flatbuffers/
vtable_writer.rs

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1/*
2 * Copyright 2018 Google Inc. All rights reserved.
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 *     http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17use core::ptr::write_bytes;
18
19use crate::endian_scalar::emplace_scalar;
20use crate::primitives::*;
21
22/// VTableWriter compartmentalizes actions needed to create a vtable.
23#[derive(Debug)]
24pub struct VTableWriter<'a> {
25    buf: &'a mut [u8],
26}
27
28impl<'a> VTableWriter<'a> {
29    #[inline(always)]
30    pub fn init(buf: &'a mut [u8]) -> Self {
31        VTableWriter { buf }
32    }
33
34    /// Writes the vtable length (in bytes) into the vtable.
35    ///
36    /// Note that callers already need to have computed this to initialize
37    /// a VTableWriter.
38    ///
39    /// In debug mode, asserts that the length of the underlying data is equal
40    /// to the provided value.
41    #[inline(always)]
42    pub fn write_vtable_byte_length(&mut self, n: VOffsetT) {
43        let buf = &mut self.buf[..SIZE_VOFFSET];
44        // Safety:
45        // Validated range above
46        unsafe {
47            emplace_scalar::<VOffsetT>(buf, n);
48        }
49        debug_assert_eq!(n as usize, self.buf.len());
50    }
51
52    /// Writes an object length (in bytes) into the vtable.
53    #[inline(always)]
54    pub fn write_object_inline_size(&mut self, n: VOffsetT) {
55        let buf = &mut self.buf[SIZE_VOFFSET..2 * SIZE_VOFFSET];
56        // Safety:
57        // Validated range above
58        unsafe {
59            emplace_scalar::<VOffsetT>(buf, n);
60        }
61    }
62
63    /// Writes an object field offset into the vtable.
64    ///
65    /// Note that this expects field offsets (which are like pointers), not
66    /// field ids (which are like array indices).
67    #[inline(always)]
68    pub fn write_field_offset(&mut self, vtable_offset: VOffsetT, object_data_offset: VOffsetT) {
69        let idx = vtable_offset as usize;
70        let buf = &mut self.buf[idx..idx + SIZE_VOFFSET];
71        // Safety:
72        // Validated range above
73        unsafe {
74            emplace_scalar::<VOffsetT>(buf, object_data_offset);
75        }
76    }
77
78    /// Clears all data in this VTableWriter. Used to cleanly undo a
79    /// vtable write.
80    #[inline(always)]
81    pub fn clear(&mut self) {
82        // This is the closest thing to memset in Rust right now.
83        let len = self.buf.len();
84        let p = self.buf.as_mut_ptr() as *mut u8;
85
86        // Safety:
87        // p is byte aligned and of length `len`
88        unsafe {
89            write_bytes(p, 0, len);
90        }
91    }
92}
flatbuffers/vtable_writer.rs

flatbuffers/
vtable_writer.rs
