In linux, is calloc exactly the same as malloc + memset or does this depend on the exact linux/kernel version?
I am particularly interested in the question of whether you can calloc more RAM than you physically have (as you can certainly malloc more RAM than you physically have, you just can't write to it). In other words, does calloc always actually write to the memory you have been allocated as the specs suggest it should.
Of course, that depends on the implementation, but on a modern day Linux, you probably can. Easiest way is to try it, but I'm saying this based on the following logic.
You can malloc more than the memory you have (physical + virtual) because the kernel delays allocation of your memory until you actually use it. I believe that's to increase the chances of your program not failing due to memory limits, but that's not the question.
calloc is the same as malloc but zero initializes the memory. When you ask Linux for a page of memory, Linux already zero-initializes it. So if calloc can tell that the memory it asked for was just requested from the kernel, it doesn't actually have to zero initialize it! Since it doesn't, there is no access to that memory and therefore it should be able to request more memory than there actually is.
As mentioned in the comments this answer provides a very good explanation.