pointers - In C: How to point to different members with offset at run time? -

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i want use single pointer pt , offset size_t offp update value of either mynfo.valuea or mynfo.valueb depends on run time conditions.

i can set pt &mynfo, not know proper type of pt , how should calculate offp.

edited:

  • i forget mention structures used allocated @ runtime.

  • i using khash, need distinguish whether key exists. if use pt = &pnfo->valuea;, have write similar codes twice. want determine member first , turn hash.

typedef struct {     uint16_t valuea;     uint16_t valueb;     const char * filename; } __attribute__ ((__packed__)) info_t;  info_t mynfo, *pnfo;  pnfo = calloc(1,sizeof(info_t));  size_t offp = &(mynfo.valueb) - &(mynfo.valuea);  if (ret==1) {     pt = pnfo; } else {     pt = pnfo + offp; }  *pt = 100;

use offsetof()

size_t offp; if (some condition) {     offp = offsetof(info_t, valuea); } else {     offp = offsetof(info_t, valueb); } *(uint16_t*)((char*)pnfo + offp) = 100;

you need pointer casting because offsetof() returns offset in bytes. first have cast struct pointer char* add bytes it, cast pointer element type assign it.


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