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发表于 2025-6-6 11:14:57
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这段代码确实没问题，有点令人意外，case 是可以放在 switch 中的代码块里面的。
Duff's device
The famous "Duff's device" in C makes use of the fact that a `case` statement is still legal within a sub-block of its matching `switch` statement. Tom Duff used this for an optimised output loop:
switch (count % 8)
{
case 0: do { *to = *from++;
case 7: *to = *from++;
case 6: *to = *from++;
case 5: *to = *from++;
case 4: *to = *from++;
case 3: *to = *from++;
case 2: *to = *from++;
case 1: *to = *from++;
} while ((count -= 8) > 0);
}
```
We can put it to a slightly different use in the coroutine trick. Instead of using a `switch` statement to decide which `goto` statement to execute, we can use the `switch` statement to perform the jump itself:
int function(void)
{
static int i, state = 0;
switch (state)
{
case 0: /* start of function */
for (i = 0; i < 10; i++)
{
state = 1; /* so we will come back to "case 1" */
return i;
case 1:; /* resume control straight after the return */
}
}
}
Now this is looking promising. All we have to do now is construct a few well chosen macros, and we can hide the gory details in something plausible-looking:
```
#define crBegin static int state=0; switch(state) { case 0:
#define crReturn(i,x) do { state=i; return x; case i:; } while (0)
#define crFinish }
int function(void)
{
static int i;
crBegin;
for (i = 0; i < 10; i++) crReturn(1, i);
crFinish;
}
```
(note the use of `do ... while(0)` to ensure that `crReturn` does not need braces around it when it comes directly between `if` and `else`)
This is almost exactly what we wanted. We can use `crReturn` to return from the function in such a way that control at the next call resumes just after the return. Of course we must obey some ground rules (surround the function body with `crBegin` and `crFinish`; declare all local variables `static` if they need to be preserved across a `crReturn`; *never* put a `crReturn` within an explicit `switch` statement); but those do not limit us very much.
The only snag remaining is the first parameter to `crReturn`. Just as when we invented a new label in the previous section we had to avoid it colliding with existing label names, now we must ensure all our state parameters to `crReturn` are different. The consequences will be fairly benign - the compiler will catch it and not let it do horrible things at run time - but we still need to avoid doing it.
Even this can be solved. ANSI C provides the special macro name `__LINE__`, which expands to the current source line number. So we can rewrite `crReturn` as
```
#define crReturn(x) do { state=__LINE__; return x; \
case __LINE__:; } while (0)
```
and then we no longer have to worry about those state parameters at all, provided we obey a fourth ground rule (never put two `crReturn` statements on the same line). |
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