github.com/pingcap/badger@v1.5.1-0.20230103063557-828f39b09b6d/y/mmap_unix.go (about)

     1  // +build !windows
     2  
     3  /*
     4   * Copyright 2017 Dgraph Labs, Inc. and Contributors
     5   *
     6   * Licensed under the Apache License, Version 2.0 (the "License");
     7   * you may not use this file except in compliance with the License.
     8   * You may obtain a copy of the License at
     9   *
    10   *     http://www.apache.org/licenses/LICENSE-2.0
    11   *
    12   * Unless required by applicable law or agreed to in writing, software
    13   * distributed under the License is distributed on an "AS IS" BASIS,
    14   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    15   * See the License for the specific language governing permissions and
    16   * limitations under the License.
    17   */
    18  
    19  package y
    20  
    21  import (
    22  	"os"
    23  	"syscall"
    24  	"unsafe"
    25  
    26  	"golang.org/x/sys/unix"
    27  )
    28  
    29  // Mmap uses the mmap system call to memory-map a file. If writable is true,
    30  // memory protection of the pages is set so that they may be written to as well.
    31  func Mmap(fd *os.File, writable bool, size int64) ([]byte, error) {
    32  	mtype := unix.PROT_READ
    33  	if writable {
    34  		mtype |= unix.PROT_WRITE
    35  	}
    36  	return unix.Mmap(int(fd.Fd()), 0, int(size), mtype, unix.MAP_SHARED)
    37  }
    38  
    39  // Munmap unmaps a previously mapped slice.
    40  func Munmap(b []byte) error {
    41  	return unix.Munmap(b)
    42  }
    43  
    44  // Madvise uses the madvise system call to give advise about the use of memory
    45  // when using a slice that is memory-mapped to a file. Set the readahead flag to
    46  // false if page references are expected in random order.
    47  func Madvise(b []byte, readahead bool) error {
    48  	flags := unix.MADV_NORMAL
    49  	if !readahead {
    50  		flags = unix.MADV_RANDOM
    51  	}
    52  	return madvise(b, flags)
    53  }
    54  
    55  // This is required because the unix package does not support the madvise system call on OS X.
    56  func madvise(b []byte, advice int) (err error) {
    57  	_, _, e1 := syscall.Syscall(syscall.SYS_MADVISE, uintptr(unsafe.Pointer(&b[0])),
    58  		uintptr(len(b)), uintptr(advice))
    59  	if e1 != 0 {
    60  		err = e1
    61  	}
    62  	return
    63  }