关于C语言中一些类型变量在计算机存储的学习

我们都知道char型变量占一个字节，int型占用4个字节，long占用8个字节，double型变量占8个字节，那么这些变量和指针在计算机中存储在哪个地址单元呢？

#include <stdio.h>
/* $end show-bytes */
#include <stdlib.h>
#include <string.h>
/* $begin show-bytes */

typedef unsigned char *byte_pointer;
void show_bytes(byte_pointer start, size_t len) {
size_t i;
for (i = 0; i < len; i++)
printf("%p\t0x%.2x\n", &start[i], start[i]);
printf("\n");
}
void show_int(int x) {
show_bytes((byte_pointer) &x, sizeof(int));
}

void show_float(float x) {
show_bytes((byte_pointer) &x, sizeof(float));
}

void show_pointer(void *x) {
show_bytes((byte_pointer) &x, sizeof(void *));
}
void test_show_bytes(int val) {
int ival = val;
double fval = (double) ival;
int *pval = &ival;
printf("Stack variable ival = %d\n", ival);
printf("(int)ival:\n");
show_int(ival);
printf("(float)ival:\n");
show_float(fval);
printf("&ival:\n");
show_pointer(pval);
}
void simple_show_a() {
/* $begin simple-show-a */
int val = 0x87654321;
byte_pointer valp = (byte_pointer) &val;
show_bytes(valp, 1); /* A. */
show_bytes(valp, 2); /* B. */
show_bytes(valp, 3); /* C. */
/* $end simple-show-a */
}

void simple_show_b() {
/* $begin simple-show-b */
int val = 0x12345678;
byte_pointer valp = (byte_pointer) &val;
show_bytes(valp, 1); /* A. */
show_bytes(valp, 2); /* B. */
show_bytes(valp, 3); /* C. */
/* $end simple-show-b */
}

void float_eg() {
int x = 3490593;
float f = (float) x;
printf("For x = %d\n", x);
show_int(x);
show_float(f);

x = 3510593;
f = (float) x;
printf("For x = %d\n", x);
show_int(x);
show_float(f);
}
void string_ueg() {
/* $begin show-ustring */
const char *s = "ABCDEF";
show_bytes((byte_pointer) s, strlen(s));
/* $end show-ustring */
}
void string_leg() {
/* $begin show-lstring */
const char *s = "abcdef";
show_bytes((byte_pointer) s, strlen(s));
/* $end show-lstring */
}
void show_twocomp()
{
/* $begin show-twocomp */
short x = 12345;
short mx = -x;
show_bytes((byte_pointer) &x, sizeof(short));
show_bytes((byte_pointer) &mx, sizeof(short));
/* $end show-twocomp */
}

int main(int argc, char *argv[])
{
int val = 12345;
if (argc > 1) {
val = strtol(argv[1], NULL, 0);
printf("calling test_show_bytes\n");
test_show_bytes(val);
} else {
printf("calling show_twocomp\n");
show_twocomp();
printf("Calling simple_show_a\n");
simple_show_a();
printf("Calling simple_show_b\n");
simple_show_b();
printf("Calling float_eg\n");
float_eg();
printf("Calling string_ueg\n");
string_ueg();
printf("Calling string_leg\n");
string_leg();
}
return 0;
}

结果如下：

hxl@hxl-virtual-machine:~/桌面/task/code$ ./sh
calling show_twocomp
0x7ffd77301fc4	0x39
0x7ffd77301fc5	0x30

0x7ffd77301fc6	0xc7
0x7ffd77301fc7	0xcf

Calling simple_show_a
0x7ffd77301fbc	0x21

0x7ffd77301fbc	0x21
0x7ffd77301fbd	0x43

0x7ffd77301fbc	0x21
0x7ffd77301fbd	0x43
0x7ffd77301fbe	0x65

Calling simple_show_b
0x7ffd77301fbc	0x78

0x7ffd77301fbc	0x78
0x7ffd77301fbd	0x56

0x7ffd77301fbc	0x78
0x7ffd77301fbd	0x56
0x7ffd77301fbe	0x34

Calling float_eg
For x = 3490593
0x7ffd77301f9c	0x21
0x7ffd77301f9d	0x43
0x7ffd77301f9e	0x35
0x7ffd77301f9f	0x00

0x7ffd77301f9c	0x84
0x7ffd77301f9d	0x0c
0x7ffd77301f9e	0x55
0x7ffd77301f9f	0x4a

For x = 3510593
0x7ffd77301f9c	0x41
0x7ffd77301f9d	0x91
0x7ffd77301f9e	0x35
0x7ffd77301f9f	0x00

0x7ffd77301f9c	0x04
0x7ffd77301f9d	0x45
0x7ffd77301f9e	0x56
0x7ffd77301f9f	0x4a

Calling string_ueg
0x55dc9b616d34	0x41
0x55dc9b616d35	0x42
0x55dc9b616d36	0x43
0x55dc9b616d37	0x44
0x55dc9b616d38	0x45
0x55dc9b616d39	0x46

Calling string_leg
0x55dc9b616d3b	0x61
0x55dc9b616d3c	0x62
0x55dc9b616d3d	0x63
0x55dc9b616d3e	0x64
0x55dc9b616d3f	0x65
0x55dc9b616d40	0x66

hxl@hxl-virtual-machine:~/桌面/task/code$ ./sh 1
calling test_show_bytes
Stack variable ival = 1
(int)ival:
0x7fff1c97f40c	0x01
0x7fff1c97f40d	0x00
0x7fff1c97f40e	0x00
0x7fff1c97f40f	0x00

(float)ival:
0x7fff1c97f40c	0x00
0x7fff1c97f40d	0x00
0x7fff1c97f40e	0x80
0x7fff1c97f40f	0x3f

&ival:
0x7fff1c97f408	0x34
0x7fff1c97f409	0xf4
0x7fff1c97f40a	0x97
0x7fff1c97f40b	0x1c
0x7fff1c97f40c	0xff
0x7fff1c97f40d	0x7f
0x7fff1c97f40e	0x00
0x7fff1c97f40f	0x00

解释一下代码运行结果。不带参数运行时，调用show_twocomp()函数，这个函数再调用 show_bytes()函数，因为定义的是short型变量x=12345；转化成16进制是0x3039，则在计算机中就是0x30放在高地址区域，0x39放在低地址区域，这就是常说的小端模式，数据高有效部分放在高地址区域。同样的-12345存储时也是如此，需要注意的就是负数在计算机中存储是用补码表示，转化成16进制就是0xcfc7，后面代码也是如此，需要注意的就是，64位机指针是8个字节的，而且输出时如果高地址对应的数据高有效区位数不足，则需要用0x00来存。
再要说的就是浮点数在计算机中的存储，浮点数在64位机上一位表示符号，11位表示阶码，52位表示尾数，对于规格化的浮点数，11位阶码是由原数转换成2进制后通过移动小数点，来获得1.xxxxxxx *2的n次方或者0.xxxxxxx *2的n次方，此时阶码等于n+偏移量，偏移量=2的阶码-1次方-1，尾数就写小数点后面的，最后写成16进制形式，按照小端模式存储。其余运行结果都可按照类似得出。