Demo: Talking to the Kernel Directly

A program that reads a line and echoes it, written in assembly, using nothing but the syscall instruction. No libc, no wrappers.

Run

cd ../../../../02-os-types-interface-full/demos/02-os-interface/syscalls
make
./read_write_syscall

Type a line and press Enter; it prints back. Then watch the system calls, which are the whole program:

strace ./read_write_syscall

Ask

• The strace output is the source. What sits between this program and the kernel?
• read returns 12 and the next write sends 12 bytes. Where did the length come from?
• Every write is its own system call here. What would libc's printf have done instead?