[Nbd] [RFC]Probelem about nbd-client's negotiation

[harryxiyou <harryxiyou@...17...>]

nbd-client.c

...
void negotiate(int sock, u64 *rsize64, u32 *flags) {
	u64 magic, size64;
	char buf[256] = "\0\0\0\0\0\0\0\0\0";

	printf("Negotiation: ");
	if (read(sock, buf, 8) < 0)
		err("Failed/1: %m");
	if (strlen(buf)==0)
		err("Server closed connection");
	if (strcmp(buf, INIT_PASSWD))
		err("INIT_PASSWD bad");
	printf(".");
	if (read(sock, &magic, sizeof(magic)) < 0)
		err("Failed/2: %m");
	magic = ntohll(magic);
	if (magic != cliserv_magic)
		err("Not enough cliserv_magic");
	printf(".");

	if (read(sock, &size64, sizeof(size64)) < 0)
		err("Failed/3: %m\n");
	size64 = ntohll(size64);

#ifdef NBD_SET_SIZE_BLOCKS
	if ((size64>>10) > (~0UL >> 1)) {
		printf("size = %luMB", (unsigned long)(size64>>20));
		err("Exported device is too big for me. Get 64-bit machine :-(\n");
	} else
		printf("size = %luKB", (unsigned long)(size64>>10));
#else
	if (size64 > (~0UL >> 1)) {
		printf("size = %luKB", (unsigned long)(size64>>10));
		err("Exported device is too big. Get 64-bit machine or newer kernel :-(\n");
	} else
		printf("size = %lu", (unsigned long)(size64));
#endif

	if (read(sock, flags, sizeof(*flags)) < 0)
		err("Failed/4: %m\n");
	*flags = ntohl(*flags);

	if (read(sock, &buf, 124) < 0)
		err("Failed/5: %m\n");
	printf("\n");

	*rsize64 = size64;
}
...

In this function, we do the following things.

1, Read the INIT_PASSWD from nbd-server
2, Read the magic number from nbd-server.
3, Read the disk size from nbd-server.
4, Read the flag from nbd-server.
5, Read the buff from nbd-server.

Some questions confuse me a lot.

1, After get the size from nbd-server, we run following codes.
 ...
#ifdef NBD_SET_SIZE_BLOCKS
	if ((size64>>10) > (~0UL >> 1)) {
		printf("size = %luMB", (unsigned long)(size64>>20));
		err("Exported device is too big for me. Get 64-bit machine :-(\n");
	} else
		printf("size = %luKB", (unsigned long)(size64>>10));
#else
	if (size64 > (~0UL >> 1)) {
		printf("size = %luKB", (unsigned long)(size64>>10));
		err("Exported device is too big. Get 64-bit machine or newer kernel :-(\n");
	} else
		printf("size = %lu", (unsigned long)(size64));
#endif
...

I don't know the meanings of them. Cloud you please understand me, thanks.

2, We read the flag from the nbd-server, but what the flag is? How can
i understand
    the flag's significances?

3, At last, we read buff like this.
...
	if (read(sock, &buf, 124) < 0)
		err("Failed/5: %m\n");
	printf("\n");
...
What are the buffs? It really makes any senses, right?


-- 
Thanks
Harry Wei