Linux System Monitor (htop clone)

What it did

Build a terminal process monitor (like htop) from scratch in C++. Reads /proc/stat, /proc/meminfo, /proc/[pid]/* continuously, renders aggregate CPU%, memory, uptime, and a sortable per-process table via ncurses.

The /proc API surface

| File | What | |------|------| | /proc/stat | aggregate CPU jiffies (user, nice, system, idle, iowait) | | /proc/meminfo | RAM total/free/cached/buffers | | /proc/uptime | seconds since boot | | /proc/[pid]/stat | per-process CPU jiffies + state + start-time | | /proc/[pid]/status | per-process memory usage (RSS, VmSize) | | /proc/[pid]/cmdline | the full command line as a null-separated string | | /proc/[pid]/comm | the short command name |

CPU% per process is (process_jiffies_now - process_jiffies_then) / (total_cpu_jiffies_now - total_cpu_jiffies_then) — a delta over a fixed sampling interval, never absolute.

What was actually tricky

• cmdline is null-separated. Reading it with >> gives you only argv[0]; you need to read the whole file and replace \0 with spaces.
• PIDs reuse. A process can die and a new one get the same PID between samples. Tracking by start-time + PID handles it.
• ncurses redraw flicker. Need to use wclear() + double-buffer rendering carefully or the screen tears.

What I'd do differently with hindsight

• Use /proc/[pid]/io for per-process disk I/O — htop's modern view shows it; the project didn't.
• Test on different distros. The /proc schema is technically documented but I've hit edge cases on Alpine, Ubuntu, and Fedora where one column was off-by-one.
• Replace ncurses with a TUI lib — notcurses or ratatui (Rust) make rendering far simpler.

What it taught me

The Linux kernel exposes a lot through /proc and /sys — everything htop, ps, top, free, and uptime shows comes from parsing these. Knowing the schema makes you faster at debugging production issues; you can read /proc by hand instead of running a tool. That's been useful many times since.