Memory layout

Their timing must be just right, or else one of the ropes will catch them on their shoulder or foot, breaking the rhythm and bringing the game to a halt. The player needs to have time to set up their entrance or exit so that everything runs smoothly. And if two or more players are going to make the transition, it is all the more important that they be ready at the same time.

Memory layout

Who should read this This page is about a technique for reducing the memory footprint of programs in compiled languages with C-like structures - manually repacking these declarations for reduced size. To read it, you will require basic knowledge of the C programming language.

You need to know this technique if Memory layout intend to write code for memory-constrained embedded systems, or operating-system kernels. Memory layout is useful if you are working with application data sets so large that your programs routinely hit memory limits.

It is good to know in any application where you really, really care about optimizing your use of memory bandwidth and minimizing cache-line misses. Finally, knowing this technique is a gateway to other esoteric C topics.

You are not an advanced C programmer until you have grasped these rules. You are not a master of C until you could have written this document yourself and can criticize it intelligently.

This document originated with "C" in the title, but many of the techniques discussed here also apply to the Go language - and, should generalize to any compiled language with C-like structures.

Memory layout

There is a note discussing Go and Rust towards the end. Why I wrote it This webpage exists because in late I found myself heavily applying an optimization technique that I had learned more than two decades previously and not used much since.

I needed to reduce the memory footprint of a program that used thousands - sometimes hundreds of thousands - of C struct instances. The program was cvs-fast-export and the problem was that it was dying with out-of-memory errors on large repositories.

There are ways to reduce memory usage significantly in situations like this, by rearranging the order of structure members in careful ways. But as I worked, and thought about what I was doing, it began to dawn on me that the technique I was using has been more than half forgotten in these latter days.

A couple of Wikipedia entries touch the topic, but I found nobody who covered it comprehensively. CS courses rightly steer people away from micro-optimization towards finding better algorithms.

The plunging price of machine resources has made squeezing memory usage less necessary. And the way hackers used to learn how to do it back in the day was by bumping their noses on strange hardware architectures - a less common experience now. But the technique still has value in important situations, and will as long as memory is finite.

This document is intended to save programmers from having to rediscover the technique, so they can concentrate effort on more important things. Alignment requirements The first thing to understand is that, on modern processors, the way your compiler lays out basic datatypes in memory is constrained in order to make memory accesses faster.

Our examples are in C, but any compiled language generates code under the same constraints. Rather, each type except char has an alignment requirement; chars can start on any byte address, but 2-byte shorts must start on an even address, 4-byte ints or floats must start on an address divisible by 4, and 8-byte longs or doubles must start on an address divisible by 8.

Signed or unsigned makes no difference. The jargon for this is that basic C types on x86 and ARM are self-aligned. Pointers, whether bit 4-byte or bit 8-byte are self-aligned too.

Memory layout

Self-alignment makes access faster because it facilitates generating single-instruction fetches and puts of the typed data.

Without alignment constraints, on the other hand, the code might end up having to do two or more accesses spanning machine-word boundaries. In fact, with sufficient determination and the right e18 hardware flag set on the processor, you can still trigger this on x Also, self-alignment is not the only possible rule.

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Historically, some processors especially those lacking barrel shifters have had more restrictive ones. If you do embedded systems, you might trip over one of these lurking in the underbrush.

Be aware this is possible. From when it was first written at the beginning of until latethis section ended with the last paragraph. It does packet analysis by reading packets off the wire directly into memory that the rest of the code sees as a struct, relying on the assumption of minimal self-aligned padding.

The interesting news is that NTP has apparently being getting away with this for decades across a very wide span of hardware, operating systems, and compilers, including not just Unixes but under Windows variants as well.

Consider the following series of variable declarations in the top level of a C module: That is, on a bit machine 4 bytes of pointer would be immediately followed by 1 byte of char and that immediately followed by 4 bytes of int.English: An attempt at mapping the virtual memory in Linux on x86 64 bit systems.

Where is the kernel and user space virtual memory areas and how is the user space virtual memory area decomposed for a process. As this is an attempt, I've uploaded the SVG so anyone can update and correct it easily.

I am basically wondering how C++ lays out the object in memory. So, I hear that dynamic casts simply adjust the object's pointer in memory with an offset; and reinterpret kind of allows us to do an. Memory Layout of C Program - Code, Data, BSS, Stack, and Heap Segments: program code stored in text or code segment.

Uninitialized static and global variable stored in BSS segment. Initialized static and global variable stored in data segment. Size command is used to check size of code, data, and bss segments on Linux.

On version , the initial binaries loaded into memory by the kernel always have the upper bits as all-zero, so there are 6 fewer bits of layout randomization. Binaries loaded within the main-binary-region are loaded into memory in the following order, immediately after .

The memory layout of one of these looks unsurprising, like this: This is the art of structure packing. The first thing to notice is that slop only happens in two places.

One is where storage bound to a larger data type (with stricter alignment requirements) follows storage bound to a smaller one. Room Layout Each Memory Care suite is a little different, but all include the same comfort and safety features.

Each apartment includes a generous living space right .

Instruction Set Manual: NXP 80C51MX Memory Layout