Try this code. It fits y = mx + b to your (x,y) data.

The arguments to linreg are

linreg(int n, REAL x[], REAL y[], REAL* b, REAL* m, REAL* r)

n = number of data points
x,y  = arrays of data
*b = output intercept
*m  = output slope
*r = output correlation coefficient (can be NULL if you don't want it)

The return value is 0 on success, !=0 on failure.

Here’s the code

#include "linreg.h"
#include <stdlib.h>
#include <math.h>                           /* math functions */

//#define REAL float
#define REAL double


inline static REAL sqr(REAL x) {
    return x*x;
}


int linreg(int n, const REAL x[], const REAL y[], REAL* m, REAL* b, REAL* r){
    REAL   sumx = 0.0;                      /* sum of x     */
    REAL   sumx2 = 0.0;                     /* sum of x**2  */
    REAL   sumxy = 0.0;                     /* sum of x * y */
    REAL   sumy = 0.0;                      /* sum of y     */
    REAL   sumy2 = 0.0;                     /* sum of y**2  */

    for (int i=0;i<n;i++){ 
        sumx  += x[i];       
        sumx2 += sqr(x[i]);  
        sumxy += x[i] * y[i];
        sumy  += y[i];      
        sumy2 += sqr(y[i]); 
    } 

    REAL denom = (n * sumx2 - sqr(sumx));
    if (denom == 0) {
        // singular matrix. can't solve the problem.
        *m = 0;
        *b = 0;
        if (r) *r = 0;
            return 1;
    }

    *m = (n * sumxy  -  sumx * sumy) / denom;
    *b = (sumy * sumx2  -  sumx * sumxy) / denom;
    if (r!=NULL) {
        *r = (sumxy - sumx * sumy / n) /    /* compute correlation coeff */
              sqrt((sumx2 - sqr(sumx)/n) *
              (sumy2 - sqr(sumy)/n));
    }

    return 0; 
}

Example

You can run this example online.

int main()
{
    int n = 6;
    REAL x[6]= {1, 2, 4,  5,  10, 20};
    REAL y[6]= {4, 6, 12, 15, 34, 68};

    REAL m,b,r;
    linreg(n,x,y,&m,&b,&r);
    printf("m=%g b=%g r=%g\n",m,b,r);
    return 0;
}

Here is the output

m=3.43651 b=-0.888889 r=0.999192    

Here is the Excel plot and linear fit (for verification).

All values agree exactly with the C code above (note C code returns r while Excel returns R**2).