a simple url shortener in Go (check it out at qurl.org)
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// +build !windows,!plan9,!solaris
package bbolt
import ( "fmt" "syscall" "time" "unsafe" )
// flock acquires an advisory lock on a file descriptor.
func flock(db *DB, exclusive bool, timeout time.Duration) error { var t time.Time if timeout != 0 { t = time.Now() } fd := db.file.Fd() flag := syscall.LOCK_NB if exclusive { flag |= syscall.LOCK_EX } else { flag |= syscall.LOCK_SH } for { // Attempt to obtain an exclusive lock.
err := syscall.Flock(int(fd), flag) if err == nil { return nil } else if err != syscall.EWOULDBLOCK { return err }
// If we timed out then return an error.
if timeout != 0 && time.Since(t) > timeout-flockRetryTimeout { return ErrTimeout }
// Wait for a bit and try again.
time.Sleep(flockRetryTimeout) } }
// funlock releases an advisory lock on a file descriptor.
func funlock(db *DB) error { return syscall.Flock(int(db.file.Fd()), syscall.LOCK_UN) }
// 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_READ, syscall.MAP_SHARED|db.MmapFlags) if err != nil { return err }
// Advise the kernel that the mmap is accessed randomly.
err = madvise(b, syscall.MADV_RANDOM) if err != nil && err != syscall.ENOSYS { // Ignore not implemented error in kernel because it still works.
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])) db.datasz = sz 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.dataref = nil db.data = nil db.datasz = 0 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 }
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