本文探讨一种基于C++的结构体赋值内存的方法，通过该方法，在进行内存赋值的同时，对代码阅读较友好，可适用于一些实际工作环境，且能提高开发效率，降低维护成本。
问题提出
某工程某次迭代，需要添加一个功能，该功能无法做成一个单独的模块，要在不同模块函数中判断并处理。初次实现版本，在对应的函数最后添加一个参数，且设置默认值，这样既能达到目的又不影响原始逻辑。但涉及模块较多，代码审查时，被上峰否了。当初如此实现，一是工程代码是首次接触，工程代码量不少，且需在较少时间内了解熟悉并掌握如何改，从函数参数入手改是最快最简单的方法。
经思考后，发现原始函数有一个较大结构体参数，内有预留字段，缓冲区够大，但已经对某些地址进行赋值和获取地，地址偏移量已是固定了，比如第68字节开始的4个字节等。
方案描述
可以在缓冲区指定偏移量赋值并读取，比如固定第0字段为一标志字段，表示是否排序。但这样不安全，万一没有赋值，但被其它数据污染恰好为1，则影响代码逻辑。
故而考虑使用sort=1;foo=xxx;bar=yyy;这样的形式，出现了sort=，其后的1个字节才是所需的。这样通过多个字节加强数据准确性的判断。好处是不限长度，直观方便，但要解析对比字符串才能得到所需内容。
然而上述形式适用于字符串，假定要传递地址指针（在C++中，传递指针是常有的事），需将地址值转成字符串，读取时，再将字符串转成数值形式，较麻烦。
最终使用自定义结构体方式，使用字段对应偏移量，对于不使用的偏移地址，使用预留字段字段。但是也有缺点，即需手动设计结构体，一旦确定不能轻易改，且内容是定长的（可适当多预留一些空间）。具体见下。
工程代码
设计思路
这里结合代码描述思路，自定义的结构体StructInfo_tt，按需求列出字段和对应值的，设置tagXXX和valXXX。前者形如foo=，则其长度为4；后者假定是指针，考虑兼容32位和64位，设置长度为8。
通过StructInfo_t *myptr = (StructInfo_t *)buffer;将内存直接映射到结构体（C++就如此便捷），通过memcpy(myptr->tagFoo, "foo=", sizeof(myptr->tagFoo)); memcpy(myptr->valFoo, &mybuf, sizeof(myptr->valFoo));拷贝对应的字段。
读取亦类似。
实现代码
完整代码：
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define DEFWIDTH 16 /* Default # chars to show per line */
#define MAXWIDTH 32 /* Maximum # of bytes per line */
/**
输出示例：
地址：字符十六进制 可打印字符
new buffer print: 70
0x0012FAE4: 89 50 4e 47 0d 0a 1a 0a 00 00 00 0d 49 48 44 52 .PNG........IHDR
0x0012FAF4: 00 00 00 f0 00 00 01 40 08 02 00 00 00 0d 8a 66 [email protected]
0x0012FB04: 04 00 00 20 00 49 44 41 54 78 da ed 7d 3d 68 24 ... .IDATx..}=h$
0x0012FB14: 01 02 03 04 05 06 07 08 09 0a 0b ...........
*/
void dump(const char *buffer, int len)
{
int i, j, n;
int line = DEFWIDTH;
char c;
unsigned char* buf = (unsigned char *)buffer; // 必须是unsigned char类型
n = len / line;
if (len % line)
n++;
for (i=0; i<n; i++)
{
//printf("0x%08x: ", (unsigned int)(buf+i*line)); // linux ok
printf("0x%8p: ", buf+i*line); // windows ok
for (j=0; j<line; j++)
{
if ((i*line+j) < len)
printf("%02x ", buf[i*line+j]);
else
printf(" ");
}
printf(" ");
for (j=0; j<line && (i*line+j)<len; j++)
{
if ((i*line+j) < len)
{
c = buf[i*line+j];
printf("%c", c >= ' ' && c < '~' ? c : '.');
}
else
printf(" ");
}
printf("\n");
}
}
// 自定义结构体
typedef struct StructInfo_tt
{
char tagFoo[4]; // foo=
char valFoo[8]; // ptr
char tagBar[4]; // bar=
char valBar[4]; // size ptr
char tagName[8]; // LaTeLee=
char valName[6]; // 250
char reserve[216]; // left
} StructInfo_t;
char buffer[250] = {0};
void set_struct(char* mybuf, int bufsize)
{
// char* bufptr = mybuf;
// int bufsize = 250;
StructInfo_t *myptr = (StructInfo_t *)buffer;
memcpy(myptr->tagFoo, "foo=", sizeof(myptr->tagFoo));
memcpy(myptr->valFoo, &mybuf, sizeof(myptr->valFoo));
memcpy(myptr->tagBar, "bar=", sizeof(myptr->tagBar));
memcpy(myptr->valBar, &bufsize, sizeof(myptr->valBar));
char tmpName[8] = "520250";
memcpy(myptr->tagName, "LaTeLee=", sizeof(myptr->tagName));
memcpy(myptr->valName, tmpName, sizeof(myptr->valName));
printf("mybuf ptr: %p\n", mybuf);
dump((const char *)myptr, 64);
}
void get_struct()
{
char *bufptr = NULL;
int bufsize = 0;
StructInfo_t *myptr = (StructInfo_t*)buffer;
if(strncmp(myptr->tagFoo, "foo=", sizeof(myptr->tagFoo)) == 0)
{
// memcpy(&bufptr, myptr->valFoo, sizeof(myptr->valFoo));
memcpy(&bufptr, myptr->valFoo, sizeof(char*));
}
if(strncmp(myptr->tagBar, "bar=", sizeof(myptr->tagBar)) == 0)
{
memcpy(&bufsize, myptr->valBar, sizeof(myptr->valBar));
}
memcpy(bufptr, "this is hello from hell", bufsize);
printf("got ptr: %p size: %d\n", bufptr, bufsize);
dump((const char *)myptr, 64);
}
测试环境
32位：
# uname -a
Linux debian 3.2.0-4-686-pae #1 SMP Debian 3.2.65-1+deb7u1 i686 GNU/Linux
# g++ --version
g++ (Debian 4.7.2-5) 4.7.2
64位：
$ uname -a
Linux master 3.10.0-957.27.2.el7.x86_64 #1 SMP Mon Jul 29 17:46:05 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
$ g++ --version
g++ (GCC) 4.8.5 20150623 (Red Hat 4.8.5-39)
测试代码
缓冲够大：
int main(void)
{
printf("struct value test, sizeof(char*): %d sizeof(int): %d\n", sizeof(char*), sizeof(int));
char mybuf[64] = {0};
int bufsize = 64;
set_struct(mybuf, bufsize);
get_struct();
printf("out mybuf [%s]\n", mybuf);
dump((const char *)mybuf, 64);
return 0;
}
测试结果：
struct value test, sizeof(char*): 8 sizeof(int): 4
mybuf ptr: 0x7fffd9638240
0x0x601080: 66 6f 6f 3d 40 82 63 d9 ff 7f 00 00 62 61 72 3d [email protected]=
0x0x601090: 40 00 00 00 4c 61 54 65 4c 65 65 3d 35 32 30 32 @...LaTeLee=5202
0x0x6010a0: 35 30 00 00 00 00 00 00 00 00 00 00 00 00 00 00 50..............
0x0x6010b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
got ptr: 0x7fffd9638240 size: 64
0x0x601080: 66 6f 6f 3d 40 82 63 d9 ff 7f 00 00 62 61 72 3d [email protected]=
0x0x601090: 40 00 00 00 4c 61 54 65 4c 65 65 3d 35 32 30 32 @...LaTeLee=5202
0x0x6010a0: 35 30 00 00 00 00 00 00 00 00 00 00 00 00 00 00 50..............
0x0x6010b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
out mybuf [this is hello from hell]
0x0x7fffd9638240: 74 68 69 73 20 69 73 20 68 65 6c 6c 6f 20 66 72 this is hello fr
0x0x7fffd9638250: 6f 6d 20 68 65 6c 6c 00 67 6f 74 20 70 74 72 3a om hell.got ptr:
0x0x7fffd9638260: 20 25 70 20 73 69 7a 65 3a 20 25 64 0a 00 6f 75 %p size: %d..ou
0x0x7fffd9638270: 74 20 6d 79 62 75 66 20 5b 25 73 5d 0a 00 00 00 t mybuf [%s]....
观察：运行系统的指针长度为8字节，int类型为4字节。指针及大小被正确传递。
缓冲区较小：
int main(void)
{
printf("struct value test, sizeof(char*): %d sizeof(int): %d\n", sizeof(char*), sizeof(int));
char mybuf[16] = {0};
int bufsize = 16;
set_struct(mybuf, bufsize);
get_struct();
printf("out mybuf [%s]\n", mybuf);
dump((const char *)mybuf, 64);
return 0;
}
测试结果：
struct value test, sizeof(char*): 8 sizeof(int): 4
mybuf ptr: 0x7ffd3992b880
0x0x601080: 66 6f 6f 3d 80 b8 92 39 fd 7f 00 00 62 61 72 3d foo=...9....bar=
0x0x601090: 10 00 00 00 4c 61 54 65 4c 65 65 3d 35 32 30 32 ....LaTeLee=5202
0x0x6010a0: 35 30 00 00 00 00 00 00 00 00 00 00 00 00 00 00 50..............
0x0x6010b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
got ptr: 0x7ffd3992b880 size: 16
0x0x601080: 66 6f 6f 3d 80 b8 92 39 fd 7f 00 00 62 61 72 3d foo=...9....bar=
0x0x601090: 10 00 00 00 4c 61 54 65 4c 65 65 3d 35 32 30 32 ....LaTeLee=5202
0x0x6010a0: 35 30 00 00 00 00 00 00 00 00 00 00 00 00 00 00 50..............
0x0x6010b0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
out mybuf [this is hello fr
0x0x7ffd3992b880: 74 68 69 73 20 69 73 20 68 65 6c 6c 6f 20 66 72 this is hello fr
0x0x7ffd3992b890: 80 b9 92 39 fd 7f 00 00 00 00 00 00 10 00 00 00 ...9............
0x0x7ffd3992b8a0: 00 00 00 00 00 00 00 00 55 15 39 d7 40 7f 00 00 ........U.9.@...
0x0x7ffd3992b8b0: 00 00 00 00 00 00 00 00 88 b9 92 39 fd 7f 00 00 ...........9....
说明：
因为mybuf较少，底层按实际大小拷贝，字符串this is hello fr就是16字节，只是其后没有\0结尾，因此无法正确换行（从二进制打印也可看出）。
32位测试
缓存区足够：
struct value test, sizeof(char*): 4 sizeof(int): 4
mybuf ptr: 0xbfcac45c
0x0x8049e20: 66 6f 6f 3d 5c c4 ca bf 40 00 00 00 62 61 72 3d foo=\[email protected]=
0x0x8049e30: 40 00 00 00 4c 61 54 65 4c 65 65 3d 35 32 30 32 @...LaTeLee=5202
0x0x8049e40: 35 30 00 00 00 00 00 00 00 00 00 00 00 00 00 00 50..............
0x0x8049e50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
got ptr: 0xbfcac45c size: 64
0x0x8049e20: 66 6f 6f 3d 5c c4 ca bf 40 00 00 00 62 61 72 3d foo=\[email protected]=
0x0x8049e30: 40 00 00 00 4c 61 54 65 4c 65 65 3d 35 32 30 32 @...LaTeLee=5202
0x0x8049e40: 35 30 00 00 00 00 00 00 00 00 00 00 00 00 00 00 50..............
0x0x8049e50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
out mybuf [this is hello from hell]
0x0xbfcac45c: 74 68 69 73 20 69 73 20 68 65 6c 6c 6f 20 66 72 this is hello fr
0x0xbfcac46c: 6f 6d 20 68 65 6c 6c 00 67 6f 74 20 70 74 72 3a om hell.got ptr:
0x0xbfcac47c: 20 25 70 20 73 69 7a 65 3a 20 25 64 0a 00 00 00 %p size: %d....
0x0xbfcac48c: 73 74 72 75 63 74 20 76 61 6c 75 65 20 74 65 73 struct value tes
缓冲区较小：
# ./a.out
struct value test, sizeof(char*): 4 sizeof(int): 4
mybuf ptr: 0xbfb3f25c
0x0x8049e20: 66 6f 6f 3d 5c f2 b3 bf 10 00 00 00 62 61 72 3d foo=\.......bar=
0x0x8049e30: 10 00 00 00 4c 61 54 65 4c 65 65 3d 35 32 30 32 ....LaTeLee=5202
0x0x8049e40: 35 30 00 00 00 00 00 00 00 00 00 00 00 00 00 00 50..............
0x0x8049e50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
got ptr: 0xbfb3f25c size: 16
0x0x8049e20: 66 6f 6f 3d 5c f2 b3 bf 10 00 00 00 62 61 72 3d foo=\.......bar=
0x0x8049e30: 10 00 00 00 4c 61 54 65 4c 65 65 3d 35 32 30 32 ....LaTeLee=5202
0x0x8049e40: 35 30 00 00 00 00 00 00 00 00 00 00 00 00 00 00 50..............
0x0x8049e50: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ................
out mybuf [this is hello fr]
0x0xbfb3f25c: 74 68 69 73 20 69 73 20 68 65 6c 6c 6f 20 66 72 this is hello fr
0x0xbfb3f26c: 10 00 00 00 30 8a 04 08 00 00 00 00 f8 f2 b3 bf ....0...........
0x0xbfb3f27c: 46 9e 53 b7 01 00 00 00 24 f3 b3 bf 2c f3 b3 bf F.S.....$...,...
0x0xbfb3f28c: c8 42 6a b7 21 38 7e b7 ff ff ff ff f4 bf 7e b7 .Bj.!8..........
说明：
get_struct函数中的语句memcpy(&bufptr, myptr->valFoo, sizeof(myptr->valFoo));只能在64位系统正常运行，因为StructInfo_t定义字段大小时，是按最大原则的，即存储指针使用8字节。而在32位系统中，bufptr只有4字节，因此必须使用memcpy(&bufptr, myptr->valFoo, sizeof(char*));方可适用于不同位数系统。
小结
经测试，上文所述方案可解决所遇问题。但效果如何，待后续观察。