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The input and output callbacks deal in bytes, not ASCII character codes. output gets each byte as a signed value (from HEAP8) and is never called with null. input returns 0-255, null for end of input, or undefined for no data yet. Fixes emscripten-core#17117.
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This pull request updates the Filesystem API documentation in Filesystem-API.rst to clarify the behavior of the input and output callbacks. Specifically, it explains that these callbacks are executed once per byte, details the expected return values and ranges (including handling of undefined for EAGAIN and signed integers for output), and provides instructions on how to retrieve unsigned bytes. There are no review comments, and I have no feedback to provide.
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Fixes #17117.
The
FS.init()docs say the callbacks deal in "ASCII character codes" and thatoutputgetsnullto flush. Testing with emcc 6.0.13 on Node, that's not what happens:outputgets one call per byte, as a signed value, becausefd_writepassesHEAP8. Writing "é" gives-61, -87.outputis never called withnull, even afterfflush,fsync, or exit.inputcan return any byte 0-255.nullmeans end of input (readreturns 0), andundefinedwith nothing read yet givesEAGAIN.This only updates the docs to match. Switching
outputto unsigned bytes would change what existing callbacks see, so I left that alone, but I'm happy to look at it if you'd prefer.