Never mind modern systems, malloc() was never a syscall :-) One of the great things about K&R is that it shows you how to implement parts of the C library, including a simple malloc(), demonstrating that the library does not need to be magical.
Yes, and often mmap() for large allocations and other parts of the heap.
There has been an interesting discussion about memory management in Ritchie’s PDP11 C compiler on the TUHS list this month https://minnie.tuhs.org/pipermail/tuhs/2020-August/thread.ht... from the era when large programs could not necessarily afford the overhead of malloc() so sometimes used sbrk() directly.
