Typecasting a char to an int (for socket)

I have probably asked this question twice since y'day, but I have still not got a favourable answer. My problem is I have an IP address which is s开发者_StackOverflow中文版tored in an unsigned char. Now i want to send this IP address via socket from client to server. People have advised me to use htonl() and ntohl() for network byte transfer, but I am not able to understand that the arguments for htonl() and ntohl() are integers...how can i use it in case of unsigned char?? if I can't use it, how can I make sure that if I send 130.191.166.230 in my buffer, the receiver will receive the same all the time?? Any inputs or guidance will be appreciated. Thanks in advance.

If you have an unsigned char array string (along the lines of "10.0.0.7") forming the IP address (and I'm assuming you do since there are very few 32-bit char systems around, making it rather difficult to store an IP address into a single character), you can just send that through as it is and let the other end use it (assuming you both encode characters the same way of course, such as with ASCII).

On the other hand, you may have a four byte array of chars (assuming chars are eight bits) containing the binary IP address.

The use of htonl and ntohl is to ensure that this binary data is sent through in an order that both big-endian and little-endian systems can understand.

To that end, network byte order (the order of the bytes "on the wire") is big-endian so these functions basically do nothing on big-endian systems. On little-endian systems, they swap the bytes around.

In other words, you may have the following binary data:

uint32_t ipaddress = 0x0a010203; // for 10.1.2.3

In big endian layout that would be stored as 0x0a,0x01,0x02,0x03, in little endian as 0x03,0x02,0x01,0x0a.

So, if you want to send it in network byte order (that any endian system will be able to understand), you can't just do:

write (fd, &ipaddress, 4);

since sending that from little endian system to a big endian one will end up with the bytes reversed.

What you need to do is:

uint32_t ipaddress = 0x0a010203; // for 10.1.2.3
uint32_t ip_netorder = htonl (ipaddress); // change if necessary.
write (fd, &ip_netorder, 4);

That forces it to be network byte order which any program at the other end can understand (assuming it uses ntohl to ensure it's correct for its purposes).

In fact, this scheme can handle more than just big and little endian. If you have a 32-bit integer coding scheme where ABCD (four bytes) is encoded as A,D,B,C or even where you have a bizarrely wild bit mixture forming your integers (like using even bits first then odd bits), this will still work since your local htonl and ntohl know about those formats and can convert them correctly to network byte order.

An array of chars has a defined ordering and is not endian dependent - they always operate from low to high addresses by convention.

Do you have a string or 4 bytes?

IP4 address is 4 bytes (aka chars). So you will be having 4 unsigned chars, in an array somewhere. cast that array to send it across.

e.g. unsigned char IP[4];

use ((char *)IP) as data buffer to send, and send 4 bytes from it.