19.14.1 Label Value Uses

One use for label-valued constants is to initialize a static array to serve as a jump table:

static void *array[] = { &&foo, &&bar, &&hack };

Then you can select a label with indexing, like this:

goto *array[i];

Note that this does not check whether the subscript is in bounds—array indexing in C never checks that.

You can make the table entries offsets instead of addresses by subtracting one label from the others. Here is an example:

static const int array[] = { &&foo - &&foo, &&bar - &&foo,
                             &&hack - &&foo };
goto *(&&foo + array[i]);

Using offsets is preferable in shared libraries, as it avoids the need for dynamic relocation of the array elements; therefore, the array can be read-only.

An array of label values or offsets serves a purpose much like that of the switch statement. The switch statement is cleaner, so use switch by preference when feasible.

Another use of label values is in an interpreter for threaded code. The labels within the interpreter function can be stored in the threaded code for super-fast dispatching.