wasm2go/benchmarks/generated/mem/goccy_pure/sharedimage_mmap.go

//go:build unix

package goccy_pure

import (
	"fmt"
	"os"
	"syscall"
)

const mmapSupported = true

// mmapImage maps the image as one instance's linear memory: MAP_PRIVATE, so
// reads share the image's physical pages with every other instance and a write
// faults in a private copy of just that page.
//
// The file is sparse-extended to the ceiling first, so the whole range is
// mappable and everything past the image reads as zeros — which is what a
// freshly grown wasm page must contain. Mapping past the end of the file
// instead would fault with SIGBUS on first access.
func mmapImage(f *os.File, ceiling int) ([]byte, error) {
	fi, err := f.Stat()
	if err != nil {
		return nil, err
	}
	if fi.Size() < int64(ceiling) {
		if err := f.Truncate(int64(ceiling)); err != nil {
			return nil, fmt.Errorf("wasm2go: sizing the shared memory image: %w", err)
		}
	}
	mem, err := syscall.Mmap(int(f.Fd()), 0, ceiling,
		syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_PRIVATE)
	if err != nil {
		return nil, fmt.Errorf("wasm2go: mapping the shared memory image: %w", err)
	}
	return mem, nil
}

// mmapShared maps the image file MAP_SHARED, for an in-place builder:
// every page the builder writes lands directly in the file's page cache
// and IS the image — no intermediate buffer, no copy, no write-out. The
// caller has already sized the file (sparse) to length.
func mmapShared(f *os.File, length int) ([]byte, error) {
	mem, err := syscall.Mmap(int(f.Fd()), 0, length,
		syscall.PROT_READ|syscall.PROT_WRITE, syscall.MAP_SHARED)
	if err != nil {
		return nil, fmt.Errorf("wasm2go: mapping the image for in-place build: %w", err)
	}
	return mem, nil
}

func unmapMemory(mem []byte) {
	if len(mem) == 0 {
		return
	}
	_ = syscall.Munmap(mem)
}