计算机系统基础日志No.5

计算机系统基础日志No.4
show bytes-打印数据的字节表示形式

一，原程序及编译运行

1.源程序

/* show-bytes - prints byte representation of data */
/* $begin show-bytes */
#include <stdio.h>
/* $end show-bytes */
#include <stdlib.h>
#include <string.h>
/* $begin show-bytes */

typedef unsigned char *byte_pointer;
//typedef char *byte_pointer;
//typedef int *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 *));
}
/* $end show-bytes */


/* $begin test-show-bytes */
void test_show_bytes(int val) {
    int ival = val;
    //float fval = (float) ival;
	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);
}
/* $end test-show-bytes */

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;
}

2.编译运行

calling show_twocomp
0x7fffc3b39ee4	0x39
0x7fffc3b39ee5	0x30

0x7fffc3b39ee6	0xc7
0x7fffc3b39ee7	0xcf

Calling simple_show_a
0x7fffc3b39edc	0x21

0x7fffc3b39edc	0x21
0x7fffc3b39edd	0x43

0x7fffc3b39edc	0x21
0x7fffc3b39edd	0x43
0x7fffc3b39ede	0x65

Calling simple_show_b
0x7fffc3b39edc	0x78

0x7fffc3b39edc	0x78
0x7fffc3b39edd	0x56

0x7fffc3b39edc	0x78
0x7fffc3b39edd	0x56
0x7fffc3b39ede	0x34

Calling float_eg
For x = 3490593
0x7fffc3b39ebc	0x21
0x7fffc3b39ebd	0x43
0x7fffc3b39ebe	0x35
0x7fffc3b39ebf	0x00

0x7fffc3b39ebc	0x84
0x7fffc3b39ebd	0x0c
0x7fffc3b39ebe	0x55
0x7fffc3b39ebf	0x4a

For x = 3510593
0x7fffc3b39ebc	0x41
0x7fffc3b39ebd	0x91
0x7fffc3b39ebe	0x35
0x7fffc3b39ebf	0x00

0x7fffc3b39ebc	0x04
0x7fffc3b39ebd	0x45
0x7fffc3b39ebe	0x56
0x7fffc3b39ebf	0x4a

Calling string_ueg
0x5557af22ad34	0x41
0x5557af22ad35	0x42
0x5557af22ad36	0x43
0x5557af22ad37	0x44
0x5557af22ad38	0x45
0x5557af22ad39	0x46

Calling string_leg
0x5557af22ad3b	0x61
0x5557af22ad3c	0x62
0x5557af22ad3d	0x63
0x5557af22ad3e	0x64
0x5557af22ad3f	0x65
0x5557af22ad40	0x66

二，程序拓展

1.把byte_pointer改为int *

1>程序结果

calling show_twocomp
0x7ffe0e250ef4	0xcfc73039
0x7ffe0e250ef8	0x1549db00

0x7ffe0e250ef6	0xdb00cfc7
0x7ffe0e250efa	0xda411549

Calling simple_show_a
0x7ffe0e250eec	0x87654321

0x7ffe0e250eec	0x87654321
0x7ffe0e250ef0	0xe250eec

0x7ffe0e250eec	0x87654321
0x7ffe0e250ef0	0xe250eec
0x7ffe0e250ef4	0x7ffe

Calling simple_show_b
0x7ffe0e250eec	0x12345678

0x7ffe0e250eec	0x12345678
0x7ffe0e250ef0	0xe250eec

0x7ffe0e250eec	0x12345678
0x7ffe0e250ef0	0xe250eec
0x7ffe0e250ef4	0x7ffe

Calling float_eg
For x = 3490593
0x7ffe0e250ecc	0x354321
0x7ffe0e250ed0	0xe250f00
0x7ffe0e250ed4	0x7ffe
0x7ffe0e250ed8	0xbbe37a6c

0x7ffe0e250ecc	0x4a550c84
0x7ffe0e250ed0	0xe250f00
0x7ffe0e250ed4	0x7ffe
0x7ffe0e250ed8	0xbbe37a7c

For x = 3510593
0x7ffe0e250ecc	0x359141
0x7ffe0e250ed0	0xe250f00
0x7ffe0e250ed4	0x7ffe
0x7ffe0e250ed8	0xbbe37aad

0x7ffe0e250ecc	0x4a564504
0x7ffe0e250ed0	0xe250f00
0x7ffe0e250ed4	0x7ffe
0x7ffe0e250ed8	0xbbe37abd

Calling string_ueg
0x5602bbe37d44	0x44434241
0x5602bbe37d48	0x61004645
0x5602bbe37d4c	0x65646362
0x5602bbe37d50	0x61630066
0x5602bbe37d54	0x6e696c6c
0x5602bbe37d58	0x65742067

Calling string_leg
0x5602bbe37d4b	0x64636261
0x5602bbe37d4f	0x63006665
0x5602bbe37d53	0x696c6c61
0x5602bbe37d57	0x7420676e
0x5602bbe37d5b	0x5f747365
0x5602bbe37d5f	0x776f6873

2>int*结果分析

为什么会是这个呢？
通过debug，发现 printf("%.2x", start[i]); start[i]被解释为了int。所以，程序每次读取4个字节(int 的大小)，并以16进制输出。
可以通过内存值来证明
start[0] 39 30 00 00 -> 3039
start[1] 5a 10 3a 01 -> 013a 105a
start[2] 5a 10 3a 01 -> 013a 105a
start[3] 00 90 f0 00 -> 00f0 0900
这也和输出是符合的。

2.把byte_pointer改为char *

1>程序结果

calling show_twocomp
0x7ffdc1d86654	0x39
0x7ffdc1d86655	0x30

0x7ffdc1d86656	0xffffffc7
0x7ffdc1d86657	0xffffffcf

Calling simple_show_a
0x7ffdc1d8664c	0x21

0x7ffdc1d8664c	0x21
0x7ffdc1d8664d	0x43

0x7ffdc1d8664c	0x21
0x7ffdc1d8664d	0x43
0x7ffdc1d8664e	0x65

Calling simple_show_b
0x7ffdc1d8664c	0x78

0x7ffdc1d8664c	0x78
0x7ffdc1d8664d	0x56

0x7ffdc1d8664c	0x78
0x7ffdc1d8664d	0x56
0x7ffdc1d8664e	0x34

Calling float_eg
For x = 3490593
0x7ffdc1d8662c	0x21
0x7ffdc1d8662d	0x43
0x7ffdc1d8662e	0x35
0x7ffdc1d8662f	0x00

0x7ffdc1d8662c	0xffffff84
0x7ffdc1d8662d	0x0c
0x7ffdc1d8662e	0x55
0x7ffdc1d8662f	0x4a

For x = 3510593
0x7ffdc1d8662c	0x41
0x7ffdc1d8662d	0xffffff91
0x7ffdc1d8662e	0x35
0x7ffdc1d8662f	0x00

0x7ffdc1d8662c	0x04
0x7ffdc1d8662d	0x45
0x7ffdc1d8662e	0x56
0x7ffdc1d8662f	0x4a

Calling string_ueg
0x55bb9bfdbd34	0x41
0x55bb9bfdbd35	0x42
0x55bb9bfdbd36	0x43
0x55bb9bfdbd37	0x44
0x55bb9bfdbd38	0x45
0x55bb9bfdbd39	0x46

Calling string_leg
0x55bb9bfdbd3b	0x61
0x55bb9bfdbd3c	0x62
0x55bb9bfdbd3d	0x63
0x55bb9bfdbd3e	0x64
0x55bb9bfdbd3f	0x65
0x55bb9bfdbd40	0x66

2>char*结果分析

再次debug此时start[i]被解释为了char
test =12345的16进制为00003039。
test=54321的16进制为0000d431
而问题就是出现在这。
在打印test的情况下，
start[0] 39 <128
start[1] 30 <128
start[2] 00 <128
start[3] 00 <128
都是在char的正常范围。但是在打印test=54321的情况下
start[0] 31 <128
start[1] d4 >128
start[2] 00 <128
start[3] 00 <127
在打印start[1]时，值超过了char的正常范围。本来也是没什么问题的，但是此时打印格式是16进制。所以程序按照补码扩充。
d4 -> 1101 0111
-> 1111 1111 1111 1111 1111 1111 1101 0111->F F F F F F D 4
所以出了错误