github.com/flowerwrong/netstack@v0.0.0-20191009141956-e5848263af28/tcpip/link/sharedmem/pipe/pipe.go (about)

     1  // Copyright 2018 The gVisor Authors.
     2  //
     3  // Licensed under the Apache License, Version 2.0 (the "License");
     4  // you may not use this file except in compliance with the License.
     5  // You may obtain a copy of the License at
     6  //
     7  //     http://www.apache.org/licenses/LICENSE-2.0
     8  //
     9  // Unless required by applicable law or agreed to in writing, software
    10  // distributed under the License is distributed on an "AS IS" BASIS,
    11  // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    12  // See the License for the specific language governing permissions and
    13  // limitations under the License.
    14  
    15  // Package pipe implements a shared memory ring buffer on which a single reader
    16  // and a single writer can operate (read/write) concurrently. The ring buffer
    17  // allows for data of different sizes to be written, and preserves the boundary
    18  // of the written data.
    19  //
    20  // Example usage is as follows:
    21  //
    22  //	wb := t.Push(20)
    23  //	// Write data to wb.
    24  //	t.Flush()
    25  //
    26  //	rb := r.Pull()
    27  //	// Do something with data in rb.
    28  //	t.Flush()
    29  package pipe
    30  
    31  import (
    32  	"math"
    33  )
    34  
    35  const (
    36  	jump           uint64 = math.MaxUint32 + 1
    37  	offsetMask     uint64 = math.MaxUint32
    38  	revolutionMask uint64 = ^offsetMask
    39  
    40  	sizeOfSlotHeader        = 8 // sizeof(uint64)
    41  	slotFree         uint64 = 1 << 63
    42  	slotSizeMask     uint64 = math.MaxUint32
    43  )
    44  
    45  // payloadToSlotSize calculates the total size of a slot based on its payload
    46  // size. The  total size is the header size, plus the payload size, plus padding
    47  // if necessary to make the total size a multiple of sizeOfSlotHeader.
    48  func payloadToSlotSize(payloadSize uint64) uint64 {
    49  	s := sizeOfSlotHeader + payloadSize
    50  	return (s + sizeOfSlotHeader - 1) &^ (sizeOfSlotHeader - 1)
    51  }
    52  
    53  // slotToPayloadSize calculates the payload size of a slot based on the total
    54  // size of the slot. This is only meant to be used when creating slots that
    55  // don't carry information (e.g., free slots or wrap slots).
    56  func slotToPayloadSize(offset uint64) uint64 {
    57  	return offset - sizeOfSlotHeader
    58  }
    59  
    60  // pipe is a basic data structure used by both (transmit & receive) ends of a
    61  // pipe. Indices into this pipe are split into two fields: offset, which counts
    62  // the number of bytes from the beginning of the buffer, and revolution, which
    63  // counts the number of times the index has wrapped around.
    64  type pipe struct {
    65  	buffer []byte
    66  }
    67  
    68  // init initializes the pipe buffer such that its size is a multiple of the size
    69  // of the slot header.
    70  func (p *pipe) init(b []byte) {
    71  	p.buffer = b[:len(b)&^(sizeOfSlotHeader-1)]
    72  }
    73  
    74  // data returns a section of the buffer starting at the given index (which may
    75  // include revolution information) and with the given size.
    76  func (p *pipe) data(idx uint64, size uint64) []byte {
    77  	return p.buffer[(idx&offsetMask)+sizeOfSlotHeader:][:size]
    78  }