问从C#到C++非托管代码的多个函数调用会导致AccessViolationException

EN

[DllImport("C:\\c_keycode.dll", EntryPoint = "generateKeyCode", 
           CallingConvention = CallingConvention.Cdecl)]
static extern IntPtr generateKeyCode(char[] serial, char[] option, char c_type);

const char* generateKeyCode(char serial[], 
                 char option[], 
                 char c_type)
{
returnBufferString = "";
SHA1_CTX context;
int optionLength = 0;
#ifdef WIN32
unsigned char buffer[16384] = {0};
#else
unsigned char buffer[256] = {0}; 

#endif
//char output[80];
char keycode[OPTION_KEY_LENGTH+1]        = "";
int digest_array_size = 10; //default value for digest array size
unsigned char digest[20] = {0};
char optx[24] = {0};
char c_type_upper;
// Combine serial # and Option or Version number

char str1[30] = {0};
int i;
int size = 0;
int pos = 0;


...
...
}

public static string genKeyCode_marshal(string serial, string option, char type)
{
    return Marshal.PtrToStringAnsi(generateKeyCode(serial.ToCharArray(), 
           option.ToCharArray(), type));
}