Passing string to dll

I'm passing some strings to a 'C' dll (which is written by myself) that expects char* argument . it's passed as 'byval string' argument. The data to be passed is not of fixed length so the string is not fixed length. but in this process sometimes it's not passed properly and the data is a garbage.
What can be other way to pass this string properly.

In your C++ code use BSTR instead of char*