良好形式:指针与局部变量与阵列索引

Question

请原谅我,如果这是一个微不足道的问题 - 我通常是一个控制系统人(plc和自动化),但最近我发现自己参与了一些嵌入式微控制器和PC项目.

假设我有一个接受指向"命令字节"数组的指针的函数,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).

这是更好的形式:

1. 创建指示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];
   
   // 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];
   
   // 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，但有点罗嗦。我一点也不喜欢2，而且3也很难理解。就我个人而言，我更喜欢使用结构来完成这类事情。

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typedef struct  {\n   unsigned char whatever[2];\n   unsigned char device_address;\n   unsigned char register_address;\n   unsigned char num_bytes;\n   }  CMD;\n\nCMD * pcmd = (CMD *)&cmd[0];\n\n// use the local variables:\nif(num_bytes \xe2\x89\xa4 0xFF) {\n    do_stuff(pcmd->device_address, pcmd->register_address, pcmd->num_bytes);\n

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