sys-internals

This repo is a systems programming project built around two core pieces:

• malloc/: a small custom heap allocator (smalloc)
• shell/: a minimal shell used as a command driver and test harness

The goal was to practice low-level memory management and process control in C, then use those pieces together in a way that is testable and debuggable.

What was implemented

• A page-backed allocator using mmap with an explicit free list
• First-fit allocation, block splitting, and adjacent block coalescing on free
• Heap/introspection helpers (get_head_pointer, get_next_pointer, get_prev_pointer) for trace-style validation
• A shell with:
  • built-ins (cd, pwd, exit)
  • external command execution via fork/execvp
  • output redirection with both > and custom >+ behavior
  • semicolon-separated command execution
• Simulated protocol commands in shell (I2C/SPI/MDIO/UART) backed by a small device model
• Command tracing to JSONL and basic post-run analysis scripts
• A pluggable allocation backend in shell (libc or smalloc) plus memory fault injection knobs

Pain points

Some of the harder parts were less about writing a lot of code and more about getting edge behavior right:

• Redirection parsing was easy to break with malformed input or ambiguous spacing
• Free-list pointer updates during split/coalesce were fragile until insertion order was made consistent
• Error handling had to stay strict and predictable (An error has occurred) across built-ins and external commands
• Failure-injection and timeout flows were useful, but they made test outcomes noisier until tracing was added

This is still a simple systems project, not a polished production shell or allocator. I hope to come back to this sometime and expand test cases, allocator robustness, and maybe more.