socket binding failed (address already in use, even with SO_REUSEADDR) - c

Question

I am working in a simple socket project. I would like to know:
why error messages appear before telnet localhost 5678?
why SO_REUSEADDR (between socket() and bind()) don't work, and what else I should try?
Code Output Message:
bind error
Error opening file: Address already in use
telnet localhost 5678
[+]Server Socket is created.
main.c
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <errno.h>
#define BUFSIZE 1024 // Buffer Size
#define PORT 5678
int main() {
printf("telnet localhost 5678\n");
int rfd; // socket descriptor
int clientfd; // client descriptor
struct sockaddr_in client; // Client Socket address
socklen_t client_len; // Length of Client Data
char input[BUFSIZE]; // Client Data -> Server
int bytes_read; // Client Bytes
// 1. socket() = create a socket, SOCK_STREAM = TCP
rfd = socket(AF_INET, SOCK_STREAM, 0);
if (rfd < 0) {
fprintf(stderr, "socket error\n");
exit(-1);
}
printf("[+]Server Socket is created.\n");
// optional
int enable = 1;
if (setsockopt(rfd, SOL_SOCKET, SO_REUSEADDR, &enable, sizeof(int)) < 0)
fprintf(stderr, "setsockopt(SO_REUSEADDR) failed");
//Initialize the server address by the port and IP
struct sockaddr_in server;
memset(&server, 0, sizeof(server));
server.sin_family = AF_INET; // Internet address family: v4 address
server.sin_addr.s_addr = INADDR_ANY; // Server IP address
server.sin_port = htons(PORT); // Server port
// 2. bind() = bind the socket to an address
int brt = bind(rfd, (struct sockaddr *) &server, sizeof(server));
if (brt < 0) {
int errnum;
errnum = errno;
fprintf(stderr, "bind error\n");
fprintf(stderr, "Error opening file: %s\n", strerror(errnum));
exit(-1);
}
printf("[+]Bind to port %d\n", PORT);
// 3. listen() = listen for connections
int lrt = listen(rfd, 50);
if (lrt < 0) {
printf("listen error\n");
exit(-1);
}
if (lrt == 0) {
printf("[+]Listening....\n");
}
// non-stop loop
while (1) {
// 4. accept() = accept a new connection on socket from client
clientfd = accept(rfd, (struct sockaddr *) &client, &client_len);
if (clientfd < 0) {
fprintf(stderr, "accept failed with error %d\n");
exit(-1);
}
printf("Client connected\n");
...
close(clientfd);
printf("Client disconnected\n");
}
close(rfd);
}

Answer 1

I'm assuming that you are using Linux. If you want to rebind to an address, you should use SO_REUSEPORT not SO_REUSEADDR. Name is really misleading. But make sure that you know how it works and whether you really want to use it or not.
You can check difference here: How do SO_REUSEADDR and SO_REUSEPORT differ?