DemoSave() / DemoLoad()

#include <stdio.h>
#define MAXPUNKTE   16

void DemoSave() {
    double feldX[MAXPUNKTE] = { 10, 20, 30, 40 };
    double feldY[MAXPUNKTE] = { 15, 25, 35, 45 };
    int feldLen = 4;

    FILE *file = fopen("demo.txt", "w");
    if (file != NULL) {
        for (int i = 0; i < feldLen; i++) {
            fprintf(file, "%.2lf %.2lf\n", feldX[i], feldY[i]);
        }
        fclose(file);
    }
}

void DemoLoad() {
    double feldX[MAXPUNKTE];
    double feldY[MAXPUNKTE];
    int feldLen = 0;

    FILE *datei = fopen("demo.txt", "r");
    if (datei != NULL) {
        while (fscanf(datei, "%lf %lf", &feldX[feldLen], &feldY[feldLen]) == 2) {
            feldLen++;
            if (feldLen >= MAXPUNKTE) {
                break; // too much
            }
        }
        fclose(datei);
    }

    for (int i = 0; i < feldLen; i++)
    {
        printf("%3i. Punkt: x : %lf\n", i + 1, feldX[i]);
        printf("            y : %lf\n", feldY[i]);
    }
}

int main(void)
{
    DemoSave();
    DemoLoad();
    getchar();
}