Hugo Hugo - 2 months ago 6
C Question

Nothing happens when listening on a TCP port

I'm trying to write a small, example app to listen to a TCP port, but it's not working for some reason.

It see seems to reach

accept(3p)
, but
lsof -i :1080
reports that nothing is listening on that port. There are not errors, or anything else remotely obvious.

Any hints?

#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>

const int THREADS = 4;
const int BUFFER_SIZE = 256;

void usage(char *app_name) {
printf("Usage: %s PORT\n", app_name);
}

int main(int argc, char **argv) {
struct sockaddr_in6 serv_addr, addr_cli;
char buffer[BUFFER_SIZE];
int sockfd, sock_cli, n, port;

if (argc < 2) {
usage(argv[0]);
return -1;
}

port = atoi(argv[1]);

if ((sockfd = socket(AF_INET6, SOCK_STREAM, 0)) < 0) {
perror("Could not open socket");
return errno;
}

serv_addr.sin6_family = AF_INET6;
serv_addr.sin6_port = htonl(port);
// other addresses: INADDR_LOOPBACK, INADDR_ANY
serv_addr.sin6_addr = in6addr_any;
serv_addr.sin6_scope_id = 0;

if (bind(sockfd, (struct sockaddr *) &serv_addr, sizeof(serv_addr)) != 0) {
perror("Could not bind to socket");
return errno;
}

if (listen(sockfd, THREADS + 1) < 0) {
perror("Could not listen to socket");
return errno;
}

printf("Listening on :%d\n", port);

unsigned long l;
if ((sock_cli = accept(sockfd, (struct sockaddr *) &addr_cli, (socklen_t*) &l)) != 0) {
perror("Could not accept connections on socket");
return errno;
}

printf("Connection accepted\n");

while ((n = recv(sock_cli, buffer, BUFFER_SIZE, 0)) > 0) {
printf("> Got data: %s\n", buffer);
}

if (n < 0) {
printf("Could not receive data\n");
return errno;
}

printf("Bye!\n");
}

Answer

I got your program to work

$ ./hugo 8888 &
[1] 25330
$ Listening on :8888
telnet -6 localhost 8888
Trying ::1...
Connected to localhost.
Escape character is '^]'.
Connection accepted
hello 
> Got data: hello
^]
telnet> quit
Connection closed.
Bye!
[1]+  Exit 10                 ./hugo 8888

making the following changes:

#include <netinet/in.h>
...
serv_addr.sin6_port = htons(port);
...
if ((sock_cli = accept(sockfd,  
                       (struct sockaddr *) &addr_cli, 
                       (socklen_t*) &l)) == -1) {

As the comment suggested, htons(3) causes the correct port to be listened to. And accept(2) returns a socket, so you have test explicitly for -1.

If I could suggest two small things,

  • return EXIT_SUCCESS from main. :-)
  • use err(3) instead of perror(3). Less typing, and you really don't want to return errno anyway.