c - 如何在C中实现return *语句？

Question

我正在阅读有关递归上升-下降解析器的文章 here 。

在第 2.1 节中，他们描述了一个 return * 语句，

Our C extension occurs with return statements. We have used the notation return * k to indicate that a k-level function return is to be made. That is, return * 1 is identical to the normal C return statement and simply returns control to the caller of the current function; return * 2 means that control is to be returned to the caller of the caller, and so on. Finally, return * 0 is to be interpreted as a null statement. We leave emulation of the return * k construct in languages that lack this operation as a simple exercise for the reader.

如何在自己的代码中实现此类 return* 语句或使用 goto 语句或/和指针模拟此行为？是否有任何语言默认提供此功能？

Answer 1

您可以使用setjmp()和longjmp()来模拟这种多级返回，只要您注意维护一个jmp_buf堆栈s 每次调用函数时。

示例:

#include <stdio.h>
#include <setjmp.h>
#include <assert.h>

#define MAXDEPTH 10
jmp_buf stack[MAXDEPTH];
int sp = 0;

#define CALL(f)                                                                \
  do {                                                                         \
    assert(sp < MAXDEPTH);                                                     \
    if (setjmp(stack[sp++]) == 0) {                                            \
      f;                                                                       \
    }                                                                          \
  } while (0)
#define RETLEVEL(n)                                                            \
  do {                                                                         \
    if ((n) > 0) {                                                             \
      sp -= (n);                                                               \
      assert(sp >= 0 && sp < MAXDEPTH);                                        \
      longjmp(stack[sp], 1);                                                   \
    }                                                                          \
  } while (0)
#define RETURN                                                                 \
  do {                                                                         \
    sp -= 1;                                                                   \
    assert(sp >= 0);                                                           \
    return;                                                                    \
  } while (0)

void f3(void) {
  printf("In f3(), sp is %d, returning back 2 levels\n", sp);
  RETLEVEL(2);
}

void f2(void) {
  printf("In f2(), calling f3(), sp is %d\n", sp);
  CALL(f3());
  printf("Returning from f2(), sp is %d\n", sp);
  RETURN;
}

void f1(void) {
  printf("In f1(), calling f2(), sp is %d\n", sp);
  CALL(f2());
  printf("Returning from f1(), sp is %d\n", sp);
  RETURN;
}

int main(void) {
  printf("In main(), calling f1(), sp is %d\n", sp);
  CALL(f1());
  printf("Returning from main(), sp is now %d\n", sp);
  return 0;
}

编译并运行时，输出:

In main(), calling f1(), sp is 0
In f1(), calling f2(), sp is 1
In f2(), calling f3(), sp is 2
In f3(), sp is 3, returning back 2 levels
Returning from f1(), sp is 1
Returning from main(), sp is now 0

不过，请仔细阅读这些函数，因为它们带有一些关于局部变量在 setjmp() 返回之间保存其值的警告。

---

对于具有内置多级返回的语言... tcl 浮现在脑海中的是 return -level N 。不过，任何具有延续性的语言(例如方案或协程)都可以轻松模拟它。