Convert tvbuff_t* to unsigned char*

Is there a nice clean way to convert tvbuff_t to an unsigned char? I am using an external library for some decryption and it accepts the unsigned char*. I was looking through the Wireshark source code but only came up with:

struct tvbuff;
typedef struct tvbuff tvbuff_t;

Which doesn't give me enough information to be able to convert, that is unless I can simply do a byte for byte conversion?

Thank you for your time, Brandon