Unix networking in C++ doesn't wait for a connection - c++

Question

I have recently been learning about networking in Unix, and have written these 2 simple programs that waits for a connection, receives a message, then sends the same message back to the sender:
Send.cpp
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <iostream>
#include <unistd.h>
int main(int argc, char const *argv[])
{
struct addrinfo hints, *res;
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE;
getaddrinfo(NULL, "2000", &hints, &res);
int sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
if (auto error = connect(sockfd, res->ai_addr, res->ai_addrlen) < 0){
std::cout << strerror(errno) << '\n';
exit(1);
}
char msg[] = "Hello, World";
send(sockfd, &msg, sizeof(msg), 0);
char input[sizeof(msg)];
recv(sockfd, &input, sizeof(input), 0);
std::cout << input << '\n';
close(sockfd);
return 0;
}
receive.cpp
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <sys/poll.h>
#include <iostream>
#include <unistd.h>
int main(int argc, char const *argv[])
{
struct addrinfo hints, *res;
int sockfd, new_fd;
memset(&hints, 0, sizeof(hints));
hints.ai_family = AF_UNSPEC;
hints.ai_socktype = SOCK_STREAM;
hints.ai_flags = AI_PASSIVE;
getaddrinfo(NULL, "2000", &hints, &res);
sockfd = socket(res->ai_family, res->ai_socktype, res->ai_protocol);
bind(sockfd, res->ai_addr, res->ai_addrlen);
listen(sockfd, 5);
struct sockaddr_storage their_addr1;
socklen_t addr_size;
addr_size = sizeof(their_addr1);
new_fd = accept(sockfd, (struct sockaddr *)&their_addr1, &addr_size);
if (new_fd < 0)
{
std::cout << "Error" << '\n';
std::cout << strerror(errno) << '\n';
exit(1);
}
std::cout << "Accepted" << '\n';
char msg[255];
auto n = recv(new_fd, &msg, 255, 0);
std::cout << msg << '\n';
send(new_fd, &msg, n, 0);
close(sockfd);
close(new_fd);
return 0;
}
If I run receive.cpp, then quickly (within about 5 or so seconds) run send.cpp, (running both from the command line) the programs work perfectly, but if I take any longer than that, then the send program just always gets a Connection Refused error and the receive program never terminates. It was my understanding that the accept() call would block the rest of the application and continue listening until a valid connection appears. Is this not how it works? Any help would be great thank you.

Answer 1

Credit to #n. 1.8e9-where's-my-share m.
The problem is that the binding was failing due to a "Address already in use" that was being caused because the TCP/IP protocol waits for a specific time to make sure all packets have arrived before allowing the same IP and Port combo to be used again.
Fix:
Wait for the kernel to give up the port.
Set the the socket options to allow for reuse of the same port using the
setsockopt()
int yes=1;
if (setsockopt(listener,SOL_SOCKET,SO_REUSEADDR,&yes,sizeof yes) == -1) {
perror("setsockopt");
exit(1);
}