[英]C++ struct alignment question
I have a predefined struct (actually several) where variables span across 32-bit word boundary. 我有一个预定义的结构(实际上是几个),其中变量跨越32位字边界。 In Linux (and Windows using GCC) I am able to get my structs to pack to the correct size using 'attribute((packed))'.
在Linux(以及使用GCC的Windows)中,我能够使用'attribute((packed))'将我的结构打包到正确的大小。 However I cannot get it to work the same way using VC++ and #pragma pack.
但是我无法使用VC ++和#pragma pack以相同的方式工作。
Using GCC this returns a correct size of 6 bytes: 使用GCC,返回正确的6字节大小:
struct
{
unsigned int a : 3;
unsigned int b : 1;
unsigned int c : 15;
unsigned int troubleMaker : 16;
unsigned short padding : 13;
} __attribute__((packed)) s;
Using VC++ this returns an incorrect size of 8 bytes 使用VC ++,这将返回不正确的8字节大小
#pragma pack(push)
#pragma pack(1)
struct
{
unsigned int a : 3;
unsigned int b : 1;
unsigned int c : 15;
unsigned int troubleMaker : 16;
unsigned short padding : 13;
} s;
#pragma pack(pop)
I can get things to work by splitting 'troubleMaker' across the boundary manually but I'd prefer not to. 我可以通过手动将“troubleMaker”分割成边界来实现工作,但我不愿意。 Any ideas?
有任何想法吗?
I would suggest not using vendor-specific C language extensions to match device or network bit formats. 我建议不要使用特定于供应商的C语言扩展来匹配设备或网络位格式。 Even if you get the fields to line up using a series of one-per-vendor language extensions, you still have byte order to worry about, and you still have a struct layout that requires extra instructions to access.
即使您使用一系列每个供应商的语言扩展来排列字段,您仍然需要担心字节顺序,并且您仍然需要一个需要额外指令才能访问的结构布局。
You can write a C99 conforming program that will work on any architecture or host and at maximum speed and cache efficiency by using the standardized C API string and memory copy functions and the Posix hton and ntoh functions. 您可以使用标准化的C API字符串和内存复制函数以及Posix hton和ntoh函数编写符合C99标准的程序,该程序可以在任何体系结构或主机上以最高速度和缓存效率运行。
A good practice is to use the following functions for which there exist published standards: 一个好的做法是使用已发布标准的以下函数:
C99: memcpy(), Posix: htonl(), htons(), ntohl(), ntohs()
Update: here is some code that should work the same everywhere. 更新:这里有一些代码应该在任何地方都一样。 You may need to get
<stdint.h>
from this project if Microsoft still hasn't implemented it for C99, or just make the usual assumptions about int sizes. 如果Microsoft 仍然没有为C99实现它,或者只是对int size做出通常的假设,那么你可能需要从这个项目中获取
<stdint.h>
。
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <stdio.h>
#include <arpa/inet.h>
struct packed_with_bit_fields { // ONLY FOR COMPARISON
unsigned int a : 3;
unsigned int b : 1;
unsigned int c : 15;
unsigned int troubleMaker : 16;
unsigned short padding : 13;
} __attribute__((packed)); // USED ONLY TO COMPARE IMPLEMENTATIONS
struct unpacked { // THIS IS THE EXAMPLE STRUCT
uint32_t a;
uint32_t b;
uint32_t c;
uint32_t troubleMaker;
}; // NOTE NOT PACKED
struct unpacked su;
struct packed_with_bit_fields sp;
char *bits = "Lorem ipsum dolor";
int main(int ac, char **av) {
uint32_t x; // byte order issues ignored in both cases
// This should work with any environment and compiler
memcpy(&x, bits, 4);
su.a = x & 7;
su.b = x >> 3 & 1;
su.c = x >> 4 & 0x7fff;
memcpy(&x, bits + 2, 4);
su.troubleMaker = x >> 3 & 0xffff;
// This section works only with gcc
memcpy(&sp, bits, 6);
printf( sp.a == su.a
&& sp.b == su.b
&& sp.c == su.c
&& sp.troubleMaker == su.troubleMaker
? "conforming and gcc implementations match\n" : "huh?\n");
return 0;
}
Alignment and ordering of bitfields are notoriously implementation-specific. 位域的对齐和排序是众所周知的特定于实现的。 It is much safer to declare a normal integer field and manipulate the "bitfields" within using masks and bitwise (| & ^) operators .
它是更安全声明一个正常的整型字段,并使用口罩和位内操纵“位域”(|&^)运营商。
I don't believe this behavior is supported in Visual Studio. 我不相信Visual Studio中支持此行为。 In addiction to the pack macro I tried using
__declspec(align(1))
and got the same behavior. 在对包宏的依赖中,我尝试使用
__declspec(align(1))
并获得了相同的行为。 I think you are stuck with 12 bytes or re-ordering your structure a bit. 我认为你被困在12个字节或者重新排序你的结构。
我相信VC ++不支持这一点,我对GCC在这方面的行为是否实际上是标准的表示严重怀疑。
如果绝对确定需要6个字节然后将其定义为3个短路并自己获取数据...它不会减慢速度......编译器就是这样做...
struct unpacked { // apparently my other example was too long and confusing
uint32_t a; // ...here is a much shorter example with only the conforming
uint32_t b; // ...code. (The other program had the gcc-specific declaration,
uint32_t c; // but only for test code. Still, it was a bit long.)
uint32_t troubleMaker;
};
struct unpacked su;
char *bits = "Lorem ipsum dolor";
void f(void) {
uint32_t x;
memcpy(&x, bits, 4);
su.a = x & 7;
su.b = x >> 3 & 1;
su.c = x >> 4 & 0x7fff;
memcpy(&x, bits + 2, 4);
su.troubleMaker = x >> 3 & 0xffff;
return 0;
}
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