Data structure alignment is the way data is arranged and accessed in computer memory. It consists of two separate but related issues: data alignment and data structure padding. When a modern computer reads from or writes to a memory address, it will do this in word sized chunks (e.g. 4 byte chunks on a 32-bit system) or larger. Data alignment means putting the data at a memory address equal to some multiple of the word size, which increases the system’s performance due to the way the CPU handles memory. To align the data, it may be necessary to insert some meaningless bytes between the end of the last data structure and the start of the next, which is data structure padding.

In order to align the data in memory, one or more empty bytes (addresses) are inserted (or left empty) between memory addresses which are allocated for other structure members while memory allocation. This concept is called structure padding. Architecture of a computer processor is such a way that it can read 1 word (4 byte in 32 bit processor) from memory at a time. To make use of this advantage of processor, data are always aligned as 4 bytes package which leads to insert empty addresses between other member’s address. Because of this structure padding concept in C, size of the structure is always not same as what we think.

填充和填充只是同一事物的两个方面:

包装或对齐是每个成员四舍五入的大小 填充是为匹配对齐而添加的额外空间

在mystruct_A中，假设默认对齐为4，则每个成员以4字节的倍数进行对齐。因为char的大小是1，所以a和c的填充是4 - 1 = 3个字节，而int b不需要填充，因为它已经是4个字节了。它对mystruct_B的工作方式相同。

结构填充抑制结构填充，当对齐最重要时使用填充，当空间最重要时使用填充。

一些编译器提供#pragma来抑制填充或将其打包为n个字节。有些公司提供关键字来做到这一点。通常，用于修改结构填充的pragma将采用以下格式(取决于编译器):

#pragma pack(n)

例如，ARM提供了__packed关键字来抑制结构填充。查阅编译器手册了解更多相关信息。

所以一个填充结构是一个没有填充的结构。

一般采用填料结构

为了节省空间 对数据结构进行格式化，以便在网络上传输 协议(这当然不是一个好的实践，因为您需要这样做 处理字节序)

填充规则:

结构体的每个成员都应该位于能被其大小整除的地址。 填充在元素之间或结构的末尾插入，以确保满足此规则。这样做是为了硬件更容易和更有效地访问总线。 结构体末尾的填充是根据结构体最大成员的大小决定的。

规则二: 考虑下面的结构，

如果我们要为这个结构体创建一个数组(包含2个结构体)， 结束时不需要填充:

因此，struct的大小= 8字节

假设我们要创建另一个结构体，如下所示:

如果我们要创建这个结构体的数组， 最后需要填充的字节数有两种可能。

A.如果我们在末尾添加3个字节，并将其对齐为int而不是Long:

B.如果我们在末尾添加7个字节并将其对齐为Long:

第二个数组的起始地址是8(i)的倍数。e 24)。 struct的大小= 24字节

因此，通过将结构体的下一个数组的起始地址对齐为最大成员(i。E如果我们要创建这个结构体的数组，第二个数组的第一个地址必须从一个地址开始，该地址必须是该结构体最大成员的倍数。这里是24(3 * 8))，我们可以计算出最后所需的填充字节数。

我知道这个问题很老了，这里的大多数答案都很好地解释了填充，但当我自己试图理解它时，我发现对正在发生的事情有一个“视觉”形象是有帮助的。

处理器以一定大小(字)的“块”读取内存。假设处理器字有8字节长。它将把内存看作一个8字节的大行构建块。每当它需要从内存中获取一些信息时，它就会到达其中一个块并获取它。

如上图所示，一个Char(1字节长)在哪里并不重要，因为它将在其中一个块中，只需要CPU处理1个字。

When we deal with data larger than one byte, like a 4 byte int or a 8 byte double, the way they are aligned in the memory makes a difference on how many words will have to be processed by the CPU. If 4-byte chunks are aligned in a way they always fit the inside of a block (memory address being a multiple of 4) only one word will have to be processed. Otherwise a chunk of 4-bytes could have part of itself on one block and part on another, requiring the processor to process 2 words to read this data.

这同样适用于8字节的double，只不过现在它必须在8的倍数内存地址中，以确保它始终在块中。

这里考虑的是8字节的字处理器，但这个概念也适用于其他大小的字。

填充通过填充这些数据之间的间隙来确保它们与这些块对齐，从而提高读取内存时的性能。

然而，正如其他人回答的那样，有时空间比性能本身更重要。也许您正在一台没有太多RAM的计算机上处理大量数据(可以使用交换空间，但速度要慢得多)。您可以在程序中排列变量，直到完成最少的填充(这在其他一些回答中得到了很好的例子)，但如果这还不够，您可以显式地禁用填充，这就是打包。

Data structure alignment is the way data is arranged and accessed in computer memory. It consists of two separate but related issues: data alignment and data structure padding. When a modern computer reads from or writes to a memory address, it will do this in word sized chunks (e.g. 4 byte chunks on a 32-bit system) or larger. Data alignment means putting the data at a memory address equal to some multiple of the word size, which increases the system’s performance due to the way the CPU handles memory. To align the data, it may be necessary to insert some meaningless bytes between the end of the last data structure and the start of the next, which is data structure padding.

In order to align the data in memory, one or more empty bytes (addresses) are inserted (or left empty) between memory addresses which are allocated for other structure members while memory allocation. This concept is called structure padding. Architecture of a computer processor is such a way that it can read 1 word (4 byte in 32 bit processor) from memory at a time. To make use of this advantage of processor, data are always aligned as 4 bytes package which leads to insert empty addresses between other member’s address. Because of this structure padding concept in C, size of the structure is always not same as what we think.