Why select do not tell me that a client wants to connect? - linux

Question

I've made a simple tcp server that I can test with telnet program.
When running it on windows, it works as expected, when running it on linux, the behavior is strange:
telnet clients understand that they are connected to the server,
the server do not see clients (select return always 0),
when I kill the server, the clients detect the disconnection.
I think I missed something in accept, listen or select.
What did I missed?
Thanks.
Here's the program source:
#include "headers.h"
#define DEFAULT_PORT 24891
/**
* test_server [ip port]
*/
int main(int argc, char *argv[])
{
sockaddr_in sin;
socket_t sock;
/* listening socket creation */
sock = socket(AF_INET, SOCK_STREAM, 0);
if (-1 == sock)
{ die("socket()"); }
/* binding */
sin.sin_family = AF_INET;
sin.sin_addr.s_addr = htonl(INADDR_ANY);
sin.sin_port = htons(DEFAULT_PORT);
if (3 == argc)
{
sin.sin_addr.s_addr = inet_addr(argv[1]);
sin.sin_port = htons(strtol(argv[2], NULL, 0));
}
if (-1 == bind(sock, (sockaddr*) &sin, sizeof(sin)))
{ die("bind()"); }
/* Listening */
if (-1 == listen(sock, SOMAXCONN))
{ die("listen()"); }
while (1)
{
timeval timeout = { 1, 0 };
fd_set in_set;
FD_ZERO(&in_set);
FD_SET(sock, &in_set);
// select the set
int cnt = select(1, &in_set, NULL, NULL, &timeout);
if (cnt > 0)
{
// ask if an event occurs on listening socket
if (FD_ISSET(sock, &in_set))
{
/* a new client wants to connect */
socket_t csock = accept(sock, NULL, NULL);
send(csock, "hello\r\n", 7, 0);
printf("new client!\n");
close(csock);
}
}
else if (cnt < 0)
{ die("select"); }
}
/* closing listen socket */
close(sock);
printf("socket closed\n");
return 0;
}

Answer 1

You simply call select incorrectly. The first parameter needs to be the highest numbered fd from the fdset, plus one. See man page:
int select(int nfds, fd_set *readfds, fd_set *writefds,
fd_set *exceptfds, struct timeval *timeout);
....
nfds is the highest-numbered file descriptor in any of the three sets, plus 1.
The code may work, or may not, that depends on the fd returned by "socket()".
In your case the value of "nfds" needs to be "sock + 1", generally you need to track the highest numbered fd when doing a select on multiple fd's.