Sorry, I didn’t mean the userland stuff above the kernel, which, along with the boot loader, is the part that is really restricted in any sense. With the standard library, there are large parts that are not kernel safe. That’s because they rely on services like memory allocation, which are different in the kernel. Or they rely on kernel services, like files. And what I work on has no kernel, just an RTOS.

And C in the kernel isn’t really a libc replacement. They often have different behaviors because they need to run without allocating memory, or be interrupt safe. For example, in some situations I use a function call instruction that specifically does not creat a new stack frame.