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Shresth Paul
Shresth Paul

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Building a Cross-Platform Disk Usage Analyzer in C

#c #systemsprogramming #linux #windows

I wanted a low-level C project that touches memory management, recursive filesystem traversal, and platform-specific APIs — so I built a terminal disk usage analyzer from scratch. No external libraries, just the C standard library and OS system calls.

What it does

  • Recursively scans a directory using nftw()
  • Reports total / used / available disk space (cross-platform)
  • Lists every file sorted by size (ascending) with a proportional ASCII bar chart 
=== Disk Usage Report ===

Total:     512.0 GB
Used:      210.3 GB
Available: 301.7 GB

[                                        ]
2.0 B      ./notes.txt

[########################################]
50.0 MB    ./video.mp4
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The interesting bugs

Bug 1 — realloc without sizeof

total_size = total_size * 2;
files = realloc(files, total_size);   // ❌ bytes, not entries!
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Should be total_size * sizeof(FileEntry). Without it, the buffer shrinks to almost nothing and every write past the original bound corrupts the heap — a great reminder that realloc takes bytes, always.

Bug 2 — mixing directories into the file list

nftw() calls your callback for every entry — files, directories, symlinks. I had to explicitly filter:

if (typeflag != FTW_F)
    return 0;
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Bug 3 — no cross-platform disk stats

statvfs() works great on Linux/macOS but doesn't exist on Windows. Solved with an #ifdef _WIN32 split: GetDiskFreeSpaceExA on Windows, statvfs everywhere else, unified behind one function signature.

Bar chart scaling

Once all files are captured, I scale each bar relative to the largest file found:

int filled = (int)((double) size / (double) max * BAR_WIDTH);
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Then qsort with a custom comparator sorts everything ascending by size before printing.

Repo

Single C file + CMakeLists, builds on Linux and Windows (MinGW tested): [https://github.com/SecByShresth/Disk-Usage-CLI.git]

Would love feedback on the traversal/performance side — currently it walks the entire tree with no depth limit or exclusion list (.git, node_modules, etc.), which is next on my list to fix.

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