Question

I need to use the C library in a C# project. How can I do?

To be more specific: for reasons of efficiency I need to use the strtod function to extract double values from a string (like this “9.63074,9.63074 -5.55708e-006 0 ,0 1477.78”). If you have suggestions about how to optimize this operation do not be shy, but the main question remains that specified by title.

Answer 1

I think it very unlikely that p/invoking to strtod would be more efficient than a pure C# solution. There is an overhead in managed/unmanaged transitions and I would think that would be significant for something as trivial as strtod. Myself I would use a C# tokenizer, combined with double.Parse.

The simplest C# tokenizer is String.Split() which yields this routine:

static List<double> getValues(string str)
{
    List<double> list = new List<double>();
    foreach (string item in str.Split(default(Char[]), StringSplitOptions.RemoveEmptyEntries))
        list.Add(double.Parse(item));
    return list;
}

---

However, since I enjoy p/invoke, here’s how you would call strtod from C#, bearing in mind that I recommend you don’t use this approach in real code.

[DllImport(@"msvcrt.dll", CallingConvention=CallingConvention.Cdecl)]
static extern double strtod(IntPtr str, ref IntPtr endptr);

You can call it like this:

IntPtr str = Marshal.StringToHGlobalAnsi(inputStr);
IntPtr endptr = IntPtr.Zero;
double val = strtod(str, ref endptr);
Marshal.FreeHGlobal(str);

I’m passing the string as an IntPtr because you would be calling strtod repeatedly to walk across the entire buffer. I didn’t show that here, but if you are going to make any use of endptr then you need to do it as I illustrate.

Of course, to use strtod remotely effectively you need to gain access to the errno global variable. The very fact that you need to deal with a global variable should be warning enough that here be dragons. What’s more, the error reporting offered through errno is exceedingly limited. However, if you want it, here it is:

[DllImport(@"msvcrt.dll", CallingConvention = CallingConvention.Cdecl)]
static extern int _get_errno();

One final point. Your suggested input string is

"9.63074,9.63074 -5.55708e-006 0 ,0 1477.78"

but strtod won’t tokenize that because of the spurious commas.