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. Since Unions are a special case of structures, we open a structure window (menu View|Structures), press Ins to create an union. Do not check “Create Union”
We create the union members using the regular data definition commands. (press D repeatedly to define a field, N to rename it) we obtain :
urecord_t union ; (sizeof=0x4) chr db ? shrt dw ? lng dd ? urecord_t ends; ----------------------------------- record_t struc ; (sizeof=0x8) type dd ? u urecord_t ? record_t ends
Switching to the disassembly window (or closing the enumeration window with Alt-F3), 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 !