Complexity of R Garbage Collector in .Call()

Cited from R’s C interface

An additional complication is the garbage collector: if you don’t protect every R object you create, the garbage collector will think they are unused and delete them.

PROTECT() calls do. They tell R that the object is in use and shouldn’t be deleted if the garbage collector is activated. (We don’t need to protect objects that R already knows we’re using, like function arguments.)

You also need to make sure that every protected object is unprotected. UNPROTECT() takes a single integer argument, n, and unprotects the last n objects that were protected. The number of protects and unprotects must match. If not, R will warn about a “stack imbalance in .Call”.