如何“ #define”结构域或结构名

    int  desc = -1;
    if ( DB_DBM_HSEARCH == 1 )  {    desc = db->fd  }   else desc = db->dbm_pagf;

   #define DESC db->fd     //and then
    int desc = DESC;

DB_DBM_HSEARCH

/* conditionally define the macros */
#if DB_DBM_HSEARCH == 1
#  define DESC(db) (db)->fd
#else
#  define DESC(db) (db)->dbm_pagf
#endif

/* then to initialize */
int desc = DESC(db);