考虑:
struct mystruct_A
{
char a;
int b;
char c;
} x;
struct mystruct_B
{
int b;
char a;
} y;
结构尺寸分别为12和8。
这些结构是填充的还是包装的?
什么时候进行填充或包装?
考虑:
struct mystruct_A
{
char a;
int b;
char c;
} x;
struct mystruct_B
{
int b;
char a;
} y;
结构尺寸分别为12和8。
这些结构是填充的还是包装的?
什么时候进行填充或包装?
当前回答
变量存储在可以被其对齐方式(通常是大小)整除的任何地址上。所以,填充/填充不仅仅是为了结构。实际上,所有数据都有自己的对齐要求:
int main(void) {
// We assume the `c` is stored as first byte of machine word
// as a convenience! If the `c` was stored as a last byte of previous
// word, there is no need to pad bytes before variable `i`
// because `i` is automatically aligned in a new word.
char c; // starts from any addresses divisible by 1(any addresses).
char pad[3]; // not-used memory for `i` to start from its address.
int32_t i; // starts from any addresses divisible by 4.
这类似于struct,但有一些区别。首先,我们可以说有两种填充——a)为了正确地从每个成员的地址开始,在成员之间插入一些字节。b)为了正确地从struct的地址启动下一个struct实例,将一些字节追加到每个struct:
// Example for rule 1 below.
struct st {
char c; // starts from any addresses divisible by 4, not 1.
char pad[3]; // not-used memory for `i` to start from its address.
int32_t i; // starts from any addresses divisible by 4.
};
// Example for rule 2 below.
struct st {
int32_t i; // starts from any addresses divisible by 4.
char c; // starts from any addresses.
char pad[3]; // not-used memory for next `st`(or anything that has same
// alignment requirement) to start from its own address.
};
The struct's first member always starts from any addresses divisible by struct's own alignment requirement which is determined by largest member's alignment requirement(here 4, alignment of int32_t). This is different with normal variables. The normal variables can start any addresses divisible by its alignment, but it is not the case for struct's first member. As you know, the address of a struct is the same as the address of its first member. There can be additional padded trailing bytes inside a struct, making next struct(or next element in an array of structs) starting from its own address. Think of struct st arr[2];. To make arr[1](arr[1]'s first member) starting from an address divisible by 4, we should append 3 bytes at the end of each struct.
这是我从《丢失的结构包装艺术》中学到的。
注意:可以通过_Alignof操作符来研究数据类型的对齐要求。同样,你也可以通过offsetof宏来获取结构中成员的偏移量。
其他回答
这件事没有但是!想要掌握这门学科必须做到以下几点:
细读埃里克·s·雷蒙德所著的《丢失的结构包装艺术》 看一下Eric的代码示例 最后但并非最不重要的是,不要忘记下面关于填充的规则,即结构体的对齐方式与最大类型的对齐方式一致 要求。
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。
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.
这些结构是填充的还是包装的?
它们填充。
最初想到的唯一可能是,如果char和int的大小相同,那么char/int/char结构的最小大小将不允许填充,int/char结构也是如此。
然而,这将要求sizeof(int)和sizeof(char)都为4(以获得12和8的大小)。由于sizeof(char)始终为1的标准保证了整个理论的崩溃。
如果char和int的宽度相同,那么大小将是1和1,而不是4和4。因此,为了得到12的大小,在最终字段之后必须有填充。
什么时候进行填充或包装?
只要编译器实现需要。编译器可以在字段之间和最后一个字段之后(但不能在第一个字段之前)插入填充。
这样做通常是为了性能,因为某些类型在特定边界上对齐时性能更好。甚至有一些架构会在你试图访问未对齐的数据时拒绝运行(即崩溃)(是的,我在看你,ARM)。
您通常可以使用特定于实现的特性(如#pragma pack)来控制打包/填充(这实际上是同一领域的两个极端)。即使您不能在特定的实现中这样做,您也可以在编译时检查代码以确保它满足您的需求(使用标准C特性,而不是特定于实现的东西)。
例如:
// C11 or better ...
#include <assert.h>
struct strA { char a; int b; char c; } x;
struct strB { int b; char a; } y;
static_assert(sizeof(struct strA) == sizeof(char)*2 + sizeof(int), "No padding allowed");
static_assert(sizeof(struct strB) == sizeof(char) + sizeof(int), "No padding allowed");
如果这些结构中有任何填充,类似这样的东西将拒绝编译。
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)