# ei_connect

### C LIBRARY

ei_connect

### C LIBRARY SUMMARY

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()`.

| 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.

### AUTHORS

Jakob Cederlund - support@erlang.ericsson.se

erl_interface 3.4
Copyright © 1991-2003 Ericsson Utvecklings AB