Connection to socket refused - c

Question

I have simple TCP server program. After I close it with CTRL+C and restart again in some cases telnet client report me "connection refused". Why it happens not always, but just in some cases? Command ps shows nothing regarding my programm.
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <stdio.h>
#include<string.h>
void printHex(char *bts);
int main() {
char str[100];
int listen_fd, comm_fd;
struct sockaddr_in servaddr;
listen_fd = socket(AF_INET, SOCK_STREAM, 0);
bzero( &servaddr, sizeof(servaddr));
servaddr.sin_family = AF_INET;
servaddr.sin_addr.s_addr = htons(INADDR_ANY);
servaddr.sin_port = htons(22000);
printf("binding\n");
bind(listen_fd, (struct sockaddr *) &servaddr, sizeof(servaddr));
printf("listening\n");
listen(listen_fd, 10);
printf("accepting\n");
comm_fd = accept(listen_fd, (struct sockaddr*) NULL, NULL);
printf("accep done\n");
int cn = 0;
while(1) {
bzero(str, 100);
printf("will read\n");
int br = read(comm_fd, str, 100);
printf("read done\n");
if (br > 0) {
printHex(str);
} else if (br <= 0) {
comm_fd = accept(listen_fd, (struct sockaddr*) NULL, NULL);
printf("accep done\n");
}
printf("error %d \n", br);
}
}
void printHex(char *bts) {
char *s = bts;
int i = 0;
do {
printf("%02X ", (unsigned char) *bts);
} while (*++bts !=0);
printf("%s\n",s);
}

Answer 1

You should check all return values from system calls:
int res;
res = bind(listen_fd, (struct sockaddr *) &servaddr, sizeof(servaddr));
if (res == -1)
// handle error
listen_fd = socket(AF_INET, SOCK_STREAM, 0);
if (listed_fd == -1)
// handle error
Most likely you will find then that your code fails on a bind system call, which means you can't reuse socket on same IP address, protocol and port. You can change this using setsockopt call:
// allow to rebind
int reuseaddr_on = 1;
res = setsockopt(listen_fd, SOL_SOCKET, SO_REUSEADDR,
&reuseaddr_on, sizeof(reuseaddr_on));
if (res < 0)
{
stderr("Setting of SO_REUSEADDR on server's"
"socket failed");
}
You should also close a socket when you are done with it.
while(1)
{
// your loop
}
res = close(comm_fd);
if (res == -1)
// handle error
As Linux Programmer's Manual says on pages devoted to close call:
NOTES
Not checking the return value of close() is a common but nevertheless serious programming error. It is quite possible that
errors on a previous write(2) operation are first reported at the final close(). Not checking the return value when closing the file may lead to silent
loss of
data.

Answer 2

Always check the return values of your syscalls.
In this case,
bind(listen_fd, (struct sockaddr *) &servaddr, sizeof(servaddr));
fails with EADDRINUSE, because you didn't set SO_REUSEADDR on the socket before binding.

Answer 3

When you do CTRL-C and restart again, is the bind sucessful?
bind(listen_fd, (struct sockaddr *) &servaddr, sizeof(servaddr));
You need to check the status of the bind call. In all likelihood, this has failed due to connection having moved to TIME_WAIT state in server. And from this follows the error "connection refused" in client.
Far the bind to be successful when the server is run immediately after CTRL-C, use the SO_REUSEADDR in your socket option. With this bind will be successful, as long as there isn't another program bound to the port.