#include <stdlib.h>
#include <stdio.h>

int stringToDouble(const char* string, double* result)
{
    /* erstes Zeichen muss Ziffer, Minuszeichen oder das Komma sein */
    if((*string < '0' || *string > '9') && *string != '.' && *string != '-')
    {
        return -1;
    }

    /* String wandeln */
    char* endPointer;
    double value = strtod(string, &endPointer);

    /* endPointer zeigt hinter den double im String -> hier muss 0 sein (Stringende) */
    if(*endPointer != 0)
    {
        return -2;
    }

    /* Ergebnis speichern */
    *result = value;
    return 0;
}

int main()
{
    {
        double d = 0.0;
        int error = stringToDouble("123.456", &d);
        printf("Returncode: %d - Double: %f\n", error, d);
    }

    {
        double d = 0.0;
        int error = stringToDouble("-123.456", &d);
        printf("Returncode: %d - Double: %f\n", error, d);
    }

    {
        double d = 0.0;
        int error = stringToDouble("-.1E1", &d);
        printf("Returncode: %d - Double: %f\n", error, d);
    }

    {
        double d = 0.0;
        int error = stringToDouble("-.1E-1", &d);
        printf("Returncode: %d - Double: %f\n", error, d);
    }

    {
        double d = 0.0;
        int error = stringToDouble(" 123.456", &d);
        printf("Returncode: %d - Double: %f\n", error, d);
    }

    {
        double d = 0.0;
        int error = stringToDouble("a123.456", &d);
        printf("Returncode: %d - Double: %f\n", error, d);
    }

    {
        double d = 0.0;
        int error = stringToDouble("123.456 ", &d);
        printf("Returncode: %d - Double: %f\n", error, d);
    }

    {
        double d = 0.0;
        int error = stringToDouble("123.456a", &d);
        printf("Returncode: %d - Double: %f\n", error, d);
    }

    {
        double d = 0.0;
        int error = stringToDouble(" 123.456 ", &d);
        printf("Returncode: %d - Double: %f\n", error, d);
    }

    {
        double d = 0.0;
        int error = stringToDouble("a123.456a", &d);
        printf("Returncode: %d - Double: %f\n", error, d);
    }

    {
        double d = 0.0;
        int error = stringToDouble(".5", &d);
        printf("Returncode: %d - Double: %f\n", error, d);
    }
}