Speaking of sum programs (let's keep Razi out of it) here's my

typedef struct KahanAdder_t {
    double sum_; /* The current working sum. */
    double carry_; /* The carry from the previous operation */
    double temp_; /* A temp used to capture residual bits of precision
*/
    double y_; /* A temp used to capture residual bits of precision */
} KahanAdder_t;

 /* After each add operation, the carry will contain the additional */
 /* bits that could be left out from the previous operation. */
void            add(const double datum, KahanAdder_t * kp)
{
    kp->y_ = datum - kp->carry_;
    kp->temp_ = kp->sum_ + kp->y_;
    kp->carry_ = (kp->temp_ - kp->sum_) - kp->y_;
    kp->sum_ = kp->temp_;
}

void reset( KahanAdder_t * kp)
{
    kp->y_ = 0;
    kp->temp_ = 0;
    kp->carry_ = 0;
    kp->sum_ = 0;
}

#define MAXLINELEN 256

#include <stdio.h>
#include <stdlib.h>
#include <time.h>
#include <float.h>
int             main(void)
{
    KahanAdder_t    k = {0};
    double          d;
    double          standard_sum = 0;
    float           standard_sum_f = 0;
    clock_t         start, end;
    unsigned        count = 0;
    char line[MAXLINELEN];

    setvbuf(stdin, NULL, _IOFBF, 1024 * 16);
    start = clock();
    while (fgets(line, sizeof line, stdin)) {
        d = atof(line);
        add(d, &k);
        standard_sum += d;
        standard_sum_f += (float) d;
        count++;
    }
    end = clock();

    printf("Standard sum (double) = %20.15f\n"
           "Kahan sum (double)    = %20.15f\n"
           "Standard sum (float)  = %20.15f\n"
           "Number of items was %u\n"
           "Time consumed was %f seconds\n",
           standard_sum,
           k.sum_,
           standard_sum_f,
           count,
           (end - start) * 1.0 / CLOCKS_PER_SEC
           );

    return 0;
}
/* Possible Output from a file of 5,000,001 numbers:
C:\tmp\Release> foo.exe < c:\tmp\n.txt
Standard sum (double) = 27104700.447471067000000
Kahan sum (double)    = 27104700.447473072000000
Standard sum (float)  = 26857582.000000000000000
Number of items was 5000001
Time consumed was 4.594000 seconds
*/