打印内存地址

Question

I ran the following program on Cygwin on Windows8.

#include<iostream>
#include <stdio.h>
using namespace std;
int main(){
char c1 = 'a';
char c2 = 'b';
int i1 = 1;
float l1 = 100;
float f1 = 3.14;
double d1 = 1.424;
int i2;
char c3;
int i3;
printf("&c1 -> %u\n", (unsigned long)&c1);
printf("&c2 -> %u\n", (unsigned long)&c2);
printf("&i1 -> %u\n", (unsigned long)&i1);
printf("&l1 -> %u\n", (unsigned long)&l1);
printf("&f1 -> %u\n", (unsigned long)&f1);
printf("&d1 -> %u\n", (unsigned long)&d1);
printf("&i2 -> %u\n", (unsigned long)&i2);
printf("&c3 -> %u\n", (unsigned long)&c3);
printf("&i3 -> %u\n", (unsigned long)&i3);
}

My laptop gave the results below.

$ ./a.exe
&c1 -> 2337487
&c2 -> 2337486
&i1 -> 2337480
&l1 -> 2337476
&f1 -> 2337472
&d1 -> 2337464
&i2 -> 2337460
&c3 -> 2337459
&i3 -> 2337452

I understand that each data type has its own size and takes up memory addresses according to the size. For example, in this case, since char type has only 1 byte size, the variable c1 takes up 1 memory address(2337487), and the next variable c2 starts from the address 2337486. However, I was confused by that i1 starts from 2337480. If c2 is also a char type variable, shouldn't it take up only one address? And i1 start from 2337485?

I guess it's something to do with a compiler, but don't understand how it works.Would anyone give me some advice?

Answer 1

This is because of data structure alignment.

For example, when the computer's word size is 4 bytes (a byte means 8 bits on most machines, but could be different on some systems), the data to be read should be at a memory offset which is some multiple of 4. When this is not the case, eg the data starts at the 14th byte instead of the 16th byte, then the computer has to read two 4 byte chunks and do some calculation before the requested data has been read, or it may generate an alignment fault. Even though the previous data structure ends at the 13th byte, the next data structure should start at the 16th byte. Two padding bytes are inserted between the two data structures to align the next data structure to the 16th byte.

Answer 2

这是编译器的最佳化：有时，如果分配的内存不是固定块，则更快。