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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"ffi: Fast API calls silently truncate out-of-range integer arguments","articleBody":"### Version\n\nmain\n\n### Platform\n\n```text\nmacOS 26.5.2\n```\n\n### Subsystem\n\nffi\n\n### What steps will reproduce the bug?\n\n`repro.c`\n```c\n#include \u003cstdint.h>\n\nint8_t i8(int8_t x) {\n  return x;\n}\n\nuint8_t u8(uint8_t x) {\n  return x;\n}\n\nint16_t i16(int16_t x) {\n  return x;\n}\n\nuint16_t u16(uint16_t x) {\n  return x;\n}\n\nint64_t i64(int64_t x) {\n  return x;\n}\n```\n\nCompile it by running\n```sh\n$ clang -dynamiclib repro.c -o librepro.dylib\n```\n\n`repro.js`\n```js\nimport { dlopen } from 'node:ffi';\n\nconst { functions } = dlopen(`./librepro.dylib`, {\n  i8: { arguments: ['i8'], return: 'i8' },\n  u8: { arguments: ['u8'], return: 'u8' },\n  i16: { arguments: ['i16'], return: 'i16' },\n  u16: { arguments: ['u16'], return: 'u16' },\n  i64: { arguments: ['i64'], return: 'i64' },\n});\n\nfunction callI8(x) { return functions.i8(x); }\nfunction callU8(x) { return functions.u8(x); }\nfunction callI16(x) { return functions.i16(x); }\nfunction callU16(x) { return functions.u16(x); }\nfunction callI64(x) { return functions.i64(x); }\n\nfunction optimize(fn, value) {\n  eval('%PrepareFunctionForOptimization(fn)');\n  fn(value);\n  fn(value);\n  eval('%OptimizeFunctionOnNextCall(fn)');\n  fn(value);\n}\n\nfor (const [fn, value] of [\n  [callI8, 0],\n  [callU8, 0],\n  [callI16, 0],\n  [callU16, 0],\n  [callI64, 0n],\n]) {\n  optimize(fn, value);\n}\n\nconsole.log('i8:', callI8(128));\nconsole.log('u8:', callU8(256));\nconsole.log('i16:', callI16(32768));\nconsole.log('u16:', callU16(65536));\nconsole.log('i64:', callI64(2n ** 63n));\n```\n\nRun it\n```sh\n$ node --no-warnings --experimental-ffi --allow-natives-syntax repro.js\n```\n\n### How often does it reproduce? Is there a required condition?\n\nAlways\n\n### What is the expected behavior? Why is that the expected behavior?\n\nEach call should throw `ERR_INVALID_ARG_VALUE`, as the generic FFI path does.\n\n### What do you see instead?\n\n```sh\ni8: -128\nu8: 0\ni16: -32768\nu16: 0\ni64: -9223372036854775808n\n```\n\noptimized FFI calls silently truncate or wrap every out-of-range argument\n\n### Additional information\n\n_No response_","author":{"url":"https://github.com/trivikr","@type":"Person","name":"trivikr"},"datePublished":"2026-07-20T06:20:19Z","url":"https://github.com/nodejs/node/issues/64613"}