Socket operation on non-socket C - c

Question

I am getting this error when trying to make an application to connect and listen to a port for data.
#include <sys/socket.h>
#include <sys/types.h>
#include <arpa/inet.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <errno.h>
#include <unistd.h>
#include <fcntl.h>
int main(int argc, char * arg[]){
int conn_s = socket(AF_INET, SOCK_STREAM, 0); //Create the socket
struct sockaddr_in servaddr;
memset(&servaddr, 0, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_port = htons(1234);
servaddr.sin_addr.s_addr = htonl(INADDR_ANY);
int res = bind(conn_s, (struct sockaddr *) &servaddr, sizeof(servaddr));
if(res < 0){
printf("Error has occured\n");
}
FILE * stream = fdopen(conn_s, "w+"); //Create a stream for the socket
FILE * file = fopen("response2.txt", "w+"); //Create a file to store the output of the stream
char * line = NULL; //Where each line of the stream will be stored in
size_t len = 0; // The length of the line
ssize_t bytes; //The size of the line in bytes
int lis = listen(conn_s, SOMAXCONN);
fcntl(lis, F_SETFL, O_NONBLOCK);
while(1) {
conn_s = accept(lis, NULL, NULL);
if(conn_s < 0){
if((errno == EAGAIN) || (errno == EWOULDBLOCK))
continue;
perror("Failed to accept connection");
exit(EXIT_FAILURE);
}
long conn_s_flags = fcntl(conn_s, F_GETFL);
fcntl(conn_s, F_SETFL, conn_s_flags & ~O_NONBLOCK);
while((bytes = getline(&line, &len, stream)) != -1) {
printf("%s\n", line);
fwrite(line, sizeof(char), bytes, file);
}
close(conn_s);
}
free(line);
return 0;
}
I am trying to connect to port 1234, listen to it and accept it to recieve data, but the error keeps occuring.
Also I am trying to test using netcat, but get a different error whenever nc is running on the port I specified.
Thanks

Answer 1

int lis = listen(conn_s, SOMAXCONN);
fcntl(lis, F_SETFL, O_NONBLOCK);
while(1) {
conn_s = accept(lis, NULL, NULL);
listen() does not return a socket FD. It returns zero or -1. The second line is therefore erroneous, as is the following accept() call. It should be:
if (listen(conn_s, SOMAXCONN) == -1)
{
perror("listen");
return; // or whatever
}
fcntl(conn_s, F_SETFL, O_NONBLOCK);
while(1) {
int conn_c = accept(conn_s, NULL, NULL);
NB Don't lose conn_s by storing the result of accept() into it.