title: In C program, how to dynamically select data type
body: <p>I have encountered a problem, and I haven't found an answer in the internet and forums. I hope you can help.</p>
  <p>There is an interface requirement to update the LOG parameter. The interface passes in the parameter index and the value to be added, and the interface adds the value to the corresponding parameter. The interface requires simple implementation and no complicated judgments.</p>
  <p>My idea is to create a mapping table that records the starting address and data type of each parameter. When the interface is called, the parameter address is obtained according to the parameter index and forced to be converted to the corresponding type, and then the addition operation is performed.</p>
  <p>The problem with this solution is that the increase_log_info function is too complicated. How to simplify the increase_log_info function in C language? How parameter types can be mapped directly, rather than through an if...else condition.</p>
  <p>Thanks in advance.</p>
  <p>Note:</p>
  <ol>
  <li>T_LOG_INFO is data structure definition and cannot be changed.</li>
  <li>LOG_INFO_INCREASE is an update parameter interface provided
  externally and cannot be changed.</li>
  <li>Other codes can be changed.</li>
  </ol>
  <pre><code>#pragma once
  
  /********************************can not be change, begin**********************************/
  /* T_LOG_INFO is data structure definition and cannot be changed. */
  typedef struct
  {
      unsigned short           tx_num;
      unsigned int             tx_bytes;
      unsigned short           rx_num;
      unsigned int             rx_bytes;
      unsigned char            discard_num;
      unsigned int             discard_bytes;
      // There are many parameters behind, not listed
  }T_LOG_INFO;
  
  T_LOG_INFO g_log_info;
  
  /* This macro is called very frequently, and efficiency needs to be considered.
  ** LOG_INFO_INCREASE is an update parameter interface provided externally and cannot be changed. */
  //#define LOG_INFO_INCREASE(para_idx, inc_val)
  /********************************can not be change, end**********************************/
  
  /********************************an alternative, begin**********************************/
  enum
  {
      LOG_PARA_IDX_TX_NUM,
      LOG_PARA_IDX_TX_BYTES,
      LOG_PARA_IDX_RX_NUM,
      LOG_PARA_IDX_RX_BYTES,
      LOG_PARA_IDX_DISCARD_NUM,
      LOG_PARA_IDX_DISCARD_BYTES,
      LOG_PARA_IDX_MAX
  };
  
  enum
  {
      DATA_TYPE_U8,
      DATA_TYPE_U16,
      DATA_TYPE_U32
  };
  
  typedef struct
  {
      /* Indicates the offset of this parameter in the structure. */
      unsigned char            offset;
      /* Indicates the data type of the parameter. */
      unsigned char            data_type;
  }T_PARA_MAPPING;
  
  /* This table can also be calculated during system initialization. */
  T_PARA_MAPPING g_para_mapping_table[LOG_PARA_IDX_MAX] =
  {
      {0, DATA_TYPE_U16},  // LOG_PARA_IDX_TX_NUM
      {4, DATA_TYPE_U32},  // LOG_PARA_IDX_TX_BYTES
      {8, DATA_TYPE_U16},  // LOG_PARA_IDX_RX_NUM
      {12, DATA_TYPE_U32}, // LOG_PARA_IDX_RX_BYTES
      {16, DATA_TYPE_U8},  // LOG_PARA_IDX_DISCARD_NUM
      {20, DATA_TYPE_U32}  // LOG_PARA_IDX_DISCARD_BYTES
  };
  
  /* How to simplify the function??? especially to remove the judgment. */
  static inline void increase_log_info(unsigned int para_idx, unsigned inc_val)
  {
      unsigned int data_type = g_para_mapping_table[para_idx].data_type;
  
      /* Get the parameter address and cast it to the corresponding type pointer before adding. */
      if (data_type == DATA_TYPE_U8)
      {
          *((unsigned char*)(((unsigned char*)&amp;g_log_info) + g_para_mapping_table[para_idx].offset)) += inc_val;
      }
      else if (data_type == DATA_TYPE_U16)
      {
          *((unsigned short*)(((unsigned char*)&amp;g_log_info) + g_para_mapping_table[para_idx].offset)) += inc_val;
      }
      else
      {
          *((unsigned int*)(((unsigned char*)&amp;g_log_info) + g_para_mapping_table[para_idx].offset)) += inc_val;
      }
  }
  
  /* This macro is called very frequently, and efficiency needs to be considered. */
  #define LOG_INFO_INCREASE(para_idx, inc_val) increase_log_info(para_idx, inc_val)
  /********************************an alternative, end**********************************/
  
  /********************************test case, begin**********************************/
  void increase_log_info_test()
  {
      LOG_INFO_INCREASE(LOG_PARA_IDX_TX_NUM, 1);
      LOG_INFO_INCREASE(LOG_PARA_IDX_TX_NUM, 2);
      LOG_INFO_INCREASE(LOG_PARA_IDX_TX_NUM, 3);
  
      LOG_INFO_INCREASE(LOG_PARA_IDX_RX_BYTES, 10);
      LOG_INFO_INCREASE(LOG_PARA_IDX_RX_BYTES, 20);
      LOG_INFO_INCREASE(LOG_PARA_IDX_RX_BYTES, 30);
  }
  /********************************test case, end**********************************/
  </code></pre>