Cinary BinaryWriter Length Prefix - UTF7 Encoding

I have a project using memory mapped files that allow two applications to exchange data with each other. The manufacturer application is written in C #, the consumer application speaks of a simple old C. Both use VS2010.

MSDN says that the “BinaryWriter.Write Method (String)” adds data using an UTF-7 encoded integer, and then writes the payload. This is exactly where I got stuck. If I write a string with a length of 256 characters, the application debugger C shows me this sequence of bytes: 0x80 0x2 and 256 times the char> payload. What is the best way to convert a length prefix to something that I can safely use in a consumer application?

Producer Application:

using System;
using System.IO;
using System.IO.MemoryMappedFiles;
using System.Threading;
using System.Text;
using System.Linq;

class Program
{
    static void Main(string[] args)
    {
        using (MemoryMappedFile mmf_read = MemoryMappedFile.CreateNew("mappedview", 4096))
        {
            using (MemoryMappedViewStream stream = mmf_read.CreateViewStream())
            {
                string str;
                BinaryWriter writer = new BinaryWriter(stream);

                str = string.Join("", Enumerable.Repeat("x", 256));

                writer.Write(str);
            }
        }
    }
}

:

#include <windows.h>
#include <stdio.h>
#include <conio.h>
#include <tchar.h>
#pragma comment(lib, "user32.lib")

#define BUF_SIZE 4096
TCHAR szName[]=TEXT("Global\\mappedview");


int _tmain()
{
    HANDLE hMapFile;
    LPCSTR pBuf;

    hMapFile = OpenFileMapping(
               FILE_MAP_ALL_ACCESS,         // read/write access
               FALSE,                       // do not inherit the name
               szName);                     // name of mapping object

    if (hMapFile == NULL)
    {
        _tprintf(TEXT("Could not open file mapping object (%d).\n"),
         GetLastError());
        return 1;
    }

    pBuf = (LPCSTR) MapViewOfFile(hMapFile,     // handle to map object
           FILE_MAP_ALL_ACCESS,             // read/write permission
           0,
           0,
           BUF_SIZE);

    if (pBuf == NULL)
    {
        _tprintf(TEXT("Could not map view of file (%d).\n"),
                GetLastError());

        CloseHandle(hMapFile);
        return 1;
    }

    printf("Proc1: %s\n\n", pBuf);              // print mapped data

    UnmapViewOfFile(pBuf);

    CloseHandle(hMapFile);

    return 0;
}

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