bit-fields

Offset in a struct with bit fields

由 久未见 提交于 2019-12-03 17:06:57
If we have a struct with bit fields, then how are the subsequent members aligned in the struct? Consider the following code: struct A{ int a:1; char b; // at offset 1 }; struct B{ int a:16; int b: 17; char c; // at offset 7 }; printf("Size of A: %d\n", (int)sizeof(struct A)); printf("Offset of b in A: %d\n", (int)offsetof(struct A, b)); printf("Size of B: %d\n", (int)sizeof(struct B)); printf("Offset of c in B: %d\n", (int)offsetof(struct B, c)); Output: Size of A: 4 Offset of b in A: 1 Size of B: 8 Offset of c in B: 7 Here, in the first case, b is allocated just in the 2nd byte of the struct

Where can I find a reference for what every bit of the CorFlags value means?

由 可紊 提交于 2019-12-03 11:36:21
I'm messing around with some rather low level things and trying to determine why I get different outputs with the CorFlags.exe utility. For reference, the outputs are as so: $ corflags test2.exe Microsoft (R) .NET Framework CorFlags Conversion Tool. Version 4.0.30319.17929 Copyright (c) Microsoft Corporation. All rights reserved. Version : v4.0.30319 CLR Header: 2.5 PE : PE32 CorFlags : 0x1 ILONLY : 1 32BITREQ : 0 32BITPREF : 0 Signed : 0 $ corflags test.exe Microsoft (R) .NET Framework CorFlags Conversion Tool. Version 4.0.30319.17929 Copyright (c) Microsoft Corporation. All rights reserved.

Size of a bitfield member?

由 前提是你 提交于 2019-12-03 11:31:21
问题 Would anyone know how to extract the size of a bit-field member. The below code naturally gives me the size of an integer, but how do I find out how many bits or bytes are in mybits.one ? I've tried sizeof(test.one) but which clearly won't work. I realize this is a measure of bits: #include <iostream> using namespace std; int main() { struct mybits { unsigned int one:15; }; mybits test; test.one = 455; cout << test.one << endl; cout << "The size of test.one is: " << sizeof(test) << endl; }

Bit field vs Bitset

由 喜夏-厌秋 提交于 2019-12-03 11:24:00
问题 I want to store bits in an array (like structure). So I can follow either of the following two approaches Approach number 1 (AN 1) struct BIT { int data : 1 }; int main() { BIT a[100]; return 0; } Approach number 2 (AN 2) int main() { std::bitset<100> BITS; return 0; } Why would someone prefer AN 2 over AN 1? 回答1: Because approach nr. 2 actually uses 100 bits of storage, plus some very minor (constant) overhead, while nr. 1 typically uses four bytes of storage per Bit structure. In general, a

Packed bit fields in c structures - GCC

由 拈花ヽ惹草 提交于 2019-12-03 08:44:39
问题 I am working with structs in c on linux. I started using bit fields and the "packed" attribute and I came across a wierd behavior: struct t1 { int a:12; int b:32; int c:4; }__attribute__((packed)); struct t2 { int a:12; int b; int c:4; }__attribute__((packed)); void main() { printf("%d\n",sizeof(t1)); //output - 6 printf("%d\n",sizeof(t2)); //output - 7 } How come both structures - that are exactly the same - take diffrent number of bytes? 回答1: Your structures are not "exactly the same". Your

Is there a portable alternative to C++ bitfields

由 岁酱吖の 提交于 2019-12-03 08:42:40
问题 There are many situations (especially in low-level programming), where the binary layout of the data is important. For example: hardware/driver manipulation, network protocols, etc. In C++ I can read/write arbitrary binary structures using char* and bitwise operations (masks and shifts), but that's tedious and error-prone. Obviously, I try to limit the scope of these operations and encapsulate them in higher-level APIs, but it's still a pain. C++ bitfields seem to offer a developer-friendly

What is the most efficient way to represent small values in a struct?

由 断了今生、忘了曾经 提交于 2019-12-03 04:06:48
问题 Often I find myself having to represent a structure that consists of very small values. For example, Foo has 4 values, a, b, c, d that, range from 0 to 3 . Usually I don't care, but sometimes, those structures are used in a tight loop; their values are read a billion times/s, and that is the bottleneck of the program; the whole program consists of a big array of billions of Foo s; In that case, I find myself having trouble deciding how to represent Foo efficiently. I have basically 4 options:

C++11 standard conformant bitmasks using enum class

由 偶尔善良 提交于 2019-12-03 03:08:01
问题 Can you implement standard conformant (as described in 17.5.2.1.3 of the n3242 draft) type safe bitmasks using enum class? The way I read it, a type T is a bitmask if it supports the |,&,^,~,|=,&= and ^= operators and further you can do if(l&r) where l and r are of type T. Missing from the list are the operator != and == and to allow sorting one probably also wants to overload <. Getting the operators to works is just annoying boilerplate code but I do not see how to do if(l&r). At least the

Size of a bitfield member?

由 大兔子大兔子 提交于 2019-12-03 02:01:14
Would anyone know how to extract the size of a bit-field member. The below code naturally gives me the size of an integer, but how do I find out how many bits or bytes are in mybits.one ? I've tried sizeof(test.one) but which clearly won't work. I realize this is a measure of bits: #include <iostream> using namespace std; int main() { struct mybits { unsigned int one:15; }; mybits test; test.one = 455; cout << test.one << endl; cout << "The size of test.one is: " << sizeof(test) << endl; } Runtime solution, the idea from this discussion: http://social.msdn.microsoft.com/Forums/en-US/7e4f01b6

Bit field vs Bitset

由 本秂侑毒 提交于 2019-12-03 01:50:27
I want to store bits in an array (like structure). So I can follow either of the following two approaches Approach number 1 (AN 1) struct BIT { int data : 1 }; int main() { BIT a[100]; return 0; } Approach number 2 (AN 2) int main() { std::bitset<100> BITS; return 0; } Why would someone prefer AN 2 over AN 1? Because approach nr. 2 actually uses 100 bits of storage, plus some very minor (constant) overhead, while nr. 1 typically uses four bytes of storage per Bit structure. In general, a struct is at least one byte large per the C++ standard. #include <bitset> #include <iostream> struct Bit {
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