"*stringPtr++"

Why are you dereferencing the pointer in the for loop when you want just to increment it?

Represents sizes of objects in memory. Always non-negative. Adapts to the computer's architecture for portability. Enforces type safety for sizes and indices.

It is actually a very good solution because asprintf stops potential buffer overflowes.
Further read: https://stackoverflow.com/questions/12746885/why-use-asprintf-instead-of-sprintf