User manual

216
mikoC PRO for PIC32
MikroElektronika
Unions
Union types are derived types sharing many of syntactic and functional features of structure types. The key difference
is that a union members share the same memory space.
Note: The mikroC PRO for PIC supports anonymous unions.
Union Declaration
Unions have the same declaration as structures, with the keyword union used instead of struct:
union tag { member-declarator-list };
Unlike structures’ members, the value of only one of union’s members can be stored at any time. Here is a simple
example:
union myunion { // union tag is ‘myunion’
int i;
double d;
char ch;
} mu, *pm;
The identier mu, of the type myunion, can be used to hold a 2-byte int, 4-byte double or single-byte char, but only
one of them at a certain moment. The identier pm is a pointer to union myunion.
Size of Union
The size of a union is the size of its largest member. In our previous example, both sizeof(union myunion) and
sizeof(mu) return 4, but 2 bytes are unused (padded) when mu holds the int object, and 3 bytes are unused when
mu holds char.
Union Member Access
Union members can be accessed with the structure member selectors (. and ->), be careful when doing this:
/* Referring to declarations from the example above: */
pm = μ
mu.d = 4.016;
tmp = mu.d; // OK: mu.d = 4.016
tmp = mu.i; // peculiar result
pm->i = 3;
tmp = mu.i; // OK: mu.i = 3
The third line is legal, since mu.i is an integral type. However, the bit pattern in mu.i corresponds to parts of the
previously assigned double. As such, it probably won’t provide an useful integer interpretation.
When properly converted, a pointer to a union points to each of its members, and vice versa.