Why doesn't accept() block?

Question

I'm new in socket programming under Linux (UNIX) sockets. I found the following code in the Internet, for a tcp-server that spawns a thread for each connection. However it doesn't work. the accept() function returns instantly, and doesn't wait for connection. What am I doing wrong ?

this is the code

int main(int argv, char *args[])
{
    struct sockaddr_in addr;
    int sd, port;

    port = htons(SERVER_PORT);

    /*--- create socket ---*/
    sd = socket(PF_INET, SOCK_STREAM, 0);
    if ( sd < 0 )
        panic("socket");

    /*--- bind port/address to socket ---*/
    memset(&addr, 0, sizeof(addr));
    addr.sin_family = AF_INET;
    addr.sin_port = port;
    addr.sin_addr.s_addr = INADDR_ANY;                   /* any interface */
    if ( bind(sd, (struct sockaddr*)&addr, sizeof(addr)) != 0 )
        panic("bind");

    /*--- make into listener with 10 slots ---*/
    if ( listen(sd, 10) != 0 )
        panic("listen")

    /*--- begin waiting for connections ---*/
    else
    {   int sd;
        pthread_t child;
        FILE *fp;

        while (1)                         /* process all incoming clients */
        {
            sd = accept(sd, 0, 0);     /* accept connection */
            fp = fdopen(sd, "wr+");           /* convert into FILE* */
            pthread_create(&child, 0, servlet, fp);       /* start thread */
            pthread_detach(child);                      /* don't track it */
        }
    }
} 

Answer 1

You are shadowing the sd variable, passing an invalid socket to accept() which causes it to fail immediately.

It will likely return EBADF to signal a bad file descriptor. You would have noticed if you checked the return value in your code.

You should enable more compiler warnings, to catch things like these. With GCC you can use the -Wshadow option to enable such a warning.

Answer 2

You're not checking the return value of accept() call. Most likely it's returning an error.