WRE explains that R_alloc() can be used to allocate memory which automatically gets released by R at the end of a .C, .Call or .External, even in the case of an error or interruption. This is a really great feature to prevent memory leaks. I was wondering if there is a way to extend this mechanism to allow for automatically running UNPROTECT and custom finalizers at the end of a .Call as well.
Currently it is all to easy for package authors to introduce a memory leak or stack imbalance by calling Rf_error() or R_CheckUserInterrupt() in a way that skips over the usual cleanup steps. This holds especially for packages interfacing C libraries (libcurl, libxml2, openssl, etc) which require xx_new() and xx_free() functions to allocate/free various types of objects, handles and contexts. Therefore we cannot use R_alloc() and we need to manually clean up when returning, which is tricky for irregular exits. Moreover package authors might benefit from an alternative of allocVector() which automatically protects the SEXP until the .Call is done. Perhaps I don't fully appreciate the complexity of the garbage collector, but one could imagine a variant of PROTECT() which automatically keeps a counter 'n' for the number of allocated objects and makes R run UNPROTECT(n) when .Call exists, along with releasing the R_alloc() memory. Yes, there are cases where it is useful to have manual control over what can be collected earlier during the .Call procedure, but these are rare. A lot of C code in packages might become safer and cleaner if authors would have an option to let this be automated. The most general feature would a hook for adding custom C functions to the .Call exit, similar to on.exit() in R: xmlNodePtr *node = xmlNewNode(...); Rf_on_exit(xmlFreeNode, node); EVP_PKEY_CTX *ctx = EVP_PKEY_CTX_new(...); Rf_on_exit(EVP_PKEY_CTX_free, ctx); SEXP out = PROTECT(allocVector(...)); Rf_on_exit(UNPROTECT, 1); I don't know R's internals well enough to estimate if something like this would be possible. I did put together a simple C example of a linked list with object pointers and their corresponding free functions, which can easily be free'd with a single call: http://git.io/vBqRA . So basically what is mostly missing at this point is a way to trigger this at the end of the .Call in a way that works for regular returns, errors and interruptions... ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
