Union tutorial

Suppose the source text looked like this:

#include <stdlib.h>

union urecord_t
{
  char c;
  short s;
  long l;
};

struct record_t
{
  int type;
#define RTYPE_CHAR      0
#define RTYPE_SHORT     1
#define RTYPE_LONG      2
  urecord_t u;
};

bool is_negative(record_t *r)
{
  switch ( r->type )
  {
    case RTYPE_CHAR:  return r->u.c < 0;
    case RTYPE_SHORT: return r->u.s < 0;
    case RTYPE_LONG:  return r->u.l < 0;
  }
  abort();
}

We have a disassembly like this:

Let’s improve it with unions. First, let’s define an union type. For this we open the Local types view (menu View|Local types), press Ins to create an union.

We create the union using the "C syntax" tab of the "Add type" dialog:

Switching to the disassembly window, we apply the defined structure through the Edit|Operand types|Struct offset menu item and select the proper representation for the operand. In the union type case, it may be necessary to select the desired union member with the Edit|Structs|Select union member command. The final disassembly looks like this:

That’s all folks !