📄 align.html
字号:
<html><!-- Mirrored from c-faq.com/struct/align.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 08:02:57 GMT --><head><title>alignment and padding</title></head><body><p>[Someone asked me why it was that structures need internal paddding.This was my reply.]<p>From: scs@eskimo.com (Steve Summit)<br>Date: Sun, 17 Mar 2002 20:56:35 -0500<br>Message-Id: <2002Mar17.2056.scs.007@aeroroot.scs.ndip.eskimo.net><p>You wrote:<br>> ...what is still haunting my brain is why structures<br>> need internal padding<p>It's for ``alignment''. Many processors can't access 2- and4-byte quantities (e.g. ints and long ints) if they're crammedin every-which-way.<p>Suppose you have this structure:<p><pre> struct { char a[3]; short int b; long int c; char d[3]; };</pre><p>Now, you might think that it ought to be possible to pack thisstructure into memory like this:<p><pre> +-------+-------+-------+-------+ | a | b | +-------+-------+-------+-------+ | b | c | +-------+-------+-------+-------+ | c | d | +-------+-------+-------+-------+</pre><p>But it's much, much easier on the processor if the compilerarranges it like this:<p><pre> +-------+-------+-------+ | a | +-------+-------+-------+ | b | +-------+-------+-------+-------+ | c | +-------+-------+-------+-------+ | d | +-------+-------+-------+</pre><p>In the ``packed'' version, notice how it's at least a little bithard for you and me to see how the b and c fields wrap around?In a nutshell, it's hard for the processor, too.Therefore, most compilers will ``pad'' the structure(as if with extra, invisible fields)like this:<p><pre> +-------+-------+-------+-------+ | a | pad1 | +-------+-------+-------+-------+ | b | pad2 | +-------+-------+-------+-------+ | c | +-------+-------+-------+-------+ | d | pad3 | +-------+-------+-------+-------+</pre></body><!-- Mirrored from c-faq.com/struct/align.html by HTTrack Website Copier/3.x [XR&CO'2008], Sat, 14 Mar 2009 08:02:57 GMT --></html>⌨️ 快捷键说明
复制代码
Ctrl + C
搜索代码
Ctrl + F
全屏模式
F11
切换主题
Ctrl + Shift + D
显示快捷键
?
增大字号
Ctrl + =
减小字号
Ctrl + -