16.4. Connection Establishment

If we're dealing with a connection-oriented network service (SOCK_STREAM or SOCK_SEQPACKET), then before we can exchange data, we need to create a connection between the socket of the process requesting the service (the client) and the process providing the service (the server). We use the connect function to create a connection.

[View full width] #include <sys/socket.h> int connect(int sockfd, const struct sockaddr *addr, socklen_t len);
Returns: 0 if OK, 1 on error

The address we specify with connect is the address of the server with which we wish to communicate. If sockfd is not bound to an address, connect will bind a default address for the caller.

When we try to connect to a server, the connect request might fail for several reasons. The machine to which we are trying to connect must be up and running, the server must be bound to the address we are trying to contact, and there must be room in the server's pending connect queue (we'll learn more about this shortly). Thus, applications must be able to handle connect error returns that might be caused by transient conditions.

Example

Figure 16.9 shows one way to handle transient connect errors. This is likely with a server that is running on a heavily loaded system.

This function shows what is known as an exponential backoff algorithm. If the call to connect fails, the process goes to sleep for a short time and then tries again, increasing the delay each time through the loop, up to a maximum delay of about 2 minutes.

Figure 16.9. Connect with retry

#include "apue.h"
#include <sys/socket.h>

#define MAXSLEEP 128

int
connect_retry(int sockfd, const struct sockaddr *addr, socklen_t alen)
{
    int nsec;
    
    /*
     * Try to connect with exponential backoff.
     */
    for (nsec = 1; nsec <= MAXSLEEP; nsec <<= 1) {
        if (connect(sockfd, addr, alen) == 0) {
            /*
             * Connection accepted.
             */
            return(0);
        }

        /*
         * Delay before trying again.
         */
        if (nsec <= MAXSLEEP/2)
            sleep(nsec);
    }
    return(-1);
}

If the socket descriptor is in nonblocking mode, which we discuss further in Section 16.8, connect will return 1 with errno set to the special error code EINPROGRESS if the connection can't be established immediately. The application can use either poll or select to determine when the file descriptor is writable. At this point, the connection is complete.

The connect function can also be used with a connectionless network service (SOCK_DGRAM). This might seem like a contradiction, but it is an optimization instead. If we call connect with a SOCK_DGRAM socket, the destination address of all messages we send is set to the address we specified in the connect call, relieving us from having to provide the address every time we transmit a message. In addition, we will receive datagrams only from the address we've specified.

A server announces that it is willing to accept connect requests by calling the listen function.

#include <sys/socket.h> int listen(int sockfd, int backlog);
Returns: 0 if OK, 1 on error

The backlog argument provides a hint to the system of the number of outstanding connect requests that it should enqueue on behalf of the process. The actual value is determined by the system, but the upper limit is specified as SOMAXCONN in <sys/socket.h>.

On Solaris, the SOMAXCONN value in <sys/socket.h> is ignored. The particular maximum depends on the implementation of each protocol. For TCP, the default is 128.

Once the queue is full, the system will reject additional connect requests, so the backlog value must be chosen based on the expected load of the server and the amount of processing it must do to accept a connect request and start the service.

Once a server has called listen, the socket used can receive connect requests. We use the accept function to retrieve a connect request and convert that into a connection.

#include <sys/socket.h> int accept(int sockfd, struct sockaddr *restrict addr, socklen_t *restrict len);
Returns: file (socket) descriptor if OK, 1 on error

The file descriptor returned by accept is a socket descriptor that is connected to the client that called connect. This new socket descriptor has the same socket type and address family as the original socket (sockfd). The original socket passed to accept is not associated with the connection, but instead remains available to receive additional connect requests.

If we don't care about the client's identity, we can set the addr and len parameters to NULL. Otherwise, before calling accept, we need to set the addr parameter to a buffer large enough to hold the address and set the integer pointed to by len to the size of the buffer. On return, accept will fill in the client's address in the buffer and update the integer pointed to by len to reflect the size of the address.

If no connect requests are pending, accept will block until one arrives. If sockfd is in nonblocking mode, accept will return 1 and set errno to either EAGAIN or EWOULDBLOCK.

All four platforms discussed in this text define EAGAIN to be the same as EWOULDBLOCK.

If a server calls accept and no connect request is present, the server will block until one arrives. Alternatively, a server can use either poll or select to wait for a connect request to arrive. In this case, a socket with pending connect requests will appear to be readable.

Example

Figure 16.10 shows a function we can use to allocate and initialize a socket for use by a server process.

We'll see that TCP has some strange rules regarding address reuse that make this example inadequate. Figure 16.20 shows a version of this function that bypasses these rules, solving the major drawback with this version.

Figure 16.10. Initialize a socket endpoint for use by a server

#include "apue.h"
#include <errno.h>
#include <sys/socket.h>

int
initserver(int type, const struct sockaddr *addr, socklen_t alen,
  int qlen)
{
    int fd;
    int err = 0;

    if ((fd = socket(addr->sa_family, type, 0)) < 0)
        return(-1);
    if (bind(fd, addr, alen) < 0) {
        err = errno;
        goto errout;
    }
    if (type == SOCK_STREAM || type == SOCK_SEQPACKET) {
        if (listen(fd, qlen) < 0) {
            err = errno;
            goto errout;
        }
    }
    return(fd);

errout:
    close(fd);
    errno = err;
    return(-1);
}