duplicate] - c++ - android, VS2017 'M_PI': undeclared identifier [duplicate] - c++. I am using MFC as shared dll, I am compiling with /MDd and checked that -D_MT flag is on. (I am also  patrikhuber changed the title Compiling the example fails on Windows because of M_PI Compiling the example fails on VS2015 because of M_PI May 3, 2017 This comment has been minimized. Sign in to view

Math constants for C++ – Code Yarns 👨‍💻, This results in code which uses such constants, say M_PI in throwing errors 1. C:\Foobar.cpp(42): error C2065: 'M_PI' : undeclared identifier  M_PI flagged as undeclared identifier. 0. Identifier not found and undeclared identifier. Hot Network Questions Why were most PC's and electronics beige back in the

  • M_PI is not actually part of the standard, so no guarantee that it's been defined in math.h
  • And please please improve your title.
  • @OJFord Done, long back i.e. :)
  • i am using Visual Studio 2013 this is why that is not running right? is it running in g++ ?
  • Dear Eunsu,if you go inside the head file <math.h> inside "Exsternal Dependencies" of Visual Studio 2013, you can see that this is part of the code: '#if defined (_USE_MATH_DEFINES) && !defined (_MATH_DEFINES_DEFINED) /* **** / #define M_PI 3.14159265358979323846 / **** */ #endif' If you want to use this #define M_PI you MUST define _USE_MATH_DEFINES with an #define. This is the reason!
  • It makes sense but doesn't work for me... I am using VS 2015 Community...
  • Thank you so much! I know this is an old post, but I would like to know why that works. I am new to C++.
  • C++ headers have macros, includes, variables, functions and classes -- declared, defined or both. Now depending on a few flags (lookup conditional compilation) the pre-processor includes / excludes them. Strictly speaking, the C++ standard doesn't mandate M_PI and hence GCC's header defaults to not including its definition. If someone needs it, they've to ask for it by undefining the guard that excludes it i.e. __STRICT_ANSI__. You can search your compiler's math.h (original C header of cmath) for M_PI and you'd see the guards surrounding it. Hope that answers your question.
  • Worked for me under Win10 + Msys2 Portable.