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。

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。

只有当你告诉编译器显式地对结构进行打包时，才会进行结构打包。你看到的是填充。您的32位系统正在填充每个字段以字对齐。如果您告诉编译器打包结构，它们将分别为6和5字节。但是不要这样做。它不可移植，使编译器生成的代码更慢(有时甚至有bug)。

(上面的答案解释的很清楚，但是对于padding的大小似乎不是很清楚，所以，我将根据我从The Lost Art of Structure Packing学到的补充一个答案，它已经发展到不局限于C，也适用于Go, Rust。)

内存对齐(用于struct)

规则:

Before each individual member, there will be padding so that to make it start at an address that is divisible by its alignment requirement. E.g., on many systems, an int should start at an address divisible by 4 and a short by 2. char and char[] are special, could be any memory address, so they don't need padding before them. For struct, other than the alignment need for each individual member, the size of whole struct itself will be aligned to a size divisible by strictest alignment requirement of any of its members, by padding at end. E.g., on many systems, if struct's largest member is int then by divisible by 4, if short then by 2.

会员顺序:

成员的顺序可能会影响结构的实际大小，所以要记住这一点。 例如，下面示例中的stu_c和stu_d具有相同的成员，但顺序不同，并导致两个结构体的大小不同。

内存中的地址(用于struct)

空的空间:

两个结构体之间的空白空间可以被适合的非结构变量使用。 例如，在下面的test_struct_address()中，变量x位于相邻的结构g和h之间。 无论是否声明x, h的地址都不会改变，x只是重用了g浪费的空间。 y也是一样。

例子

(适用于64位系统)

memory_align.c:

/**
 * Memory align & padding - for struct.
 * compile: gcc memory_align.c
 * execute: ./a.out
 */ 
#include <stdio.h>

// size is 8, 4 + 1, then round to multiple of 4 (int's size),
struct stu_a {
    int i;
    char c;
};

// size is 16, 8 + 1, then round to multiple of 8 (long's size),
struct stu_b {
    long l;
    char c;
};

// size is 24, l need padding by 4 before it, then round to multiple of 8 (long's size),
struct stu_c {
    int i;
    long l;
    char c;
};

// size is 16, 8 + 4 + 1, then round to multiple of 8 (long's size),
struct stu_d {
    long l;
    int i;
    char c;
};

// size is 16, 8 + 4 + 1, then round to multiple of 8 (double's size),
struct stu_e {
    double d;
    int i;
    char c;
};

// size is 24, d need align to 8, then round to multiple of 8 (double's size),
struct stu_f {
    int i;
    double d;
    char c;
};

// size is 4,
struct stu_g {
    int i;
};

// size is 8,
struct stu_h {
    long l;
};

// test - padding within a single struct,
int test_struct_padding() {
    printf("%s: %ld\n", "stu_a", sizeof(struct stu_a));
    printf("%s: %ld\n", "stu_b", sizeof(struct stu_b));
    printf("%s: %ld\n", "stu_c", sizeof(struct stu_c));
    printf("%s: %ld\n", "stu_d", sizeof(struct stu_d));
    printf("%s: %ld\n", "stu_e", sizeof(struct stu_e));
    printf("%s: %ld\n", "stu_f", sizeof(struct stu_f));

    printf("%s: %ld\n", "stu_g", sizeof(struct stu_g));
    printf("%s: %ld\n", "stu_h", sizeof(struct stu_h));

    return 0;
}

// test - address of struct,
int test_struct_address() {
    printf("%s: %ld\n", "stu_g", sizeof(struct stu_g));
    printf("%s: %ld\n", "stu_h", sizeof(struct stu_h));
    printf("%s: %ld\n", "stu_f", sizeof(struct stu_f));

    struct stu_g g;
    struct stu_h h;
    struct stu_f f1;
    struct stu_f f2;
    int x = 1;
    long y = 1;

    printf("address of %s: %p\n", "g", &g);
    printf("address of %s: %p\n", "h", &h);
    printf("address of %s: %p\n", "f1", &f1);
    printf("address of %s: %p\n", "f2", &f2);
    printf("address of %s: %p\n", "x", &x);
    printf("address of %s: %p\n", "y", &y);

    // g is only 4 bytes itself, but distance to next struct is 16 bytes(on 64 bit system) or 8 bytes(on 32 bit system),
    printf("space between %s and %s: %ld\n", "g", "h", (long)(&h) - (long)(&g));

    // h is only 8 bytes itself, but distance to next struct is 16 bytes(on 64 bit system) or 8 bytes(on 32 bit system),
    printf("space between %s and %s: %ld\n", "h", "f1", (long)(&f1) - (long)(&h));

    // f1 is only 24 bytes itself, but distance to next struct is 32 bytes(on 64 bit system) or 24 bytes(on 32 bit system),
    printf("space between %s and %s: %ld\n", "f1", "f2", (long)(&f2) - (long)(&f1));

    // x is not a struct, and it reuse those empty space between struts, which exists due to padding, e.g between g & h,
    printf("space between %s and %s: %ld\n", "x", "f2", (long)(&x) - (long)(&f2));
    printf("space between %s and %s: %ld\n", "g", "x", (long)(&x) - (long)(&g));

    // y is not a struct, and it reuse those empty space between struts, which exists due to padding, e.g between h & f1,
    printf("space between %s and %s: %ld\n", "x", "y", (long)(&y) - (long)(&x));
    printf("space between %s and %s: %ld\n", "h", "y", (long)(&y) - (long)(&h));

    return 0;
}

int main(int argc, char * argv[]) {
    test_struct_padding();
    // test_struct_address();

    return 0;
}

执行结果- test_struct_padding():

stu_a: 8
stu_b: 16
stu_c: 24
stu_d: 16
stu_e: 16
stu_f: 24
stu_g: 4
stu_h: 8

执行结果- test_struct_address():

stu_g: 4
stu_h: 8
stu_f: 24
address of g: 0x7fffd63a95d0  // struct variable - address dividable by 16,
address of h: 0x7fffd63a95e0  // struct variable - address dividable by 16,
address of f1: 0x7fffd63a95f0 // struct variable - address dividable by 16,
address of f2: 0x7fffd63a9610 // struct variable - address dividable by 16,
address of x: 0x7fffd63a95dc  // non-struct variable - resides within the empty space between struct variable g & h.
address of y: 0x7fffd63a95e8  // non-struct variable - resides within the empty space between struct variable h & f1.
space between g and h: 16
space between h and f1: 16
space between f1 and f2: 32
space between x and f2: -52
space between g and x: 12
space between x and y: 12
space between h and y: 8

因此，每个变量的地址起始为g:d0 x:dc h:e0 y:e8