问题描述
我写了一个小程序,以检查在我的内存中占用多少字节,并且它显示char实际上占用内存中的4个字节。我理解这主要是因为字对齐,并没有看到char只有1个字节的优势。为什么不使用4字节的char? int main(void)
{
int a;
char b;
int c;
a = 0;
b ='b';
c = 1;
printf(%p\\\
,& a);
printf(%p\\\
,& b);
printf(%p\\\
,& c);
return 0;
}
输出:
0x7fff91a15c58
0x7fff91a15c5f
0x7fff91a15c54
更新:
我不相信malloc只会为char分配1个字节,即使sizeof(char)作为参数传递,因为malloc包含一个标题将确保标题是字对齐的。任何意见?
Update2:
如果被要求有效地使用内存而不填充,唯一的办法是创建一个特殊的内存分配器?或者是否可以禁用填充?
你有int,char,int
请参见为什么限制字节对齐?
字节0字节1字节2字节3
0x1000
0x1004 X0 X1 X2 X3
0x1008
0x100C Y0 Y1 Y2
如果将它们存储在4字节,字节和4字节的形式,它将采取2个cpu周期来检索 int c
和一些位移,以获得c的实际值正确对齐,用作int 。
I wrote a small program to check how many bytes char occupies in my memory and it shows char actually occupies 4 bytes in memory. I understand it's mostly because of word alignment and don't see advantage of a char being only 1 byte. Why not use 4 bytes for char?
int main(void)
{
int a;
char b;
int c;
a = 0;
b = 'b';
c = 1;
printf("%p\n",&a);
printf("%p\n",&b);
printf("%p\n",&c);
return 0;
}
Output: 0x7fff91a15c58 0x7fff91a15c5f 0x7fff91a15c54
Update:I don't believe that malloc will allocate only 1 byte for char, even though sizeof(char) is passed as argument because, malloc contains a header will makes sure that header is word aligned. Any comments?
Update2:If you are asked to effectively use memory without padding, is the only way is to create a special memory allocator? or is it possible to disable padding?
You have int, char, int
See the image here under "Why Restrict Byte Alignment?"http://www.eventhelix.com/realtimemantra/ByteAlignmentAndOrdering.htm
Byte 0 Byte 1 Byte 2 Byte 3
0x1000
0x1004 X0 X1 X2 X3
0x1008
0x100C Y0 Y1 Y2
If it had stored them in 4-byte, 1-byte and 4-byte form, it would have taken 2 cpu cycles to retrieve int c
and some bit-shifting to get the actual value of c aligned properly for use as an int.
这篇关于C问题:为什么char实际上占用内存中的4个字节?的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持!