Implement malloc and free

What glibc Hides From You

When you call malloc(100) on Linux, glibc's ptmalloc searches free lists, may mmap fresh pages from the kernel, and returns a pointer aligned to 16 bytes. Your exercise strips that down to a fixed char heap[4096] and an explicit Block header: size, used flag, and next pointer. That header-before-payload layout is the same idea jemalloc and tcmalloc use, just with fancier bookkeeping.

First-Fit and Coalescing

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• First-fit: walk the list, take the first free block large enough
• Alignment: round requested size up to 8 bytes before comparing
• Free: mark the block unused, then merge with adjacent free neighbors to fight fragmentation

Returning user memory means skipping the header: (char*)block + sizeof(Block). Finding the header from a user pointer reverses that subtraction.

For this exercise, you will finish my_malloc and implement my_free with coalescing on a static buffer. This task asks you to see allocation as list surgery, because heap exploitation CVEs and allocator tuning both start with understanding these block headers.

Keep the relevant man page, ABI doc, or Rust reference chapter open while you work. When your output disagrees with the reference implementation on the same machine, the mismatch is usually an alignment rule, an off-by-one terminator, or a register slot you misread in GDB.