How to reverse endianness?

Question

Can someone help me understand this code?

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;
}

I can't understand this part of code:

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

Answer 1

The following causes destination to point to the byte that follows resultat (as long as resultat is an int):

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

It could also have been written as follows:

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

---

The following is just a simple memory copy loop:

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

It's equivalent to the following:

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.
}

---

Program flow (assuming 32-bit int and 8-bit char)

After setup:

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

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

After one pass of the loop:

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

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

When it's done:

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

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