Simple UDP socket code, sending and receiving messages

Question

I am just learning UDP sockets and this is my first code involving it. I have two programs which send and receive messages back and forth. My question is it seems I have to declare which IP address I am sending/receiving from multiple times throughout the code as it changes but I feel there is a better way to do this without changing the inet_addr manually within the codes. From my reading it looks like sendto and recvfrom may be able to help but I am unsure how to use them in this context. If anyone could show me how to fix my simple problem I would greatly appreciate it! Thanks

CODE 1: Send then Receive

int main(int argc, char *argv[])
{
//initialize socket and structure
int socket_info;
struct sockaddr_in server;
char message[100];
char incoming_message[100];

printf("Input Message: ");
fgets(message, 100, stdin);

    //create socket
    socket_info = socket(AF_INET, SOCK_DGRAM, 0);
    if (socket_info == -1) {
    printf("Could not create socket");
    }

//assign local values
    server.sin_addr.s_addr = inet_addr("172.21.8.178");
    server.sin_family = AF_INET;
    server.sin_port = htons( 1100 );

    //binds connection
    if (bind(socket_info, (struct sockaddr *)&server, sizeof(server)) < 0) {    
perror("Connection error");
       return 1;
    }
    puts("Bind");

    //assign new value to connect to
    server.sin_addr.s_addr = inet_addr("172.21.8.179");

    //checks connection 
    if (connect(socket_info, (struct sockaddr *)&server, sizeof(server)) <       0) {
    perror("Connection error");
       return 1;
    }
    puts("Connected");

    //sends message
if(send(socket_info, message, strlen(message), 0) <0) {        
perror("Send failed");
    return 1;
    }
    puts("Message Sent");

//receives message back    
if(recv(socket_info, incoming_message, sizeof(incoming_message), 0) <0) {    
puts("Received failed");
    return 1;
    }
    puts("Message received");
    puts(incoming_message);

close(socket_info);

}

CODE 2: Receive then Send

int main(int argc, char *argv[])
{
//initialize socket and structure
int socket_info;
struct sockaddr_in server;
char incoming_message[100];

    //create socket
    socket_info = socket(AF_INET, SOCK_DGRAM, 0);
    if (socket_info == -1) {
    printf("Could not create socket");
    }

    //assign values
    server.sin_addr.s_addr = inet_addr("172.21.8.179");
    server.sin_family = AF_INET;
    server.sin_port = htons( 1100 );

    //checks connection
    if (bind(socket_info, (struct sockaddr *)&server, sizeof(server)) < 0) {
    perror("Connection error");
       return 1;
    }
    puts("Bind");

    //Receive an incoming message
if( recv(socket_info, incoming_message, sizeof(incoming_message), 0) < 0) {      
puts("Received failed");
    return 1;
    }
    puts("Message received");
    puts(incoming_message);

server.sin_addr.s_addr = inet_addr("172.21.8.178");

if (connect(socket_info, (struct sockaddr *)&server, sizeof(server)) < 0) {
    perror("Connection error");
     return 1;
    }
    puts("Connected");

//Sends message back
char message[100];

printf("Input Message: ");
fgets(message, 100, stdin);

if(send(socket_info, message, strlen(message), 0) <0) {        
perror("Send failed");
    return 1;
    }
    puts("Message Sent");

close(socket_info);
}

Binding is not connecting, and printing 'connection error' on a bind error is just misleading yourself. You can use sendto() instead of connecting the socket. — user207421
I thought connect API is used for TCP socket. What purpose does it serve here? — Tahlil
For UDP socket program you no need to use connect.it is not an proper UDP code.have a look on simple echo server.it will help you. — BEPP
@NaveenKumar It isn't necessary but it's possible, and doing so is not improper coding. — user207421
this looks more like a TCP socket hiding behind SOCK_DGRAM — Beyondo

slangley slangley · Accepted Answer · 2018-05-21T23:36:55

If you use the function recvfrom()

    ssize_t recvfrom(int sockfd, void *buf, size_t len, int flags,
             struct sockaddr *src_addr, socklen_t *addrlen);

What this function does is it fills a structure of sockaddr with the IP and port information of the packet that it has just received. For example, if your code that sends first then receives sends a packet to the receiver, the receiver should be able to fill the structure values of sin_addr and sin_port with the correct values. You can then make a call of sendto() with this information in order to send it to the correct machine.

Here's the man pages for these functions:

https://linux.die.net/man/2/recvfrom

https://linux.die.net/man/2/sendto

Simple UDP socket code, sending and receiving messages

2 Answers