I am slightly embarrassed I didn’t know overallocate was the term for that. TIL!
Memory Allocation
81–90 of 182 posts
Re: Memory Allocation
#82Sam, this is such a wonderful resource that you've put out into the world. Thank you for the time and care you've put into each paragraph and interactive component. You've not only given me a lot to think about in terms of my basic assumptions about memory, but also about how to write and teach better online. I'm really excited to start following you!
Re: Memory Allocation
#83Oh man, this brings back the days when I wrote special debug-version malloc and free code to try to track down heap corruption due to malloc / free misuse (in code I had contributed to). Stuff like kbyte-long boundary buffers with bit patterns in them, and all sorts of lookaside lists run in parallel with libc's default code. Those bug-detectors worked OK. Hard-nosed code inspection worked far better.
As an old-timer in writing code, I think my generation's most-challenging legacies (=== the things we f**ked up) are the non-robust malloc/free concept and null-terminated text strings. Bugs in code using those conventions have been exploitable to a really damaging extent. I learned to program in C from K&R. And getting data-structure code right, and safe to deploy, in that language and its derivatives is HARD.
The C inventors are (were in Dennis Ritchie's case) brilliant and Bell Labs was great. Their ideas shaped the the stuff the global internet runs on. But these parts of what thy invented ..... ouch. (All OSs had the same problems.)
I wish the wonderful article presented here carried a more prominent warning about this. Many will read it as they learn to code. The history of our profession can teach about what NOT to do as well as what to do.
Re: Memory Allocation
#84This is absolute gold. When I use things like this, I am reminded how powerful digital learning can be. So much more capable then just text or video. But very little content is this well put together. Probably because it is so time intensive.
This feedback has made my day. Thank you. I'm inspired by Bartosz Ciechanowski and Julia Evans. The web is such a powerful toolbox. So many concepts are more complex than text alone can hope to explain. Those two are so creative and full of energy. And you're right, it's incredibly time intensive to put these together. Thousands of lines of code, plus the text content, plus reaching out to domain experts for reviews…
Such great work. I learned things and have a clearer understanding that I think I will come back to in the future. And I say that as someone who has programmed for 15 years! I think your effort paid off, and the inspiration shows through.
Re: Memory Allocation
#85So good! I wish I had an article like this to learn from when I implemented my language's memory allocator a few months ago! Wonderful, thank you!
Re: Memory Allocation
#86> When we free memory, we should make sure that if the block we return to the free list is next to any other free blocks, we combine them together. This is called "coalescing." A little offtopic but the default Delphi 7 memory allocator did this, except that it also merged blocks that it obtained from different OS allocation calls. This worked fine for regular usage, but if that memory was ever used for Bitmaps for U…
You mean a bug this deep took one day to debug?
Re: Memory Allocation
#87This is a nice read too (the whole blog actually)
http://igoro.com/archive/volatile-keyword-in-c-memory-model-...
It's a bit old by now (2010), but I always remembered the mental model Igor created.
Re: Memory Allocation
#88It's somewhat incomplete without a discussion of how one actually gets the memory allocate to a program.
Could you elaborate on what you mean?
It talks about how a simple malloc implementation would doll out entries in a buffer of memory and manage free lists, but not how the implementation actually gets that buffer from the operating system, or return it to the system when done (mmap, munmap, for example).
Re: Memory Allocation
#89Re: Memory Allocation
#90I love this so much, thank you for putting this together! My only piece of feedback would be for the "Inline Bookkeeping" section ( https://samwho.dev/memory-allocation/#inline-bookkeeping ), it took a while for me to grok the numbered list to figure out which block corresponded to address + X. I wonder if there is a better way to visualize the 4 numbered bullet points? Maybe just arrows and text pointing to the visu…
Yes, this is one of the things I look back on as a weak point in the writing. I wanted to make this a lot more clear but by the time I'd gotten to this point in the article, I'd sort of coded myself into a corner with the visualisation code. In another universe I'd hook in to the page scroll and highlight each block being referred to as I talk about it in the text. I'm probably not explaining that well here, but imag…
Couldn't you highlight each block being referred to as you mouse over / click on relevant text? (The latter might not be terribly hard to do with and CSS :target, if you can give the blocks ids.)