ceph/src/seastar/src/core/vla.hh

   1 /*
   2  * This file is open source software, licensed to you under the terms
   3  * of the Apache License, Version 2.0 (the "License").  See the NOTICE file
   4  * distributed with this work for additional information regarding copyright
   5  * ownership.  You may not use this file except in compliance with the License.
   6  *
   7  * You may obtain a copy of the License at
   8  *
   9  *   http://www.apache.org/licenses/LICENSE-2.0
  10  *
  11  * Unless required by applicable law or agreed to in writing,
  12  * software distributed under the License is distributed on an
  13  * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
  14  * KIND, either express or implied.  See the License for the
  15  * specific language governing permissions and limitations
  16  * under the License.
  17  */
  18 /*
  19  * Copyright (C) 2014 Cloudius Systems, Ltd.
  20  */
  21
  22 #pragma once
  23
  24 #include <seastar/core/aligned_buffer.hh>
  25 #include <memory>
  26 #include <new>
  27 #include <assert.h>
  28 #include <type_traits>
  29
  30 namespace seastar {
  31
  32 // Some C APIs have a structure with a variable length array at the end.
  33 // This is a helper function to help allocate it.
  34 //
  35 // for a structure
  36 //
  37 //   struct xx { int a; float b[0]; };
  38 //
  39 // use
  40 //
  41 //   make_struct_with_vla(&xx::b, number_of_bs);
  42 //
  43 // to allocate it.
  44 //
  45 template <class S, typename E>
  46 inline
  47 std::unique_ptr<S, free_deleter>
  48 make_struct_with_vla(E S::*last, size_t nr) {
  49     auto fake = reinterpret_cast<S*>(0);
  50     size_t offset = reinterpret_cast<uintptr_t>(&(fake->*last));
  51     size_t element_size = sizeof((fake->*last)[0]);
  52     assert(offset == sizeof(S));
  53     auto p = std::unique_ptr<char, free_deleter>(
  54             reinterpret_cast<char*>(::malloc(offset + element_size * nr)));
  55     auto s = std::unique_ptr<S, free_deleter>(new (p.get()) S());
  56     p.release();
  57     return s;
  58 }
  59
  60 }