Write Hello World Using Only Raw syscall() Instructions

Bypassing libc

Every libc call like write() eventually lands in a syscall instruction. Calling syscall() directly shows the register ABI the kernel expects: syscall number in rax, arguments in rdi, rsi, rdx, and friends on x86-64.

x86-64 Linux ABI

Common pattern:

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• SYS_write = 1
• fd 1 is stdout
• return value in rax (negative errno on error)

For example, hello world needs only write and exit, no printf formatting layer.

Why This Matters in Production

Bypassing libc to call syscall() directly is exactly how musl and Google's Fuchsia libc bootstrap their runtimes, and it's the same register convention (RAX for the number, RDI/RSI/RDX for arguments) that strace decodes and gdb inspects when debugging a crash. Static binaries built this way are also how minimal Docker base images and Go's early runtime avoided depending on a dynamically linked glibc entirely.

Debugging Checklist

Before you call the implementation done, walk failure modes on purpose. Test empty structures, single-element edge cases, maximum concurrency, and errno paths that must not crash the program. OS code usually fails in production when happy-path tests pass but invariants break under contention or memory pressure.

Implementation Notes

Keep structures small and name fields after kernel counterparts when possible. That lets you read man pages and kernel source side by side while you work. Print observable events during development; remove noisy logs once tests pass reliably.

For This Exercise

You will print a message using only syscall() wrappers with no libc stdio. This task asks you to list each register assignment so you can read strace output without guessing.