The difficulty is that the lack of memory might be discovered days after it was allocated (an over-committing allocator simply doesn’t know at the time whether

The difficulty is that the lack of memory might be discovered days after it was allocated (an over-committing allocator simply doesn’t know at the time whether there will be memory or not) - how do you asynchronously tell the program that this allocation has now failed in a high-level way? Generally, UNIX will let you know about the missing memory by sending the process a signal. But by that point, there’s not much that can be done to fix things up - and remember, all the fix up code would have to run without allocating any more memory itself. That’s extremely tricky in C, and nigh-on impossible in other languages.