c - 良好形式 : Pointer vs. 局部变量与数组索引

Question

如果这是一个微不足道的问题，请原谅我 - 我通常是控制系统专家(plc 和自动化)，但最近发现自己参与了一些嵌入式微 Controller 和 PC 项目。

假设我有一个函数接受指向“命令字节”数组的指针，通常长度为 5 或 10 个字节，如下所示:

char cmd_i2c_read(unsigned char *cmd, unsigned short cmd_len) { ... }

我想解码命令字节 (*cmd)。

更好的形式是:

1. 创建表明每个字节用途的局部变量:

   unsigned char device_address = cmd[2];
   unsigned char register_address = cmd[3];
   unsigned char num_bytes = cmd[4];<br/>
   // use the local variables:
   if(num_bytes &le 0xFF) {
       do_stuff(device_address, register_address, num_bytes);
   }

2. 创建本地指针:

   unsigned char *device_address = &cmd[2];
   unsigned char *register_address = &cmd[3];
   unsigned char *num_bytes = &cmd[4];<br/>
   // use the pointers:
   if(*num_bytes &le 0xFF) {
       do_stuff(*device_address, *register_address, *num_bytes);
   }

3. 直接索引*cmd数组:
   if(cmd[4] <= 0xFF) { do_stuff(cmd[2], cmd[3], cmd[4]); }

Answer 1

选项 1 很清楚，但有点冗长。我一点都不喜欢2，3很难理解。就我个人而言，我更喜欢将结构用于此类事情。

typedef struct  {
   unsigned char whatever[2];
   unsigned char device_address;
   unsigned char register_address;
   unsigned char num_bytes;
   }  CMD;

CMD * pcmd = (CMD *)&cmd[0];

// use the local variables:
if(num_bytes ≤ 0xFF) {
    do_stuff(pcmd->device_address, pcmd->register_address, pcmd->num_bytes);