why the sendto function needs the third parameter, in socket programming

Question

I was new to socket program. When learning the sendto function, as the prototype:

ssize_t sendto(int socket, const void *message, size_t length,
                int flags, const struct sockaddr *dest_addr,
                socklen_t dest_len);

I get to know that the "message" has contain the target IP, and the dest_addr argument also specifies the target IP address. Is there other usage of the dest_addr argument?

Answer 1

The message does not contain the destination address, only the payload bytes.

Unless you are using raw sockets... And you should not do that if you are new to socket programming.

Answer 2

I think you are confusing "message" and "dest_addr".

Let's look at the prototype for sendto in expanded form:

ssize_t sendto   (int sockfd,
                 const void *buf,
                 size_t length,
                 int flags,
                 const struct sockaddr *dest_addr, 
                 socklen_t addrlen);

sockfd - this is the socket you created with a call to socket()

buf - this is a pointer to an ARRAY OF BYTES (i.e. they could have made buf of type char* ). That is, this is the data that you want to send across the wire encapsulated in a UDP packet.

length - this is how many bytes are in that array. If you didn't pass "length", it wouldn't know if "buf" was 1 byte or 10000 bytes.

flags - Typically 0. This is advanced stuff

dest_addr - this is a pointer to the destination address. Typically you initialize a sockaddr_in instance and cast its pointer value to a sockaddr* type.

addrlen - the size of the dest_addr. typically, sizeof(sockaddr_in). Address length is variable because dest_addr could pointer to an IPV4 address (sockaddr_in type) or an IPV6 address (sockaddr_in6 type), or some other type.

Example of sending a packet from local port 9999 to remote host "1.2.3.4" on its port 8888. Error checking of return codes left out for brevity.

int s;
sockaddr_in addrDest;
sockaddr_in addrLocal;
char* msg = "Hello World";

// create the socket
s = socket(AF_INET, SOCK_DGRAM, 0); // UDP socket


addrLocal.sin_family = AF_INET;
addrLocal.sin_port = htons(9999);
addrLocal.sin_addr = INADDR_ANY; // zero-init sin_addr to tell it to use all available adapters on the local host

// associate this socket with local UDP port 9999
result = bind(s, (struct sockaddr*)&addrLocal, 0);

// send "Hello world" from local port 9999 to the host at 1.2.3.4 on its port 8888
addrDest.sin_family = AF_INET;
addrDest.sin_port = htons(8888);
addrDest.sin_addr.s_addr = inet_addr("1.2.3.4");

// strlen(msg)+1 for terminating null char
result = sendto(s, msg, strlen(msg)+1, 0, (struct sockaddr*)&addrDest, sizeof(addrDest));

Answer 3

No, Message Contains what you will send, here is an example:

int spatula_count = 3490;
char *secret_message = "The Cheese is in The Toaster";

int stream_socket, dgram_socket;
struct sockaddr_in dest;
int temp;

// first with TCP stream sockets:

// assume sockets are made and connected
//stream_socket = socket(...
//connect(stream_socket, ...

// convert to network byte order
temp = htonl(spatula_count);
// send data normally:
send(stream_socket, &temp, sizeof temp, 0);

// send secret message out of band:
send(stream_socket, secret_message, strlen(secret_message)+1, MSG_OOB);

// now with UDP datagram sockets:
//getaddrinfo(...
//dest = ...  // assume "dest" holds the address of the destination
//dgram_socket = socket(...

// send secret message normally:
sendto(dgram_socket, secret_message, strlen(secret_message)+1, 0, 
       (struct sockaddr*)&dest, sizeof dest);