c - 如何反转字节顺序？

Question

谁能帮我理解这段代码？

int reverse_endianess(int value) {
    int resultat = 0;
    char *source, *destination;
    int i;

    source = (char *) &value;
    destination = ((char *) &resultat) + sizeof(int);
    for (i = 0; i < sizeof(int); i++)
        *(--destination) = *(source++);
    return resultat;
}

我无法理解这部分代码:

destination = ((char *) &resultat) + sizeof(int);
for (i = 0; i < sizeof(int); i++)
    *(--destination) = *(source++);

Answer 1

以下导致destination指向resultat之后的字节(只要resultat是一个int):

destination = ((char *) &resultat) + sizeof(int);

也可以这样写:

destination = (char *)(&resultat + 1);

---

下面只是一个简单的内存复制循环:

for (i = 0; i < sizeof(int); i++)
        *(--destination) = *(source++);

等同于:

for (i = 0; i < sizeof(int); i++) {
   --destination;             // Point to the one byte earlier.
   *destination = *source;    // Copy one byte.
   source++;                  // Point to one byte later.
}

---

程序流程(假设32位int和8位char)

设置后:

source               value
+----------+         +---+---+---+---+
|        -------+    | a | b | c | d |
+----------+    |    +---+---+---+---+
                |      ^
                +------+

destination          resultat 
+----------+         +---+---+---+---+
|        -------+    | 0 | 0 | 0 | 0 |
+----------+    |    +---+---+---+---+
                |                      ^
                +----------------------+

循环一次后:

source               value
+----------+         +---+---+---+---+
|        -------+    | a | b | c | d |
+----------+    |    +---+---+---+---+
                |          ^
                +----------+

destination          resultat 
+----------+         +---+---+---+---+
|        -------+    | 0 | 0 | 0 | a |
+----------+    |    +---+---+---+---+
                |                  ^
                +------------------+

完成后:

source               value
+----------+         +---+---+---+---+
|        -------+    | a | b | c | d |
+----------+    |    +---+---+---+---+
                |                      ^
                +----------------------+

destination          resultat 
+----------+         +---+---+---+---+
|        -------+    | d | c | b | a |
+----------+    |    +---+---+---+---+
                |      ^
                +------+