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QCBOR

QCBOR is a powerful, commercial quality implementation of nearly everything in RFC8949. This RFC defines the Concise Binary Object Representation (CBOR). Since RFC 8949 is fully compatible with RFC 7049, this is also a near-complete implementation of it.

QCBOR also implements RFC8742 which defines a CBOR sequence.

New Version With Spiffy Decode

This new version of QCBOR adds a more powerful decoding API called Spiffy Decode.

  • Much easier implementation of decoding of CBOR protocols
  • Decoding implementations parallels encoding implementation
  • Overall smaller code size for implementations decoding multiple and / or complex maps
  • Backwards compatible with previous versions of QCBOR

See section below for more details.

QCBOR Characteristics

Implemented in C with minimal dependency – The only dependencies are C99, <stdint.h>, <stddef.h>, <stdbool.h> and <string.h> making it highly portable. <math.h> and <fenv.h> are used too, but their use can disabled. No #ifdefs or compiler options need to be set for QCBOR to run correctly.

Focused on C / native data representation – Careful conversion of CBOR data types in to C data types, carefully handling over and underflow, strict typing and such so the caller doesn't have to worry so much about this and so code using QCBOR passes static analyzers easier. Simpler code because there is no support for encoding/decoding to/from JSON, pretty printing, diagnostic notation... Only encoding from native C representations and decoding to native C representations is supported.

Small simple memory model – Malloc is not needed. The encode context is 174 bytes, decode context is 312 bytes and the description of decoded data item is 56 bytes. Stack use is light and there is no recursion. The caller supplies the memory to hold the encoded CBOR and encode/decode contexts so caller has full control of memory usage making it good for embedded implementations that have to run in small fixed memory.

Supports most of RFC 8949 – With some size limits, all data types and formats in the specification are supported. Map sorting is main CBOR feature that is not supported. The same decoding API supports both definite and indefinite-length map and array decoding. Decoding indefinite length strings is supported but requires a string allocator be set up. Encoding of indefinite length strings is planned, but not yet supported.

Extensible and general – Provides a way to handle data types that are not directly supported.

Secure coding style – Uses a construct called UsefulBuf as a discipline for very safe coding and handling of binary data.

Small code size – In the smallest configuration the object code is less than 4KB on 64-bit x86 CPUs. The design is such that object code for QCBOR APIs not used is not referenced.

Clear documented public interface – The public interface is separated from the implementation. It can be put to use without reading the source.

Comprehensive test suite – Easy to verify on a new platform or OS with the test suite. The test suite dependencies are minimal and the same as the library's.

Code Status

This version with spiffy decode in fall of 2020 is a big change from the previous versions but is thoroughly tested including regression for backwards compatibility with the previous version.

Should the previous version be necessary, it is available in the branch BeforeSpiffyDecode. Please file an issue in GitHub to report any problems.

QCBOR was originally developed by Qualcomm. It was open sourced through CAF with a permissive Linux license, September 2018 (thanks Qualcomm!).

This code in Laurence's GitHub has diverged from the CAF source with some simplifications, tidying up and feature additions.

Building

There is a simple makefile for the UNIX style command line binary that compiles everything to run the tests.

These eleven files, the contents of the src and inc directories, make up the entire implementation.

  • inc
    • UsefulBuf.h
    • qcbor_private.h
    • qcbor_common.h
    • qcbor_encode.h
    • qcbor_decode.h
    • qcbor_spiffy_decode.h
  • src
    • UsefulBuf.c
    • qcbor_encode.c
    • qcbor_decode.c
    • ieee754.h
    • ieee754.c

For most use cases you should just be able to add them to your project. Hopefully the easy portability of this implementation makes this work straight away, whatever your development environment is.

The test directory includes the tests that are nearly as portable as the main implementation. If your development environment doesn't support UNIX style command line and make, you should be able to make a simple project and add the test files to it. Then just call RunTests() to invoke them all.

While this code will run fine without configuration, there are several C pre processor macros that can be #defined in order to:

  • use a more efficient implementation
  • to reduce code size
  • to improve performance (a little)
  • remove features to reduce code size

See the comment sections on "Configuration" in inc/UsefulBuf.h and the pre processor defines that start with QCBOR_DISABLE_XXX.

Spiffy Decode

In Fall 2020 a large addition makes the decoder more powerful and easy to use. Backwards compatibility with the previous API is retained as the new decoding features layer on top of it.

The first noticable addition are functions to get particular data types. These are an alternative to and built on top of QCBORDecode_GetNext() that does the type checking and in some cases sophisticated type conversion. They track an error state internally so the caller doesn't need to. They also handle the CBOR tagged data types thoroughly and properly.

In line with all the new get functions for non-aggregate types there are new functions for aggregate types. When a map is expected, QCBORDecode_EnterMap() can be called to descend into and searched by label. Duplicate detection of map items is performed. There is a similar facility for arrays and byte-string wrapped CBOR.

An outcome of all this is that now the decoding implementation of some data can look very similar to the encoding of some data and is generally easier to implement. Following is an example of first encoding a map with three items and then decoding it.

 /* Encode */
 QCBOREncode_Init(&EncodeCtx, Buffer);
 QCBOREncode_OpenMap(&EncodeCtx);
 QCBOREncode_AddTextToMap(&EncodeCtx, "Manufacturer", pE->Manufacturer);
 QCBOREncode_AddInt64ToMap(&EncodeCtx, "Displacement", pE->uDisplacement);
 QCBOREncode_AddInt64ToMap(&EncodeCtx, "Horsepower", pE->uHorsePower);
 QCBOREncode_CloseMap(&EncodeCtx);
 uErr = QCBOREncode_Finish(&EncodeCtx, &EncodedEngine);

 /* Decode */
 QCBORDecode_Init(&DecodeCtx, EncodedEngine, QCBOR_DECODE_MODE_NORMAL);
 QCBORDecode_EnterMap(&DecodeCtx);
 QCBORDecode_GetTextStringInMapSZ(&DecodeCtx, "Manufacturer", &(pE->Manufacturer));
 QCBORDecode_GetInt64InMapSZ(&DecodeCtx, "Displacement", &(pE->uDisplacement));
 QCBORDecode_GetInt64InMapSZ(&DecodeCtx, "Horsepower", &(pE->uHorsePower));
 QCBORDecode_ExitMap(&DecodeCtx);
 uErr = QCBORDecode_Finish(&DecodeCtx);

The spiffy decode version of QCBOR also handles CBOR tags in a simpler and more thorough way.

The spiffy decode functions will handle definite and indefinite length maps and arrays without the caller having to do anything. This includes mixed definite and indefinte maps and arrays. (Some work remains to support map searching with indefinite length strings.)

See the PR in GitHub for a more detailed list of changes.

Uncompatible Changes

Encoding of MIME tags now uses tag 257 instead of 36. Tag 257 accommodates binary and text-based MIME messages where tag 36 does not. Decoding supports either.

The number of nested tags on a data item is limited to four. Previously it was unlimited.

Some of the error codes have changed.

Floating Point Support

By default, all QCBOR floating-point features are enabled. This includes encoding and decoding of half-precision, CBOR preferred serialization for floating-point and floating-point epoch dates.

If full floating-point is not needed the following #defines can be used to reduce object code size.

QCBOR_DISABLE_FLOAT_HW_USE -- Avoid all use of floating-point hardware and inclusion of <math.h> and <fenv.h>

QCBOR_DISABLE_PREFERRED_FLOAT -- Eliminates support for half-precision and CBOR preferred serialization.

Even if these are #defined, QCBOR can still encode and decode basic floating point numbers.

Defining QCBOR_DISABLE_PREFERRED_FLOAT will reduce object code size by about 900 bytes, though 550 of these bytes can be avoided without the #define by just not calling any of the functions that encode floating-point numbers.

Defining QCBOR_DISABLE_FLOAT_HW_USE will save a small amount of object code. Its main use is on CPUs that have no floating-point hardware.

See discussion in qcbor_encode.h for details.

Code Size

These are approximate sizes on a 64-bit x86 CPU with the -Os optimization.

|               | smallest | largest |  
|---------------|----------|---------|
| encode only   |      850 |    2100 |
| decode only   |     2000 |   13300 |
| combined      |     2850 |   15500 |

From the table above, one can see that the amount of code pulled in from the QCBOR library varies a lot, ranging from 1KB to 15KB. The main factor is in this is the number of QCBOR functions called and which ones they are. QCBOR is constructed with less internal interdependency so only code necessary for the called functions is brought in.

Encoding is simpler and smaller. An encode-only implementation may bring in only 1KB of code.

Encoding of floating-point brings in a little more code as does encoding of tagged types and encoding of bstr wrapping.

Basic decoding using QCBORDecode_GetNext() brings in 3KB.

Use of the supplied MemPool by calling QCBORDecode_SetMemPool() to setup to decode indefinite-length strings adds 0.5KB.

Basic use of spiffy decode to brings in about 3KB. Using more spiffy decode functions, such as those for tagged types bstr wrapping brings in more code.

Finally, use of all of the integer conversion functions will bring in about 5KB, though you can use the simpler ones like QCBORDecode_GetInt64() without bringing in very much code.

In addition to using fewer QCBOR functions, the following are some ways to make the code smaller.

The gcc compiler output is usually smaller than llvm because stack guards are off by default (be sure you actually have gcc and not llvm installed to be invoked by the gcc command). You can also turn off stack gaurds with llvm. It is safe to turn off stack gaurds with this code because Usefulbuf provides similar defenses and this code was carefully written to be defensive.

Disable features with defines like: QCBOR_DISABLE_EXP_AND_MANTISSA (saves about 400 bytes) QCBOR_DISABLE_ENCODE_USAGE_GUARDS (saves about 150), and QCBOR_DISABLE_PREFERRED_FLOAT (saves about 900 bytes), and QCBOR_DISABLE_INDEFINITE_LENGTH_STRINGS (saves about 400 bytes).
QCBOR_DISABLE_INDEFINITE_LENGTH_ARRAYS (saves about 200 bytes). QCBOR_DISABLE_UNCOMMON_TAGS (saves about 100 bytes).

If QCBOR is installed as a shared library, then of course only one copy of the code is in memory no matter how many applications use it.

Size of spiffy decode

When creating a decode implementation, there is a choice of whether or not to use spiffy decode features or to just use QCBORDecode_GetNext().

The implementation using spiffy decode will be simpler resulting in the calling code being smaller, but the amount of code brought in from the QCBOR library will be larger. Basic use of spiffy decode brings in about 2KB of object code. If object code size is not a concern, then it is probably better to use spiffy decode because it is less work, there is less complexity and less testing to worry about.

If code size is a concern, then use of QCBORDecode_GetNext() will probably result in smaller overall code size for simpler CBOR protocols. However, if the CBOR protocol is complex then use of spiffy decode may reduce overall code size. An example of a complex protocol is one that involves decoding a lot of maps or maps that have many data items in them. The overall code may be smaller because the general purpose spiffy decode map processor is the one used for all the maps.

Other Software Using QCBOR

Changes from CAF Version

  • Float support is restored
  • Minimal length float encoding is added
  • indefinite length arrays/maps are supported
  • indefinite length strings are supported
  • Tag decoding is changed; unlimited number of tags supported, any tag value supported, tag utility function for easier tag checking
  • Addition functions in UsefulBuf
  • QCBOREncode_Init takes a UsefulBuf instead of a pointer and size
  • QCBOREncode_Finish takes a UsefulBufC and EncodedCBOR is remove
  • bstr wrapping of arrays/maps is replaced with OpenBstrwrap
  • AddRaw renamed to AddEncoded and can now only add whole arrays or maps, not partial maps and arrays (simplification; was a dangerous feature)
  • Finish cannot be called repeatedly on a partial decode (some tests used this, but it is not really a good thing to use in the first place)
  • UsefulOutBuf_OutUBuf changed to work differently
  • UsefulOutBuf_Init works differently
  • The "_3" functions are replaced with a small number of simpler functions
  • There is a new AddTag functon instead of the "_3" functions, making the interface simpler and saving some code
  • QCBOREncode_AddRawSimple_2 is removed (the macros that referenced still exist and work the same)

Credits

  • Ganesh Kanike for porting to QSEE
  • Mark Bapst for sponsorship and release as open source by Qualcomm
  • Sachin Sharma for release through CAF
  • Tamas Ban for porting to TF-M and 32-bit ARM
  • Michael Eckel for Makefile improvements
  • Jan Jongboom for indefinite length encoding
  • Peter Uiterwijk for error strings and other
  • Michael Richarson for CI set up and additional compiler warnings

Copyright and License

QCBOR is available under what is essentially the 3-Clause BSD License.

Files created inside Qualcomm and open-sourced through CAF (The Code Aurora Forum) have a slightly modified 3-Clause BSD License. The modification additionally disclaims NON-INFRINGEMENT.

Files created after release to CAF use the standard 3-Clause BSD License with no modification. These files have the SPDX license identifier, "SPDX-License-Identifier: BSD-3-Clause" in them.

BSD-3-Clause license

  • Redistributions of source code must retain the above copyright notice, this list of conditions and the following disclaimer.

  • Redistributions in binary form must reproduce the above copyright notice, this list of conditions and the following disclaimer in the documentation and/or other materials provided with the distribution.

  • Neither the name of the copyright holder nor the names of its contributors may be used to endorse or promote products derived from this software without specific prior written permission.

THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Copyright for this README

Copyright (c) 2018-2021, Laurence Lundblade. All rights reserved.


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