Running a Real C Compiler in the Browser Using WebAssembly

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A developer built an online C compiler that keeps user code inside the browser instead of sending it to a server. The tool runs Clang 21 and LLD compiled to WebAssembly, loaded as an npm package and cached after the first use. Compilation and execution happen in a module Web Worker, with a minimal WASI host that provides stdin, stdout, stderr, clocks and randomness but no file system or network access. A timeout terminates the worker to stop runaway loops, and testing revealed differences such as 32-bit type sizes, NULL reads not crashing, and missing fortified libc checks. C++ compilation was initially slow, but a precompiled header of common standard headers brought typical compile times down to 1-3 seconds.

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