c udp socket not receiving/sending correctly - c++

Question

I am currently experiencing some trouble while testing UDP sockets.
So I have 2 c programs, one sends and receives, the other receives and sends a message.
Here is send_recv.c:
#define PORT 1010
int main(int argc, const char *argv[]) {
int socketInfo, opt;
struct sockaddr_in server;
char msgOut[30], msgIn[30];
opt = 1;
printf("Input message: ");
fgets(msgOut, 30, stdin);
socketInfo = socket(AF_INET, SOCK_DGRAM, 0);
if(socketInfo < 0) {
perror("socket error");
return EXIT_FAILURE;
}
if(setsockopt(socketInfo, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt))) {
perror("setsockopt failed");
return EXIT_FAILURE;
}
server.sin_family = AF_INET;
server.sin_addr.s_addr = INADDR_ANY;
server.sin_port = htons(PORT);
if(bind(socketInfo, (struct sockaddr *)&server, sizeof(server)) < 0) {
perror("bind error");
return EXIT_FAILURE;
}
if(connect(socketInfo, (struct sockaddr *)&server, sizeof(server)) < 0) {
perror("connection error");
return EXIT_FAILURE;
}
puts("connected");
send(socketInfo, msgOut, strlen(msgOut), 0);
puts("sleeping for 5");
sleep(5);
puts("message sent");
recv(socketInfo, msgIn, sizeof(msgIn), 0);
printf("received msg: %s\n", msgIn);
close(socketInfo);
return EXIT_SUCCESS;
}
And here is recv_send.c:
#define PORT 1010
int main(int argc, const char *argv[]) {
int socketInfo, opt;
struct sockaddr_in server;
char msgOut[30], msgIn[30];
opt = 1;
socketInfo = socket(AF_INET, SOCK_DGRAM, 0);
if(socketInfo < 0) {
perror("socket error");
return EXIT_FAILURE;
}
if(setsockopt(socketInfo, SOL_SOCKET, SO_REUSEADDR | SO_REUSEPORT, &opt, sizeof(opt))) {
perror("setsockopt failed");
return EXIT_FAILURE;
}
server.sin_family = AF_INET;
server.sin_port = htons(PORT);
server.sin_addr.s_addr = INADDR_ANY;
if(bind(socketInfo, (struct sockaddr *)&server, sizeof(server)) < 0) {
perror("bind error");
return EXIT_FAILURE;
}
if(connect(socketInfo, (struct sockaddr *)&server, sizeof(server)) < 0) {
perror("connection error");
return EXIT_FAILURE;
}
puts("connected");
recv(socketInfo, msgIn, sizeof(msgIn), 0);
printf("received msg: %s\n", msgIn);
printf("Input message: ");
fgets(msgOut, 30, stdin);
send(socketInfo, msgOut, strlen(msgOut), 0);
puts("message sent");
close(socketInfo);
return EXIT_SUCCESS;
}
The problem is that the receive program never receives anything, and the send program sends and then just receives the message it already received.
The output of send_rcv is this:
Input message: hello there
connected
message sent
received msg: hello there
and the output of rcv_send is this:
connected
(ctrl+z) Job 6, 'sudo ./rcv_send' has stopped
Another problem is that when I don't run the program with sudo it just says this: bind error: Permission denied

Answer 1

The problem is that both endpoints are listening on the same port while running on the same machine, and you have both sides sending to INADDR_ANY.
Also, port numbers below 1024 are privileged ports and therefore require root access to bind to.
Use a different port (1024 or higher) for each side of the connection, and choose a specific IP address to connect to. For example:
#define LOCAL_PORT 1100
#define REMOTE_PORT 1101
...
struct sockaddr_in local, remote;
local.sin_family = AF_INET;
local.sin_addr.s_addr = INADDR_ANY;
local.sin_port = htons(LOCAL_PORT);
remote.sin_family = AF_INET;
remote.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
remote.sin_port = htons(LOCAL_PORT);
if(bind(socketInfo, (struct sockaddr *)&local, sizeof(local)) < 0) {
perror("bind error");
return EXIT_FAILURE;
}
if(connect(socketInfo, (struct sockaddr *)&remote, sizeof(remote)) < 0) {
perror("connection error");
return EXIT_FAILURE;
}
The values of LOCAL_PORT and REMOTE_PORT would be swapped in the two programs.
Note also that you can get rid of the setsockopt call as you don't have two programs open on the same port.