golang.org/x/sys v0.31.0: unix.EFD_NONBLOCK

Clean-room public code sample — generation instructions

Write a brand-new, minimal, self-contained code sample in this clean-room directory.
Do not copy, paraphrase, or reference any existing project source. Work only from this spec.

A csx.json manifest scaffold already exists. Do not recreate it from memory. Preserve its case.goal, packages and symbols; fill its empty case.contract with exact assertions and correct its environment, commands and verifierAdapter for the files you generate.

Goal: verify golang.org/x/sys/unix.EFD_NONBLOCK in pkg:golang/golang.org/x/sys@v0.31.0
Kind: HOW

Use EXACTLY these public packages and versions:
  - pkg:golang/golang.org/x/sys@v0.31.0
Demonstrate these symbols/APIs:
  - golang.org/x/sys/unix.EFD_NONBLOCK

Rules:
  - One focused purpose; the smallest project that proves the goal.
  - Include a contract test (test/contract.*) that runs OFFLINE and exits 0 exactly when the goal behavior works.
  - Pin every dependency with a lockfile so resolution is reproducible.
  - No secrets, credentials, or tokens. No real URLs (only example.com or localhost). No absolute paths.
  - No personal names, emails, company names, or project identifiers of any kind.
  - No binaries and no generated output (node_modules, dist, target, venv, .git, .env).
  - Keep it under 200 files and 256KB packed.

{"case":{"caseId":"case:sha256:2071119373309075533dc6da14f77f747267f4987422346c2aa02ee0415bb5b4","contract":["Eventfd created with EFD_NONBLOCK returns EAGAIN on empty read instead of blocking","Eventfd created with EFD_NONBLOCK successfully reads value after write","Eventfd created with EFD_NONBLOCK returns EAGAIN on subsequent read after counter is consumed","Eventfd created with EFD_NONBLOCK and initial counter \u003e 0 allows immediate read without blocking"],"goal":"verify golang.org/x/sys/unix.EFD_NONBLOCK in pkg:golang/golang.org/x/sys@v0.31.0","kind":"HOW","packages":["pkg:golang/golang.org/x/sys@v0.31.0"],"schemaVersion":1,"symbols":["golang.org/x/sys/unix.EFD_NONBLOCK"]},"contractCommand":["go","run","./test"],"environment":{"arch":"x64","distro":"ubuntu","ecosystem":"golang","libc":"glibc","libcVersion":"2.39","os":"linux","osVersionBucket":"24","packageManager":"go","schemaVersion":1},"license":"MIT-0","packages":["pkg:golang/golang.org/x/sys@v0.31.0"],"schemaVersion":1,"subject":"pkg:golang/golang.org/x/sys@v0.31.0","symbols":["golang.org/x/sys/unix.EFD_NONBLOCK"],"verifierAdapter":"golang@1"}

module sample

go 1.23.0

require golang.org/x/sys v0.31.0

golang.org/x/sys v0.31.0 h1:ioabZlmFYtWhL+TRYpcnNlLwhyxaM9kWTDEmfnprqik=
golang.org/x/sys v0.31.0/go.mod h1:BJP2sWEmIv4KK5OTEluFJCKSidICx8ciO85XgH3Ak8k=

package main

import (
	"encoding/binary"
	"errors"
	"fmt"
	"os"

	"golang.org/x/sys/unix"
)

// NonBlockingEventFD wraps a Linux eventfd configured with non-blocking mode.
type NonBlockingEventFD struct {
	fd int
}

// NewNonBlockingEventFD creates an eventfd descriptor with EFD_NONBLOCK flag.
func NewNonBlockingEventFD(initval uint) (*NonBlockingEventFD, error) {
	fd, err := unix.Eventfd(initval, unix.EFD_NONBLOCK)
	if err != nil {
		return nil, fmt.Errorf("eventfd creation failed: %w", err)
	}
	return &NonBlockingEventFD{fd: fd}, nil
}

// Write adds a value to the counter.
func (e *NonBlockingEventFD) Write(val uint64) error {
	var buf [8]byte
	binary.NativeEndian.PutUint64(buf[:], val)
	_, err := unix.Write(e.fd, buf[:])
	return err
}

// Read attempts to read the current counter value without blocking.
// Returns unix.EAGAIN if no event has occurred (counter is 0).
func (e *NonBlockingEventFD) Read() (uint64, error) {
	var buf [8]byte
	n, err := unix.Read(e.fd, buf[:])
	if err != nil {
		return 0, err
	}
	if n != 8 {
		return 0, fmt.Errorf("unexpected read size: %d", n)
	}
	return binary.NativeEndian.Uint64(buf[:]), nil
}

// Close closes the underlying file descriptor.
func (e *NonBlockingEventFD) Close() error {
	return unix.Close(e.fd)
}

func main() {
	efd, err := NewNonBlockingEventFD(0)
	if err != nil {
		fmt.Fprintf(os.Stderr, "failed to create non-blocking eventfd: %v\n", err)
		os.Exit(1)
	}
	defer efd.Close()

	// Initial non-blocking read on empty counter returns EAGAIN
	_, err = efd.Read()
	if !errors.Is(err, unix.EAGAIN) {
		fmt.Fprintf(os.Stderr, "expected EAGAIN on empty eventfd, got: %v\n", err)
		os.Exit(1)
	}

	// Write value to eventfd
	if err := efd.Write(1); err != nil {
		fmt.Fprintf(os.Stderr, "failed to write to eventfd: %v\n", err)
		os.Exit(1)
	}

	// Read value back
	val, err := efd.Read()
	if err != nil {
		fmt.Fprintf(os.Stderr, "failed to read from eventfd: %v\n", err)
		os.Exit(1)
	}
	fmt.Printf("eventfd read value: %d\n", val)
}

{
  "schemaVersion": 1,
  "goal": "verify golang.org/x/sys/unix.EFD_NONBLOCK in pkg:golang/golang.org/x/sys@v0.31.0",
  "kind": "HOW",
  "packages": [
    "pkg:golang/golang.org/x/sys@v0.31.0"
  ],
  "symbols": [
    "golang.org/x/sys/unix.EFD_NONBLOCK"
  ]
}

package main

import (
	"encoding/binary"
	"errors"
	"fmt"
	"os"

	"golang.org/x/sys/unix"
)

func main() {
	// 1. Eventfd created with EFD_NONBLOCK returns EAGAIN on empty read instead of blocking
	{
		fd, err := unix.Eventfd(0, unix.EFD_NONBLOCK)
		if err != nil {
			fmt.Fprintf(os.Stderr, "FAIL: failed to create eventfd with EFD_NONBLOCK: %v\n", err)
			os.Exit(1)
		}
		defer unix.Close(fd)

		var buf [8]byte
		_, err = unix.Read(fd, buf[:])
		if !errors.Is(err, unix.EAGAIN) {
			fmt.Fprintf(os.Stderr, "FAIL: expected EAGAIN on empty non-blocking eventfd read, got %v\n", err)
			os.Exit(1)
		}
	}

	// 2. Eventfd created with EFD_NONBLOCK successfully reads value after write
	{
		fd, err := unix.Eventfd(0, unix.EFD_NONBLOCK)
		if err != nil {
			fmt.Fprintf(os.Stderr, "FAIL: failed to create eventfd with EFD_NONBLOCK: %v\n", err)
			os.Exit(1)
		}
		defer unix.Close(fd)

		var writeBuf [8]byte
		expectedVal := uint64(42)
		binary.NativeEndian.PutUint64(writeBuf[:], expectedVal)

		n, err := unix.Write(fd, writeBuf[:])
		if err != nil || n != 8 {
			fmt.Fprintf(os.Stderr, "FAIL: expected write to succeed, got n=%d, err=%v\n", n, err)
			os.Exit(1)
		}

		var readBuf [8]byte
		n, err = unix.Read(fd, readBuf[:])
		if err != nil || n != 8 {
			fmt.Fprintf(os.Stderr, "FAIL: expected read to succeed, got n=%d, err=%v\n", n, err)
			os.Exit(1)
		}

		gotVal := binary.NativeEndian.Uint64(readBuf[:])
		if gotVal != expectedVal {
			fmt.Fprintf(os.Stderr, "FAIL: expected value %d, got %d\n", expectedVal, gotVal)
			os.Exit(1)
		}
	}

	// 3. Eventfd created with EFD_NONBLOCK returns EAGAIN on subsequent read after counter is consumed
	{
		fd, err := unix.Eventfd(0, unix.EFD_NONBLOCK)
		if err != nil {
			fmt.Fprintf(os.Stderr, "FAIL: failed to create eventfd with EFD_NONBLOCK: %v\n", err)
			os.Exit(1)
		}
		defer unix.Close(fd)

		var writeBuf [8]byte
		binary.NativeEndian.PutUint64(writeBuf[:], 1)
		if _, err := unix.Write(fd, writeBuf[:]); err != nil {
			fmt.Fprintf(os.Stderr, "FAIL: write failed: %v\n", err)
			os.Exit(1)
		}

		var readBuf [8]byte
		if _, err := unix.Read(fd, readBuf[:]); err != nil {
			fmt.Fprintf(os.Stderr, "FAIL: first read failed: %v\n", err)
			os.Exit(1)
		}

		// Subsequent read should immediately fail with EAGAIN
		_, err = unix.Read(fd, readBuf[:])
		if !errors.Is(err, unix.EAGAIN) {
			fmt.Fprintf(os.Stderr, "FAIL: expected EAGAIN on second read, got %v\n", err)
			os.Exit(1)
		}
	}

	// 4. Eventfd created with EFD_NONBLOCK and initial counter > 0 allows immediate read without blocking
	{
		initialVal := uint(10)
		fd, err := unix.Eventfd(initialVal, unix.EFD_NONBLOCK)
		if err != nil {
			fmt.Fprintf(os.Stderr, "FAIL: failed to create eventfd with initial value: %v\n", err)
			os.Exit(1)
		}
		defer unix.Close(fd)

		var readBuf [8]byte
		n, err := unix.Read(fd, readBuf[:])
		if err != nil || n != 8 {
			fmt.Fprintf(os.Stderr, "FAIL: expected immediate read to succeed, got n=%d, err=%v\n", n, err)
			os.Exit(1)
		}

		gotVal := binary.NativeEndian.Uint64(readBuf[:])
		if gotVal != uint64(initialVal) {
			fmt.Fprintf(os.Stderr, "FAIL: expected initial value %d, got %d\n", initialVal, gotVal)
			os.Exit(1)
		}
	}

	fmt.Println("PASS: all contract assertions passed")
}