结构的大小 - 完全混淆 [英] Size of a struct - totally confused
问题描述
我很确定这非常完全无足轻重，标准C ++足够
可以在这里发布，不过它可能最初看起来像是如何制作一个
电脑游戏？问题，虽然我可能错了。
我正在加载一个位图，因为我正在使用Linux我必须定义
BITMAPFILEHEADER我自己（也许更好的做法是给我的版本
a新名称或其他什么，我不知道）。 Windows使用WORD和DWORD来定义它，我使用了SDL提供的一些标准尺寸类型（一个
图形/声音库的东西）来定义它：
struct BITMAPFILEHEADER
{
Uint16 bfType;
Uint32 bfSize;
Uint16 bfReserved1;
Uint16 bfReserved2;
Uint32 bfOffBits;
};
由于某种原因，我完全无法理解，但是，调试器显示我的
位图加载器函数认为sizeof（BITMAPFILEHEADER）== 16.
当然应该是14？
然后我尝试重新定义它几次：
BITMAPFILEHEADER
{
Uint32 bfType; //从16改为32
Uint32 bfSize;
Uint16 bfReserved1;
Uint16 bfReserved2;
Uint32 bfOffBits ;
};
这给出sizeof（BITMAPFILEHEADER）== 16，现在是正确的。
BITMAPFILEHEADER
{
Uint32 bfType;
Uint32 bfSize;
Uint32 bfReserved1;
Uint32 bfReserved2;
Uint32 bfOffBits;
};
这给出了sizeof（BITMAPFILEHEADER）== 20，这也是正确。
我很可能只是误解了某个地方，但我不知道
是什么。
谢谢，
James
I''m pretty sure this is non-totally-trivial enough and standard-C++ enough
to post here, however much it may initially look like a "how do I make a
computer game?" question, though I may be wrong.
I''m loading a bitmap, and as I''m using Linux I''ve had to define
BITMAPFILEHEADER myself (maybe better practice would be to give my version
a new name or whatever, I dunno). Windows uses WORDs and DWORDs to
define it, I''ve used some standard-size types provided by SDL (a
graphics/sound library thing) to define it thus:
struct BITMAPFILEHEADER
{
Uint16 bfType;
Uint32 bfSize;
Uint16 bfReserved1;
Uint16 bfReserved2;
Uint32 bfOffBits;
};
For some reason I utterly can''t fathom, however, a debugger shows that my
bitmap loader function believes that sizeof(BITMAPFILEHEADER) == 16.
Surely that should be 14?
I then tried redefining it a couple of times:
BITMAPFILEHEADER
{
Uint32 bfType; //changed from 16 to 32
Uint32 bfSize;
Uint16 bfReserved1;
Uint16 bfReserved2;
Uint32 bfOffBits;
};
This gives sizeof(BITMAPFILEHEADER) == 16, which is now correct.
BITMAPFILEHEADER
{
Uint32 bfType;
Uint32 bfSize;
Uint32 bfReserved1;
Uint32 bfReserved2;
Uint32 bfOffBits;
};
This gives sizeof(BITMAPFILEHEADER) == 20, which is also correct.
Most likely I just misunderstand something somewhere, but I''ve no idea
what.
Thanks,
James
推荐答案
James Gregory写道：
James Gregory wrote:
I 我非常肯定这不是完全无足轻重的，标准的C ++足以在这里发布，不过它最初可能看起来像是我怎么做电脑游戏？问题，虽然我可能是错的。
我正在加载一个位图，因为我正在使用Linux，我必须自己定义
BITMAPFILEHEADER（也许更好的做法会是给我的
版本一个新名称或其他什么，我不知道）。 Windows使用WORD和DWORD来定义它，我使用了SDL提供的一些标准尺寸类型（
图形/声音库）来定义它：
struct BITMAPFILEHEADER
{Uint16 bfType;
Uint32 bfSize;
Uint16 bfReserved1;
Uint16 bfReserved2;
Uint32 bfOffBits;
};
由于某种原因，我完全无法理解，但是，调试器显示我的
位图加载器函数认为sizeof（BITMAPFILEHEADER）== 16.
当然应该是14？
I''m pretty sure this is non-totally-trivial enough and standard-C++
enough to post here, however much it may initially look like a "how do I
make a computer game?" question, though I may be wrong.
I''m loading a bitmap, and as I''m using Linux I''ve had to define
BITMAPFILEHEADER myself (maybe better practice would be to give my
version a new name or whatever, I dunno). Windows uses WORDs and DWORDs
to
define it, I''ve used some standard-size types provided by SDL (a
graphics/sound library thing) to define it thus:
struct BITMAPFILEHEADER
{
Uint16 bfType;
Uint32 bfSize;
Uint16 bfReserved1;
Uint16 bfReserved2;
Uint32 bfOffBits;
};
For some reason I utterly can''t fathom, however, a debugger shows that my
bitmap loader function believes that sizeof(BITMAPFILEHEADER) == 16.
Surely that should be 14?
[SNIP]
不一定：
bfType：2
填充以使bfSize在int边界上重新开始：2
bfSize：4
Res1：2
Res2：2
bfOffBits：4
那是16.您可能要查看gcc / g ++文档如何询问包装for
a struct。
-
WW aka Attila
:::
加粗在你所代表什么，并小心你的目标。
[SNIP]
Not necessarily:
bfType: 2
Padding to get bfSize to start again on int boundary: 2
bfSize: 4
Res1: 2
Res2: 2
bfOffBits: 4
That is 16. You may want to look at the gcc/g++ docs how to ask packing for
a struct.
--
WW aka Attila
:::
Be bold in what you stand for and careful what you fall for.
" James Gregory" < JA **** @ f2s.com>写道...
"James Gregory" <ja****@f2s.com> wrote...
我很确定这非常完全无足轻重，标准C ++足以在这里发布，不过它可能最初看起来像一个怎么样我做一个电脑游戏吗？问题，虽然我可能是错的。
我正在加载一个位图，因为我正在使用Linux，我必须自己定义
BITMAPFILEHEADER（也许更好的做法会是给我的版本
一个新的名字或其他什么，我不知道）。 Windows使用WORD和DWORD来定义它，我使用了SDL提供的一些标准尺寸类型（
图形/声音库）来定义它：
struct BITMAPFILEHEADER
{
Uint16 bfType;
Uint32 bfSize;
Uint16 bfReserved1;
Uint16 bfReserved2;
Uint32 bfOffBits;
};
出于某种原因，我完全无法理解，但是，调试器显示我的
位图加载器函数认为sizeof（BITMAPFILEHEADER）== 16.
当然应该是14？
然后我尝试重新定义它几次：
BITMAPFILEHEADER
{Uint32 bfType; //从16改为32
Uint32 bfSize;
Uint16 bfReserved1;
Uint16 bfReserved2;
Uint32 bfOffBits;
};
这给出sizeof（BITMAPFILEHEADER）== 16，现在是正确的。
BITMAPFILEHEADER
{Uint32 bfType;
Uint32 bfSize;
Uint32 bfReserved1; < brint> bintReserved2;
Uint32 bfOffBits;
};
这给了sizeof（BITMAPFILEHEADER）== 20，这也是正确的。
很可能我只是误解了某个地方，但我不知道是什么。
I''m pretty sure this is non-totally-trivial enough and standard-C++ enough
to post here, however much it may initially look like a "how do I make a
computer game?" question, though I may be wrong.
I''m loading a bitmap, and as I''m using Linux I''ve had to define
BITMAPFILEHEADER myself (maybe better practice would be to give my version
a new name or whatever, I dunno). Windows uses WORDs and DWORDs to
define it, I''ve used some standard-size types provided by SDL (a
graphics/sound library thing) to define it thus:
struct BITMAPFILEHEADER
{
Uint16 bfType;
Uint32 bfSize;
Uint16 bfReserved1;
Uint16 bfReserved2;
Uint32 bfOffBits;
};
For some reason I utterly can''t fathom, however, a debugger shows that my
bitmap loader function believes that sizeof(BITMAPFILEHEADER) == 16.
Surely that should be 14?
I then tried redefining it a couple of times:
BITMAPFILEHEADER
{
Uint32 bfType; //changed from 16 to 32
Uint32 bfSize;
Uint16 bfReserved1;
Uint16 bfReserved2;
Uint32 bfOffBits;
};
This gives sizeof(BITMAPFILEHEADER) == 16, which is now correct.
BITMAPFILEHEADER
{
Uint32 bfType;
Uint32 bfSize;
Uint32 bfReserved1;
Uint32 bfReserved2;
Uint32 bfOffBits;
};
This gives sizeof(BITMAPFILEHEADER) == 20, which is also correct.
Most likely I just misunderstand something somewhere, but I''ve no idea
what.
对象的大小可以包括所谓的填充。确保数组中的对象
从特定的字节边界开始。例如，
在某些平台上应该是4的倍数，以便更快地访问内存。
14不是4的倍数，因此它选择下一个4的较大倍数。
可能有[编译器特定的]方法来控制填充或
对齐因子，请查阅编译器文档以了解如何
那样做。
Victor
Sizes of object can include so called "padding" to ensure that the objects
in an array begin at a particular multiple of bytes boundary. For example,
on some platforms it should be multiple of 4 to make memory access faster.
14 is not multiple of 4, so it chooses the next larger multiple of 4.
There probably is a [compiler-specific] way to control the padding or the
alignment factor, consult your compiler documentation to find out how to
do that.
Victor
James Gregory写道：
James Gregory wrote:
我很确定这非常完全无足轻重，标准C ++足以在这里发布，不过它可能最初看起来像是如何制作
电脑游戏？问题，虽然我可能是错的。
放松 - 填充是关于主题的。
我正在加载位图，因为我正在使用Linux我必须定义
自己BITMAPFILEHEADER（也许更好的做法是给我的版本
一个新名称或其他什么，我不知道）。
必须有一个可以使用的图形库。从ImageMagick开始。
Windows使用WORD和DWORD来定义它，我使用了SDL提供的一些标准尺寸类型（
图形/声音库的东西）因此定义它：
struct BITMAPFILEHEADER
{Uint16 bfType;
Uint32 bfSize;
Uint16 bfReserved1;
Uint16 bfReserved2;
Uint32 bfOffBits;
};
由于某种原因，我完全无法理解，但是，调试器显示我的
位图加载器函数认为sizeof（BITMAPFILEHEADER） == 16.
当然应该是14？
I''m pretty sure this is non-totally-trivial enough and standard-C++ enough
to post here, however much it may initially look like a "how do I make a
computer game?" question, though I may be wrong.
Relax - padding is on-topic.
I''m loading a bitmap, and as I''m using Linux I''ve had to define
BITMAPFILEHEADER myself (maybe better practice would be to give my version
a new name or whatever, I dunno).
There must be a graphics library you can use. Start with ImageMagick.
Windows uses WORDs and DWORDs to
define it, I''ve used some standard-size types provided by SDL (a
graphics/sound library thing) to define it thus:
struct BITMAPFILEHEADER
{
Uint16 bfType;
Uint32 bfSize;
Uint16 bfReserved1;
Uint16 bfReserved2;
Uint32 bfOffBits;
};
For some reason I utterly can''t fathom, however, a debugger shows that my
bitmap loader function believes that sizeof(BITMAPFILEHEADER) == 16.
Surely that should be 14?
大量的硬件最适合读取四字对齐的整数
边界。换句话说，bfSize的地址可以平均除以4
，余数为0。
编译器在bfType之间插入一个不可见的16位填充元素和
bfSize。允许编译器执行此操作，并且需要通知他们如何执行这些操作。它是指定的实现。
现在询问涵盖编译器的文档或论坛如何将其转为
off。他们可能会指出一个已经读取BMP文件的库，
。
-
Phlip
http://www.xpsd.org/cgi-bin/wiki？ Tes ... UserInterfaces
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