What is this function going to return? You need to specify the return type.
Next you don't want to surround the void* with parentheses. By using the parentheses you are indicating a cast operation. So this function prototype and function implementation should look like: void cube_updater( void* rs232_cube)
This saying that this function will not return anything and it takes one parameter (a void pointer).
It looks as though jlb assumed that you wanted your function to return nothing. A perfectly natural assumption, since your function doesn't return anything, and doesn't specify a return type!
What that error message is telling you is that your function needs to return a void*, because the pthread library expects that. So you need to give your function that return type in both the prototype and the implementation, and make it return the appropriate pointer.