In the IDL help regarding SHMMAP and the OFFSET keyword, there is a mention of the alignment and the need to be careful when specifying the offset that seems related to your error. Perhaps this will help: Most computer hardware is not able to access arbitrary data types at arbitrary memory addresses. Data must be properly aligned for its type or the program will crash with an alignment error (often called a bus error) when the data is accessed. The specific rules differ between machines, but in many cases the address of a data object must be evenly divisible by the size of that object. IDL will issue an error if you specify an offset that is not valid for the array specified.