Python-有人可以告诉我这两行做什么吗？

Question

I'm trying to convert this Python code to C. But for the life of me, I can't figure out what this line here does. The rest of the program seems simple.

self.payload = "\x02\x00%s%s" % (
    pack(">b", length),
    "".join(random.choice(string.printable) for i in range(length)))

If anybody could give me a rough idea of what this is doing, it'd be much appreciated!

Answer 1

First line:

• The correct translation of length = random.randint(18, 20) is int length = rand() % 3 + 18 .

Now let's dissect the dense second line piece by piece.

• "\\x02\\x00%s%s" % (x, y) means to substitute the format string with the given arguments (like sprintf() ). In this case it means concatenating the bytes 0x02, 0x00 with two strings x and y to follow.

• x = pack(">b", length) uses struct.pack() . In this case, it means to convert the integer value length into one byte representing its value. It's almost equivalent to using chr() .

• y = "".join(z) means to take each element in the list z (which must be a string) and concatenate them with "" (nothing) between them. (For example, "@".join(["a","b","c"]) --> "a@b@c" .)

• z = (random.choice(string.printable) for i in range(length)) returns a generator object. You can think of it as a list whose elements are computed on demand. In this case, it generates length elements where each element is one character randomly chosen from the string string.printable .

All in all, the second line yields a string that starts wxth 0x02 0x00, followed by (char)length , followed by length random characters each uniformly chosen from the set of chars string.printable .