CODEWITHSUNDEEP
cmacroswarnings
Ewan Ahmed
Ewan Ahmed

Reputation: 23

Macro Redefined Warning

I'm writing this code and a warning appear when I tried to compile. 

#include <stdio.h>
#include <math.h>
#define EPS 1.5e-6
#define M_PI 3.14159265358979
int main()
{
double x1,x2,xm,y1,y2,ym; 
int m;
for(m=0;m<11;m++){
        x1=1.450;
        x2=1.489;
        y1=atan(pow(x1*x1-1.5*1.5,0.5)/pow(1.489*1.489-x1*x1,0.5))\
        + atan(pow(x1*x1-1.450*1.450,0.5)/pow(1.489*1.489-x1*x1,0.5))\
        - 4.5 * EPS * ((2 * M_PI)/(1.5 * EPS)) * pow(1.489*1.489-x1*x1,0.5)\
        + m*M_PI*1.5;
        y2=atan(pow(x2*x2-1.5*1.5,0.5)/pow(1.489*1.489-x2*x2,0.5))\
        + atan(pow(x2*x2-1.450*1.450,0.5)/pow(1.489*1.489-x2*x2,0.5))\
        - 4.5 * EPS * ((2 * M_PI)/(1.5 * EPS)) * pow(1.489*1.489-x2*x2,0.5)\
        + m*M_PI*1.5;
        if(y1*y2>0){
                printf("change initial values\n");
             }
        else{
        while(fabs(x1-x2)>EPS){
        xm=(x1+x2)/2;
        ym=atan(pow(xm*xm-1.5*1.5,0.5)/pow(1.489*1.489-xm*xm,0.5))\
        + atan(pow(xm*xm-1.450*1.450,0.5)/pow(1.489*1.489-xm*xm,0.5))\
        - 4.5 * EPS * ((2 * M_PI)/(1.5 * EPS)) * pow(1.489*1.489-xm*xm,0.5)\
        + m*M_PI*1.5;
        if(y1*ym>0){
 x1=xm;
 }
else{
x2=xm;
}
        }
        printf("n[%d] = %.9f;\n",m, xm);
}
}
return 0; }

The warning is :

warning: 'M_PI' macro redefined [-Wmacro-redefined]

I can't figure out how to make the warning disappear

Upvotes: 1

Views: 23270

Answers (2)

P.P
P.P

Reputation: 121387

POSIX defines a macro M_PI as the value of Pi as an extension to C standard. So, if you are on POSIX system, you don't need to define your own M_PI.

But if you don't want to then you can compile only in standard C mode such as:

gcc -Wall -Wextra -std=c11 file.c

Upvotes: 1

Kerrek SB
Kerrek SB

Reputation: 477020

If there's a possibility that a macro you want to use has already been defined (and presumably defined to a useful value), then you can simply check for that:

#ifndef M_PI
#  define M_PI my_value_here
#endif

Or, if you don't trust the existing value, you can abort translation in that case:

#ifdef M_PI
#  error Macro M_PI must not be defined
#else
#  define M_PI my_value_here
#endif

It seems like the GNU C library defines those macros (conditionally) because the "Unix98 standard" requires them.

Upvotes: 6

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