Network Sockets

The network-socket API provides a common interface for using sockets on network devices. It’s a class-based interface, which should be familiar to users experienced with other socket APIs.


Here is an example of an HTTP client program. The program brings up Ethernet as the underlying network interface, and uses it to perform an HTTP transaction over a TCPSocket:


#include "mbed.h"
#include "EthernetInterface.h"

// Network interface
EthernetInterface net;

// Socket demo
int main() {
    // Bring up the ethernet interface
    printf("Ethernet socket example\n");

    // Show the network address
    const char *ip = net.get_ip_address();
    printf("IP address is: %s\n", ip ? ip : "No IP");

    // Open a socket on the network interface, and create a TCP connection to
    TCPSocket socket;;
    socket.connect("", 80);

    // Send a simple http request
    char sbuffer[] = "GET / HTTP/1.1\r\nHost:\r\n\r\n";
    int scount = socket.send(sbuffer, sizeof sbuffer);
    printf("sent %d [%.*s]\n", scount, strstr(sbuffer, "\r\n")-sbuffer, sbuffer);

    // Recieve a simple http response and print out the response line
    char rbuffer[64];
    int rcount = socket.recv(rbuffer, sizeof rbuffer);
    printf("recv %d [%.*s]\n", rcount, strstr(rbuffer, "\r\n")-rbuffer, rbuffer);

    // Close the socket to return its memory and bring down the network interface

    // Bring down the ethernet interface

The Socket classes

You can use the Socket classes are used for managing network sockets. Once opened, a socket provides a pipe through which data can be sent and received to a specific endpoint. The type of the instantiated socket indicates the underlying protocol to use:

  • The UDPSocket class provides the ability to send packets of data over UDP, using the sendto and recvfrom member functions. Packets can be lost or arrive out of order, so we suggest using a TCPSocket (described below) when guaranteed delivery is required.

  • The TCPSocket class provides the ability to send a stream of data over TCP. TCPSockets maintain a stateful connection that starts with the connect member function. After successfully connecting to a server, you can use the send and recv member functions to send and receive data (similar to writing or reading from a file).

  • The TCPServer class provides the ability to accept incoming TCP connections. The listen member function sets up the server to listen for incoming connections, and the accept member function sets up a stateful TCPSocket instance on an incoming connection.

The NetworkInterface classes

A socket requires a NetworkInterface instance when opened to indicate which NetworkInterface the socket should be created on. The NetworkInterface provides a network stack that implements the underlying socket operations.

Existing network interfaces:

The SocketAddress class

Use the SocketAddress class to represent the IP address and port pair of a unique network endpoint. Most network functions are also overloaded to accept string representations of IP addresses, but SocketAddress can be used to avoid the overhead of parsing IP addresses during repeated network transactions, and can be passed around as a first class object.

Network errors

The convention of the network-socket API is for functions to return negative error codes to indicate failure. On success, a function may return zero or a non-negative integer to indicate the size of a transaction. On failure, a function must return a negative integer, which should be one of the error codes in the nsapi_error_t enum (here):

/** Enum of standardized error codes
 *  Valid error codes have negative values and may
 *  be returned by any network operation.
 *  @enum nsapi_error
enum nsapi_error {
    NSAPI_ERROR_OK                  =  0,        /*!< no error */
    NSAPI_ERROR_WOULD_BLOCK         = -3001,     /*!< no data is not available but call is non-blocking */
    NSAPI_ERROR_UNSUPPORTED         = -3002,     /*!< unsupported functionality */
    NSAPI_ERROR_PARAMETER           = -3003,     /*!< invalid configuration */
    NSAPI_ERROR_NO_CONNECTION       = -3004,     /*!< not connected to a network */
    NSAPI_ERROR_NO_SOCKET           = -3005,     /*!< socket not available for use */
    NSAPI_ERROR_NO_ADDRESS          = -3006,     /*!< IP address is not known */
    NSAPI_ERROR_NO_MEMORY           = -3007,     /*!< memory resource not available */
    NSAPI_ERROR_NO_SSID             = -3008,     /*!< ssid not found */
    NSAPI_ERROR_DNS_FAILURE         = -3009,     /*!< DNS failed to complete successfully */
    NSAPI_ERROR_DHCP_FAILURE        = -3010,     /*!< DHCP failed to complete successfully */
    NSAPI_ERROR_AUTH_FAILURE        = -3011,     /*!< connection to access point failed */
    NSAPI_ERROR_DEVICE_ERROR        = -3012,     /*!< failure interfacing with the network processor */
    NSAPI_ERROR_IN_PROGRESS         = -3013,     /*!< operation (eg connect) in progress */
    NSAPI_ERROR_ALREADY             = -3014,     /*!< operation (eg connect) already in progress */
    NSAPI_ERROR_IS_CONNECTED        = -3015,     /*!< socket is already connected */
    NSAPI_ERROR_CONNECTION_LOST     = -3016,     /*!< connection lost */
    NSAPI_ERROR_CONNECTION_TIMEOUT  = -3017,     /*!< connection timed out */

Nonblocking operation

The network-socket API also supports nonblocking operations. The set_blocking member function changes the state of a socket. When a socket is in nonblocking mode, socket operations return NSAPI_ERROR_WOULD_BLOCK when a transaction cannot be immediately completed.

To allow efficient use of nonblocking operations, the socket classes provide an attach member function to register a callback on socket state changes. When the socket can successfully receive, send or accept, or when an error occurs, the system triggers a callback. It may call the callback spuriously without reason.

The callback may be called in interrupt context and should not perform operations such as receiving and sending calls. Do not make any read or write calls until it is on a thread.

Example applications

Here are example applications that are built on top of the network-socket API: