# miscellaneous

## 1

你是否可以設計一個容許接受任何型態參數的函數，並且傳回一個整數？另外這個函數是否有辦法讓多個參數一次傳入？

:::spoiler Answer
一個容許接受任何型態參數的函數可以這樣設計：

int func(void *ptr)

如果想要一次傳入多個參數，可以將所有要傳入的參數放在一個 structure object 中，再將這個 structure object 傳入此函數。
:::

## 2

#include <stdio.h>
#include <stdlib.h>
#define PrintInt(expr) printf("%s : %dn",#expr,(expr))

int main()  
{
    int y = 100;
    int *p;
    p = malloc(sizeof(int));
    *p = 10;
    y = y/*p; /*dividing y by *p */;
    PrintInt(y);
    return 0;
}

:::spoiler Answer
Output:

y : 100

Explaination:

答案不是 10！並非 `y = y / (*p)`，因為 `/*p; /*dividing y by *p */` 是一整串註解！
:::

## 3

以下的程式碼使用__interrupt來定義一個中斷service routine。評論這個程式碼：

__interrupt double compute_area(double radius)
{
    double area = PI*radius*radius;
    printf("\nArea=%f", area);
    return area;
}

:::spoiler Answer
這個函數有太多錯誤了，問題如下：

ISR不能返回一個值
ISR不能傳遞參數
在許多編譯器/處理器中，浮點數操作是不可重入的(re-entrant)。有些處理器/編譯器需要讓多餘的暫存器入棧(PUSH入堆疊)，有些處理器/編譯器就是不允許在ISR中做浮點運算。此外，ISR應該是短而有效率的，在ISR中做浮點運算是不明智的。
與第三點類似，printf通常會有可重入和效能的問題。

- [7.5.4 The __interrupt Keyword](https://downloads.ti.com/docs/esd/SPRUI04/the---interrupt-keyword-stdz0559860.html)

The compiler extends the C/C++ language by adding the __interrupt keyword, which specifies that a function is treated as an interrupt function. This keyword is an IRQ interrupt. The alternate keyword, "interrupt", may also be used except in strict ANSI C or C++ modes.

Note that the interrupt function attribute described in Section 7.9.21 is the recommended syntax for declaring interrupt functions.

Functions that handle interrupts follow special register-saving rules and a special return sequence. The implementation stresses safety. The interrupt routine does not assume that the C run-time conventions for the various CPU register and status bits are in effect; instead, it re-establishes any values assumed by the run-time environment. When C/C++ code is interrupted, the interrupt routine must preserve the contents of all machine registers that are used by the routine or by any function called by the routine. When you use the __interrupt keyword with the definition of the function, the compiler generates register saves based on the rules for interrupt functions and the special return sequence for interrupts.

**You can only use the __interrupt keyword with a function that is defined to return void and that has no parameters. The body of the interrupt function can have local variables and is free to use the stack or global variables**. For example:

`__interrupt void int_handler() { unsigned int flags; ... }`
:::