How to "#define" struct field or struct name

I want to do something like:

    int  desc = -1;
    if ( DB_DBM_HSEARCH == 1 )  {    desc = db->fd  }   else desc = db开发者_开发问答->dbm_pagf;

This is impossible because of a compiler error.

Is it possible to do something like:

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

?

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Sure. Though I think this would be a more suitable way to do it using macro functions (so you can change your variable names if you need to). Assuming DB_DBM_HSEARCH is a macro:

/* 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);

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You can, but don't use macros for this sort of thing, just use nicely named variables.