rt0

I like suckless style Makefiles https://git.suckless.org/dmenu/file/Makefile.html https://git.suckless.org/sprop/file/Makefile.html with file rule variables (my name for them, `OBJ = ${SRC:.c=.o}`). https://github.com/lpsantil/rt0/blob/master/Makefile https://github.com/lpsantil/oop0/blob/master/Makefile

glibc’s pre-main code is still far larger even with optimized flags. It’s at least a couple KB of disk. You can see the flags I used. I think only ‘-nostdlib’ has to be removed. Some of the code & data glibc adds has purpose (like TLS (thread level storage) bits). https://github.com/lpsantil/rt0/blob/master/STATS.md

Also worth noting that you should be able get the binary close to 2KB if you pass some other options. https://github.com/lpsantil/rt0/blob/master/STATS.md

I’d also argue > decent amount of stuff that CAN be done in _start before you run main It’s all optional. Before I found the Linux kernel’s nolibc [0], I wrote a minimal C runtime based on bunch of stuff I read [1]. Only thing you really need to do is align the stack (at least on x86, amd64). You don’t get argc/argv/envp but main will still work. [0] https://github.com/torvalds/linux/tree/master/tools/include/... [1] https://github.com/lpsantil/rt0/blob/master/src/lib/00_start...

Locating argc, argv, and envp on the stack is done by the runtime [0]. [0] https://github.com/lpsantil/rt0/blob/master/src/lib/00_start...

Great post for learning. I posted a similar test of sbrk() coding in my rt0 lib [0]. With less trivial examples, it's interesting to see how various addresses change as they're used to keep track of what the kernel is doing to the program's virtual address space. [0] https://github.com/lpsantil/rt0/blob/master/t/sbrk.c

A little self promotion but mostly because it addresses some of the other commenters concerns about malloc (or the lower-level api around sbrk): a couple years ago I wrote rt0 [0], a small (mostly minimal) C runtime for i386 & amd64 that makes it easier to replace libc & crt0 (as long as you have the kernel headers installed). Also, as part of the examples, I wrote wrappers around the sbrk syscall. Pretty easy to do and all documented in the repo. I expect to eventually port the lib to arm (raspberry pi) and aarm64. There's also lots of references to other small c runtimes. I'll be adding this one as well. [0] https://github.com/lpsantil/rt0

This seems a lot more complicated than what I implemented in rt0 [0]. [0] https://github.com/lpsantil/rt0

You can extract the differences by looking at my librt0 [0] project. You can also see a minimal amd64 version in C here [1]. [0] https://github.com/lpsantil/rt0/blob/master/src/lib/syscall.... [1] https://gist.github.com/lunixbochs/462ee21c3353c56b910f

For me, I remember the notation by correlating it to its "high-level" equivalent. mov eax, [ ptr ] is like eax = *ptr; // or, eax = ptr[ 0 ]; The offset/multiplier memory addressing format for AT&T syntax was always more troubling for me. Coming from a TASM/MASM/NASM/PASCAL/x86 background first, it felt "icky" to put offsets outside of the "brackets" (or parenthesis, as it were) [0][1]. [0] https://github.com/lpsantil/rt0/blob/master/src/lib/00_start... [1] https://github.com/lpsantil/rt0/blob/master/src/lib/00_start...

I like your watch target. I implemented something similar [0] with watch (bash builtin or GNU watch) & time. start_ci: watch time -p make clean all & echo $$! > tmp.ci.pid stop_ci: kill -9 `cat tmp.ci.pid` Then you can start continuous integration by doing make start_ci Then you can stop continuous integration by doing make stop_ci Obviously, this is inspired by rc.d scripts. [0] https://github.com/lpsantil/rt0/blob/master/Makefile#L117

I think I've fixed it [0]. Ran into the issue where gcc was kindly re-aligning the stack to 16-bytes on i386 for no good reason (stack was already aligned?!?!). But that's fixed. Just hope gcc on i386 doesn't stop doing `sub ESP, 0x1C` upon entry to _start. [0] https://github.com/lpsantil/rt0/blob/master/lib/00_start.c