Building a Cross-Platform Disk Usage Analyzer in C

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

The interesting bugs

Bug 1 - realloc without sizeof

total_size = total_size * 2;
files = realloc(files, total_size);   // ❌ bytes, not entries!

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;

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);

Then qsort with a custom comparator sorts everything ascending by size before printing.

Repo

Single C file + CMakeLists.txt, 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.