This is awesome! To anyone interested in learning more about this, I wrote https://cpu.land/ a couple years ago. It doesn't go as in-depth into e.g. memory layout as OP does but does cover multitasking and how the code is loaded in the first place.
The Journey Before main()
31–40 of 145 posts
Re: The Journey Before main()
#32Re: The Journey Before main()
#33It's also possible to pack a whole codebase into "before main()" - or with no main() at all. I was recently experimenting doing this, as well as a whole codebase that only uses main() and calls itself over and over. Good fun: https://joshua.hu/packing-codebase-into-single-function-disr...
That is a really fun read and honestly doesn't even seem to be complicated and brittle. Just rename every function to main(100+n, ...).
Re: The Journey Before main()
#34As someone who teaches this stuff at university, I see students getting confused every single year by how textbooks draw memory. The problem is mostly visual, not conceptual.
Most diagrams in books and slides use an old hardware-centric convention: they draw higher addresses at the top of the page and lower addresses at the bottom. People sometimes justify this with an analogy like “floors in a building go up,” so address 0x7fffffffe000 is drawn “higher” than 0x400000.
But this is backwards from how humans read almost everything today. When you look at code in VS Code or any other IDE, line 1 is at the top, then line 2 is below it, then 3, 4, etc. Numbers go up as you go down. Your brain learns: “down = bigger index.”
Memory in a real Linux process actually matches the VS Code model much more closely than the textbook diagrams suggest.
You can see it yourself with:
cat /proc/$$/maps
(pick any PID instead of $$).
...
...
[0x00643000] HEAP extended ↓ (more allocations => higher addresses)
...
↑ the stack pointer starts here and moves upward
(toward lower addresses) when you push ...
...
The phrases “the heap grows up” and “the stack grows down” aren't wrong. They're just describing what happens to the numeric addresses: the heap expands toward higher addresses, and the stack moves into lower addresses.
The real problem is how we draw it. We label “up” on the page as “higher address,” which is the opposite of how people read code or even how /proc//maps is printed. So students have to mentally flip the diagram before they can even think about what the stack and heap are doing.
If we just drew memory like an editor (low addresses at the top, high addresses further down) it would click instantly. Scroll down, addresses go up, and the stack sits at the bottom. At that point it’s no longer “the stack grows down”: it’s just the stack pointer being decremented, moving to lower addresses (which, in the diagram, means moving upward).
Re: The Journey Before main()
#35Earlier quoted context omitted.
Obviously only a requirement if you intend your software to run under windows. But if you don't, why bother. Not all software is intended to be distributed to users far and wide. Some of it is just for yourself, and some of it will only ever run on linux servers.
> some of it will only ever run on linux servers. I’ve spent quite a lot of time dealing with code that will ever run on Linux which did not in fact only ever run on Linux! Obviously for hobby projects anyone can do what they want. But adult projects should support Windows imho and consider Windows support from the start. Cross-platform is super easy unless you choose to make it hard.
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Re: The Journey Before main()
#36Earlier quoted context omitted.
Obviously only a requirement if you intend your software to run under windows. But if you don't, why bother. Not all software is intended to be distributed to users far and wide. Some of it is just for yourself, and some of it will only ever run on linux servers.
> some of it will only ever run on linux servers. I’ve spent quite a lot of time dealing with code that will ever run on Linux which did not in fact only ever run on Linux! Obviously for hobby projects anyone can do what they want. But adult projects should support Windows imho and consider Windows support from the start. Cross-platform is super easy unless you choose to make it hard.
I don't think we are talking about the same type of software? The type I was talking about will only ever run on Linux because it's a (HTTP-ish) server that will only ever run on Linux.
Probably a server that is only ever run by a single company on a single CPU type. That company will have complete control of the OS stack, so if it says no Windows, then no Windows has to be supported.
Re: The Journey Before main()
#37I wonder how many C projects prefer to avoid standard library, just invoking Linux syscalls directly. Much more fun to write software this way, IMO.
Not exactly the same, but on Windows if you use entirely Win32 calls you can avoid linking any C runtime library. Win32 is below the C standard library on Windows and the C runtime is optional.
This is one of the cornerstones that guarantee Windows can easily upgrade the C runtime and make performance and security upgrades. Win32 APIs have a different function calling ABI too.
So only part of that gets "bloated" is Win32 API itself (which is spread across multiple DLLs and don't actually bloat RAM usage). Most of the time even those functions and structures are carefully designed to have some future-proofness but it is usual to see APIs like CreateFile, CreateFile2, CreateFile3. Internally the earlier versions are upgraded to call the latest version. So not so much bloating there either.
When the C runtime and the OS system calls are combined into the single binary like POSIX, it creates the ABI hell we're in with the modern Unix-likes. Either the OSes have to regularly break the C ABI compatibility for the updates or we have to live with terrible implementations.
GNU libc and Linux combo is particularly bad. On GNU/Linux (or any other current libc replacements), the dynamic loading is also provided by the C library. This makes "forever" binary file compatibility particularly tricky to achieve. Glibc broke certain games / Steam by removing some parts of their ELF implementation: https://sourceware.org/bugzilla/show_bug.cgi?id=32653 . They backed due to huge backlash from the community.
If "the year of Linux desktop" would ever happen, they need to either do an Android and change the definition of what a software package is, or split Glibc into 3 parts: syscalls, dynamic loader and the actual C library.
PS: There is actually a catch to your " C runtime is optional." argument. Microsoft still intentionally holds back the ability of compiling native ABI Windows programs without Visual Studio.
The structured exception handlers (equivalent of Windows for SIGILL, SIGBUS etc.. not for SIGINT or SIGTERM though) are populated by the object files from the C runtime libraries (called VCRuntime/VCStartup). So it is actually not possible to have official Windows binaries without MSVC or any other C runtime like Mingw-64 that provides those symbols. It looks like some developers in Microsoft wanted to open-source VCRuntime / VCStartup but it was ~vetoed~ not fully approved by some people: https://github.com/microsoft/STL/issues/4560#issuecomment-23... , https://www.reddit.com/r/cpp/comments/1l8mqlv/is_msvc_ever_g...
Re: The Journey Before main()
#38Earlier quoted context omitted.
I generally try to stay portable, but file descriptors are just to nice, to not use them.
File descriptors are part of the linux syscall API, not libc. Are you thinking of FILE?
The "syscall API" is part of libc too. The read syscall is a trap, you put arguments in the right registers and issue the correct instruction[1] to enter the kernel. That's not something that can be expressed in C. The read() function that your C code actually uses is a C function provided by the C library.
[1] "svc 0" on ARM, "int 0x80" on i386, etc...
Re: The Journey Before main()
#39As someone who teaches this stuff at university, I see students getting confused every single year by how textbooks draw memory. The problem is mostly visual, not conceptual. Most diagrams in books and slides use an old hardware-centric convention: they draw higher addresses at the top of the page and lower addresses at the bottom. People sometimes justify this with an analogy like “floors in a building go up,” so ad…
That's how stacks on my desk grow and how everything grows in reality. I wouldn't numerate stacked things on my desk from the top, since this constantly changes. You also wouldn't name the first branch of a tree (the plant) to be the top-most one.
In your example "the stack grows down", seems to be wrong in the image.
Re: The Journey Before main()
#40As someone who teaches this stuff at university, I see students getting confused every single year by how textbooks draw memory. The problem is mostly visual, not conceptual. Most diagrams in books and slides use an old hardware-centric convention: they draw higher addresses at the top of the page and lower addresses at the bottom. People sometimes justify this with an analogy like “floors in a building go up,” so ad…
I think I got stuck in the same rut that I learned address space in whilst writing that diagram. I would tend to agree with you that your model makes much more sense to the student.
Related: In notation, one thing that I used to struggle with is how addresses (e.g. 0xAB_CD) actually have the bit representation of [0xCD, 0xAB]. Wonder if there's a common way to address that?