Tonight I got a hello world app to use uClibc's puts() routine! This
is a big deal because it's the first time I've had system calls coming
in from userland. I haven't checked the changes in yet, because they're
a mess, but here's a basic run-down of what I had to do...
- First, uClibc had to be taught how to make system calls to the moxie uClinux kernel. This was straight forward, except I came across one surprise which I'll describe below.
- Next, I needed to add more files to my initfs. Specifically, I needed a /dev/console. Fortunately, the kernel build process makes this easy. I decided to use the "text file" approach to populating the initramfs as described in this document.
- Finally, I had to create a tty device for my default console that spoke through the gdb simulator via software interrupts. Fortunately the ia64 port had a similar tty device for talking through one of HP's simulators that I was able to mostly copy.
Once all this was done, I was able to build a standard Hello World app with moxie-uclinux-gcc, and it just worked!
What about the system call surprise? Despite what I read somewhere that said that Linux system calls had a maximum of 5 parameters -- that's not quite true. Some take 6 (are there any with 7? more?). This thwarted my attempt to get busybox running tonight, because it uses mmap, and mmap is one of those 6-argument system calls. There are a few ways to fix this. I think I'll just hack the compiler to use 6 register arguments and see what that does to code size/performance.
If there are any GDB hackers reading this... I have one question for you... The kernel is loading and relocating my "init" program, then execve'ing it. When I run the kernel in gdb, it would be nice for gdb to load the debug info for init so I could see what it's doing when I step into userland. Is there some way to do this manually?