ei_connect(3)

NAME
ei_connect - Communicate with distributed erlang

DESCRIPTION

This module enables C programs to communicate with erlang nodes, using the erlang distribution over TCP/IP.

A C node appears to Erlang as a hidden node. That is, Erlang processes that know the name of the C node are able to communicate with it in a normal manner, but the node name will not appear in the listing provided by the Erlang function nodes/0.

EXPORTS

int ei_connect_init(ei_cnode* ec, const char* this_node_name, const char *cookie, short creation)
int ei_connect_xinit(ei_cnode* ec, const char *thishostname, const char *thisalivename, const char *thisnodename, Erl_IpAddr thisipaddr, const char *cookie, short creation)

    These function initializes the ec structure, to identify the node name and cookie of the server. One of them has to be called before other functions that works on the type ei_cnode or a file descriptor accociated with a connection to another node are used.

    ec is a structure containing information about the C-node. It is used in other ei functions for connecting and receiving data.

    this_node_name is the registered name of the process (the name before '@').

    cookie is the cookie for the node.

    creation identifies a specific instance of a C node. It can help prevent the node from receiving messages sent to an earlier process with the same registered name.

    thishostname is the name of the machine we're running on. If long names are to be used, it should be fully qualified (i.e. durin.erix.ericsson.se instead of durin).

    thisalivename is the registered name of the process.

    thisnodename is the full name of the node, i.e. einode@durin.

    thispaddr if the IP address of the host.

    A C node acting as a server will be assigned a creation number when it calls ei_publish().

    A connection is closed by simply closing the socket. Refer to system documentation to close the socket gracefully (when there are outgoing packets before close).

    This function return a negative value indicating that an error occurred.

    Example 1:
    int n = 0;
    struct in_addr addr;
    ei_cnode ec;
    addr = inet_addr("150.236.14.75");
    if (ei_connect_xinit(&ec,
                         "chivas",
                         "madonna",
                         "madonna@chivas.du.etx.ericsson.se",
                         &addr;
                         "cookie..."),
                         n++) < 0) {
        fprintf(stderr,"ERROR when initializing: %d",erl_errno);
        exit(-1);
    }

    Example 2:
    if (ei_connect_init(&ec, "madonna", "cookie...", n++) < 0) {
        fprintf("ERROR when initializing: %d",erl_errno);
        exit(-1);
    }

int ei_connect(ei_cnode* ec, char *nodename)
int ei_xconnect(ei_cnode* ec, Erl_IpAddr adr, char *alivename)

    These functions set up a connection to an Erlang node.

    ei_xconnect() requires the IP address of the remote host and the alive name of the remote node to be specified. ei_connect() provides an alternative interface, and determines the information from the node name provided.

    addr is the 32-bit IP address of the remote host.

    alive is the alivename of the remote node.

    node is the name of the remote node.

    These functions return an open file descriptor on success, or a negative value indicating that an error occurred --- in which case they will set erl_errno to one of:

        EHOSTUNREACH:
            The remote host node is unreachable

        ENOMEM:
            No more memory available.

        EIO:
            I/O error.

    Additionally, errno values from socket(2) and connect(2) system calls may be propagated into erl_errno.

    Example:
    #define NODE   "madonna@chivas.du.etx.ericsson.se"
    #define ALIVE  "madonna"
    #define IP_ADDR "150.236.14.75"
    /*** Variant 1 ***/
    int fd = ei_connect(&ec, NODE);
    /*** Variant 2 ***/
    struct in_addr addr;
    addr = inet_addr(IP_ADDR);
    fd = ei_xconnect(&ec, &addr, ALIVE);

int ei_receive(int fd, unsigned char* bufp, int bufsize)

    This function receives a message consisting of a sequence of bytes in the Erlang external format.

    fd is an open descriptor to an Erlang connection. It is obtained from a previous ei_connect or ei_accept.

    bufp is a buffer large enough to hold the expected message.

    bufsize indicates the size of bufp.

    If a tick occurs, i.e., the Erlang node on the other end of the connection has polled this node to see if it is still alive, the function will return ERL_TICK and no message will be placed in the buffer. Also, erl_errno will be set to EAGAIN.

    On success, the message is placed in the specified buffer and the function returns the number of bytes actually read. On failure, the function returns ERL_ERROR and will set erl_errno to one of:

        EAGAIN:
            Temporary error: Try again.

        EMSGSIZE:
            Buffer too small.

        EIO:
            I/O error.

int ei_receive_msg(int fd, erlang_msg* msg, ei_x_buff* x)
int ei_xreceive_msg(int fd, erlang_msg* msg, ei_x_buff* x)

    These functions receives a message to the buffer in x. ei_xreceive_msg allows the buffer in x to grow, but ei_receive_msg fails if the message is bigger than the preallocated buffer in x.

    fd is an open descriptor to an Erlang connection.

    msg is a pointer to an erlang_msg structure and contains information on the message received.

    x is buffer obtained from ei_x_new.

    On success, the function returns ERL_MSG and the msg struct will be initialized. erlang_msg is defined as follows:
    typedef struct {
        long msgtype;
        erlang_pid from;
        erlang_pid to;
        char toname[MAXATOMLEN+1];
        char cookie[MAXATOMLEN+1];
        erlang_trace token;
    } erlang_msg;

    msgtype identifies the type of message, and is one of ERL_SEND, ERL_REG_SEND, ERL_LINK, ERL_UNLINK and ERL_EXIT.

    If msgtype is ERL_SEND this indicates that an ordinary send operation has taken place, and msg->to contains the Pid of the recipient (the C-node). If type is ERL_REG_SEND then a registered send operation took place, and msg->from contains the Pid of the sender.

    If msgtype is ERL_LINK or ERL_UNLINK, then msg->to and msg->from contain the pids of the sender and receipient of the link or unlink.

    If msgtype is ERL_EXIT, then this indicates that a link has been broken. In this case, msg->to and msg->from contain the pids of the linked processes.

    The return value is the same as for ei_receive, see above.

intei_send(int fd, erlang_pid* to, char* buf, int len)

    This function sends an Erlang term to a process.

    fd is an open descriptor to an Erlang connection.

    to is the Pid of the intended recipient of the message.

    buf is the buffer containing the term in binary format.

    len is the length of the message in bytes.

    The function returns 0 if successful, otherwise -1, in the latter case it will set erl_errno to EIO.

int ei_reg_send(ei_cnode* ec, int fd, char* server_name, char* buf, int len)

    This function sends an Erlang term to a registered process.

    This function sends an Erlang term to a process.

    fd is an open descriptor to an Erlang connection.

    server_name is the registered name of the intended recipient.

    buf is the buffer containing the term in binary format.

    len is the length of the message in bytes.

    The function returns 0 if successful, otherwise -1, in the latter case it will set erl_errno to EIO.

    Example, send the atom "ok" to the process "worker":
    ei_x_buff x;
    ei_x_new_with_version(&x);
    ei_x_encode_atom(&x, "ok");
    if (ei_reg_send(&ec, fd, x.buff, x.index) < 0)
        handle_error();

int ei_rpc(ei_cnode *ec, int fd, char *mod, char *fun, const char *argbuf, int argbuflen, ei_x_buff *x)
int ei_rpc_to(ei_cnode *ec, int fd, char *mod, char *fun, const char *argbuf, int argbuflen)
int ei_rpc_from(ei_cnode *ec, int fd, int timeout, erlang_msg *msg, ei_x_buff *x)

    These functions support calling Erlang functions on remote nodes. ei_rpc_to() sends an rpc request to a remote node and ei_rpc_from() receives the results of such a call. ei_rpc() combines the functionality of these two functions by sending an rpc request and waiting for the results. See also rpc:call/4.

    ec is the C-node structure previously initiated by a call to ei_connect_init() or ei_connect_xinit()

    fd is an open descriptor to an Erlang connection.

    timeout is the maximum time (in ms) to wait for results. Specify ERL_NO_TIMEOUT to wait forever. ei_rpc() will wait infinitely for the answer, i.e. the call will never time out.

    mod is the name of the module containing the function to be run on the remote node.

    fun is the name of the function to run.

    argbuf is a pointer to a buffer with an encoded Erlang list, without a version magic number, containing the arguments to be passed to the function.

    argbuflen is the length of the buffer containing the encoded Erlang list.

    msg structure of type erlang_msg and contains information on the message received. See ei_receive_msg() for a description of the erlang_msg format.

    x points to the dynamic buffer that receives the result. For for ei_rpc() this will be the result without the version magic number. For ei_rpc_from() the result will return a version magic number and a 2-tuple {rex, Reply}.

    ei_rpc() returns the number of bytes in the result on success and -1 on failure. ei_rpc_from() returns number of bytes or one of ERL_TICK, ERL_TIMEOUT and ERL_ERROR otherwise. When failing, all three functions set erl_errno to one of:

        EIO:
            I/O error.

        ETIMEDOUT:
            Timeout expired.

        EAGAIN:
            Temporary error: Try again.

    Example, check to see if an erlang process is alive:
    int index = 0, is_alive;
    ei_x_buff args, result;
    ei_x_new(&result);
    ei_x_new(&args);
    ei_x_encode_list_header(&args, 1);
    ei_x_encode_pid(&args, &check_pid);
    ei_x_encode_empty_list(&args);
    if (ei_rpc(&ec, fd, "erlang", "is_process_alive",
               args.buff, args.index, &result) < 0)
        handle_error();
    if (ei_decode_version(result.buff, &index) < 0
        || ei_decode_bool(result.buff, &index, &is_alive) < 0)
        handle_error();

int ei_publish(ei_cnode *ec, int port)

    These functions are used by a server process to register with the local name server epmd, thereby allowing other processes to send messages by using the registered name. Before calling either of these functions, the process should have called bind() and listen() on an open socket.

    ec is the C-node structure.

    port is the local name to register, and should be the same as the port number that was previously bound to the socket.

    addr is the 32-bit IP address of the local host.

    To unregister with epmd, simply close the returned descriptor. See also ei_unpublish().

    On success, the functions return a descriptor connecting the calling process to epmd. On failure, they return -1 and set erl_errno to EIO.

    Additionally, errno values from socket(2) and connect(2) system calls may be propagated into erl_errno.

int ei_accept(ei_cnode *ec, int listensock, ErlConnect *conp)

    This function is used by a server process to accept a connection from a client process.

    ec is the C-node structure.

    listensock is an open socket descriptor on which listen() has previously been called.

    conp is a pointer to an ErlConnect struct, described as follows:
    typedef struct {
      char ipadr[4];
      char nodename[MAXNODELEN];
    } ErlConnect;

    On success, conp is filled in with the address and node name of the connecting client and a file descriptor is returned. On failure, ERL_ERROR is returned and erl_errno is set to EIO.

int ei_unpublish(ei_cnode *ec)

    This function can be called by a process to unregister a specified node from epmd on the localhost. This may be useful, for example, when epmd has not detected the failure of a node, and will not allow the name to be reused. If you use this function to unregister your own process, be sure to also close the descriptor that was returned by ei_publish().

  Note:

Careless use of this function may have unpredictable results, if the registered node is in fact still running.

ec is the node structure of the node to unregister.

If the node was successfully unregistered from epmd, the function returns 0. Otherwise, it returns -1 and sets erl_errno is to EIO.

const char *ei_thisnodename(ei_cnode *ec)
const char *ei_thishostname(ei_cnode *ec)
const char *ei_thisalivename(ei_cnode *ec)

    These functions can be used to retrieve information about the C Node. These values are initially set with ei_connect_init() or ei_connect_xinit().

    They simply fetches the appropriate field from the ec structure. Read the field directly will probably be safe for a long time, so these functions are not really needed.

erlang_pid *ei_self(ei_cnode *ec)

    This function retrieves the Pid of the C-node. Every C-node has a (pseudo) pid used in ei_send_reg, ei_rpc and others. This is contained in a field in the ec structure. It will be safe for a long time to fetch this field directly from the ei_cnode structure.

Debug Information

If a connection attempt fails, the following can be checked:

    *   erl_errno

    *   that the right cookie was used

    *   that epmd is running

    *   the remote Erlang node on the other side is running the same version of Erlang as the ei library.

AUTHOR
Jakob Cederlund - support@erlang.ericsson.se