mmap_linux.go

package bigqueue

import (
	"fmt"
	"syscall"
	"unsafe"
)

// fdatasync flushes written data to a file descriptor.
func fdatasync(db *DB) error {
	return syscall.Fdatasync(int(db.file.Fd()))
}

// mmap memory maps a DB's data file.
func mmap(db *DB, sz int) error {
	// Map the data file to memory.
	b, err := syscall.Mmap(int(db.file.Fd()), 0, sz, syscall.PROT_WRITE|syscall.PROT_READ, syscall.MAP_SHARED|db.MmapFlags)
	if err != nil {
		return err
	}

	// Advise the kernel that the mmap is accessed randomly.
	if err := madvise(b, syscall.MADV_RANDOM); err != nil {
		return fmt.Errorf("madvise: %s", err)
	}

	// Save the original byte slice and convert to a byte array pointer.
	db.dataref = b
	db.data = (*[maxMapSize]byte)(unsafe.Pointer(&b[0]))
	return nil
}

// munmap unmaps a DB's data file from memory.
func munmap(db *DB) error {
	// Ignore the unmap if we have no mapped data.
	if db.dataref == nil {
		return nil
	}

	// Unmap using the original byte slice.
	err := syscall.Munmap(db.dataref)
	db.data = nil
	return err
}

// NOTE: This function is copied from stdlib because it is not available on darwin.
func madvise(b []byte, advice int) (err error) {
	_, _, e1 := syscall.Syscall(syscall.SYS_MADVISE, uintptr(unsafe.Pointer(&b[0])), uintptr(len(b)), uintptr(advice))
	if e1 != 0 {
		err = e1
	}
	return
}