computing Computing is any goal-oriented activity requiring, benefiting from, or creating computer, computing machinery. It includes the study and experimentation of algorithmic processes, and the development of both computer hardware, hardware and softw ...

, shadow memory is a technique used to track and store information on

computer memory Computer memory stores information, such as data and programs, for immediate use in the computer. The term ''memory'' is often synonymous with the terms ''RAM,'' ''main memory,'' or ''primary storage.'' Archaic synonyms for main memory include ...

used by a

program Program (American English; also Commonwealth English in terms of computer programming and related activities) or programme (Commonwealth English in all other meanings), programmer, or programming may refer to: Business and management * Program m ...

during its execution. Shadow memory consists of shadow bytes that map to individual bits or one or more bytes in main memory. These shadow bytes are typically invisible to the original program and are used to record information about the original piece of data.

Technique

The technique is utilized by memory-error checkers that can store information on which parts of memory have been allocated to the program being checked. This shadow memory is then used for detecting and reporting incorrect accesses of memory, even though the program may not be crashing due to a

segmentation fault In computing, a segmentation fault (often shortened to segfault) or access violation is a Interrupt, failure condition raised by hardware with memory protection, notifying an operating system (OS) the software has attempted to access a restricted ...

or similar. An error checker may also store additional information on memory such as which bits have been defined and which ones do not.

Memcheck Valgrind () is a programming tool for memory debugging, memory leak detection, and profiling. Valgrind was originally designed to be a freely licensed memory debugging tool for Linux on x86, but has since evolved to become a generic framework f ...

, part of the

Valgrind Valgrind () is a programming tool for memory debugging, memory leak detection, and profiling. Valgrind was originally designed to be a freely licensed memory debugging tool for Linux on x86, but has since evolved to become a generic framework ...

suite, uses this to detect undefined behavior resulting from acting on or printing undefined memory values. Use of shadow memory is however not limited to memory-error checkers, as what information is stored in these shadow bytes is not fixed. It is for instance used by ThreadSanitizer, a

data race A race condition or race hazard is the condition of an electronics, software, or other system where the system's substantive behavior is dependent on the sequence or timing of other uncontrollable events, leading to unexpected or inconsistent ...

detector. Shadow memory can be both implemented and used a lot of different ways, and have different performance characteristics. Memcheck for instance tracks values with

bit The bit is the most basic unit of information in computing and digital communication. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represented as ...

precision, while

AddressSanitizer A code sanitizer is a programming tool that detects bugs in the form of undefined or suspicious behavior by a compiler inserting instrumentation code at runtime. The class of tools was first introduced by Google's AddressSanitizer (or ASan) of 20 ...

, part of the

clang Clang () is a compiler front end for the programming languages C, C++, Objective-C, Objective-C++, and the software frameworks OpenMP, OpenCL, RenderScript, CUDA, SYCL, and HIP. It acts as a drop-in replacement for the GNU Compiler ...

compiler, is comparatively very fast. Memcheck, like all Valgrind tools, uses

binary translation In computing, binary translation is a form of binary recompilation where sequences of instruction (computer science), instructions are translated from a source instruction set (ISA) to the target instruction set with respect to the operating syste ...

and

instrumentation Instrumentation is a collective term for measuring instruments, used for indicating, measuring, and recording physical quantities. It is also a field of study about the art and science about making measurement instruments, involving the related ...

to run code manipulating the shadow memory corresponding to program memory use. AddressSanitizer on the other hand is created on compile-time and inserts error-checking code inline into a program during compilation. Its shadow-memory implementation uses a huge reservation of virtual memory for its shadow memory, giving very different performance characteristics.

References

General

*{{Cite book, first1=N. , last1=Nethercote , first2=J. , last2=Seward , s2cid=10263496 , year=2007 , title=How to shadow every byte of memory used by a program. In Proceedings of the 3rd international Conference on Virtual Execution Environments (San Diego, California, USA) , publication-date=June 13–15, 2007 , work=VEE '07 , publisher=ACM New York , pages=65–74 , doi=10.1145/1254810.1254820, chapter=How to shadow every byte of memory used by a program , isbn=9781595936301 , citeseerx=10.1.1.643.7117 * http://research.google.com/pubs/pub37752.html Memory management