feat!: errno always-capture — CallFunction returns (syscall.Errno, error) (v0.6.0)#61
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…60) Add CallFunctionErrno/CallFunctionErrnoContext to the public API. Errno is captured inside the assembly trampoline immediately after the C function returns, before the Go runtime can migrate the goroutine to another OS thread. Architecture: - Extend syscallArgs struct with errno/errnoFn fields at the end (no ABI break) - AMD64 Unix: call __errno_location/__error in assembly after saving returns, guarded by TESTQ so regular CallFunction has zero extra cost - ARM64: same pattern using CBZ guard; R19/R20 are callee-saved across the call - FunctionCallerErrno optional interface allows type assertion without changing existing FunctionCaller; unsupported arches fall back to plain Execute - errno_stubs_amd64.s / errno_stubs_arm64.s: JMP trampolines to dynamic symbols goffi_errno_location (same pattern as internal/dl stubs) - Windows: stub returns errno=0; use GetLastError/syscall.SyscallN third return value for Win32 errors instead Platforms: Linux/macOS/FreeBSD amd64+arm64 (errno capture); Windows (stub). All 8 cross-compile targets verified. Tests: TestCallFunctionErrnoBasic validates ENOENT capture; TestCallFunctionErrnoSuccess validates zero errno.
BREAKING: CallFunction/CallFunctionContext now return (syscall.Errno, error). Callers that don't need errno use: _, err := ffi.CallFunction(...) errno is captured inside the assembly trampoline immediately after the C function returns, before Go runtime can migrate to a different OS thread. Cost: ~3-5 ns per call (CALL __errno_location + MOVL). This makes goffi the first pure-Go FFI with correct errno capture on Linux. Opt-in CallFunctionErrno removed -- always-capture is the enterprise approach.
- CHANGELOG: v0.6.0 entry with breaking changes and migration guide - README: update all CallFunction examples to (_, err) pattern, add errno feature - ARCHITECTURE: document errno capture in Layer 2 section - PERFORMANCE: document +3-5ns errno overhead
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Summary
CallFunctionandCallFunctionContextnow return(syscall.Errno, error)— errno is always capturedexitsyscallcan migrate goroutine)__errno_location(Linux glibc/musl),__error(macOS/FreeBSD)Why Always-Capture (not opt-in)
Opt-in
CallFunctionErrnois a pit of failure — you don't know you need errno until the function fails, and by then the goroutine may be on a different OS thread. Pre-1.0 = do it right.Cost: ~3-5 ns per call (
CALL __errno_location+MOVL). At 200 calls/frame × 60 FPS = 0.02% of frame budget.Migration
Test plan
go build ./...— cleango test ./...— all passCloses #60