Method ReadAt

cpp/src/arrow/io/file.cc:664–680

bool MemoryMappedFile::closed() const { return memory_map_->closed(); }

Result<std::shared_ptr<Buffer>> MemoryMappedFile::ReadAt(int64_t position,
                                                         int64_t nbytes) {
  RETURN_NOT_OK(memory_map_->CheckClosed());
  // if the file is writable, we acquire the lock before creating any slices
  // in case a resize is triggered concurrently, otherwise we wouldn't detect
  // a change in the use count
  auto guard_resize = memory_map_->writable()
                          ? std::unique_lock<std::mutex>(memory_map_->resize_lock())
                          : std::unique_lock<std::mutex>();

  ARROW_ASSIGN_OR_RAISE(
      nbytes, internal::ValidateReadRange(position, nbytes, memory_map_->size()));
  // Arrange to page data in
  RETURN_NOT_OK(::arrow::internal::MemoryAdviseWillNeed(
      {{memory_map_->data() + position, static_cast<size_t>(nbytes)}}));
  return memory_map_->Slice(position, nbytes);
}

Result<int64_t> MemoryMappedFile::ReadAt(int64_t position, int64_t nbytes, void* out) {
  RETURN_NOT_OK(memory_map_->CheckClosed());