# 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`.

### Timeout functions

Most functions appear in a version with the suffix `_tmo` appended to the function name. Those function take an additional argument, a timeout in milliseconds. The semantics is this; for each communication primitive involved in the operation, if the primitive does not complete within the time specified, the function will return an error and `erl_errno` will be set to `ETIMEDOUT`. With communication primitive is ment an operation on the socket, like `connect`, `accept`, `recv` or `send`.

Obviously the timeouts are for implementing fault tolerance, not to keep hard realtime promises. The `_tmo` functions are for detecting non-responsive peers and to avoid blocking on socket operations.

A timeout value of `0` (zero), means that timeouts are disabled. Calling a `_tmo`-function with the last argument as `0` is therefore exactly the same thing as calling the function without the `_tmo` suffix.

As with all other ei functions, you are not expected to put the socket in non blocking mode yourself in the program. Every use of non blocking mode is embedded inside the timeout functions. The socket will always be back in blocking mode after the operations are completed (regardless of the result). To avoid problems, leave the socket options alone. Ei will handle any socket options that need modification.

In all other senses, the `_tmo` functions inherit all the return values and the semantics from the functions without the `_tmo` suffix.

### 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_connect_tmo(ei_cnode* ec, char *nodename, unsigned timeout_ms)`
`int ei_xconnect_tmo(ei_cnode* ec, Erl_IpAddr adr, char *alivename, unsigned timeout_ms)`

ei_connect and ei_xconnect with an optional timeout argument, see the description at the beginning of this document.

`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_tmo(int fd, unsigned char* bufp, int bufsize, unsigned timeout_ms)`

ei_receive with an optional timeout argument, see the description at the beginning of this document.

`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.

`int ei_receive_msg_tmo(int fd, erlang_msg* msg, ei_x_buff* x, unsigned imeout_ms)`
`int ei_xreceive_msg_tmo(int fd, erlang_msg* msg, ei_x_buff* x, unsigned timeout_ms)`

ei_receive_msg and ei_xreceive_msg with an optional timeout argument, see the description at the beginning of this document.

`int ei_receive_encoded(int fd, char **mbufp, int *bufsz, erlang_msg *msg, int *msglen)`

This function is retained for compatibility with code generated by the interface compiler and with code following examples in the same application.

In essence the function performs the same operation as `ei_xreceive_msg`, but instead of using an ei_x_buff, the function expects a pointer to a character pointer (`mbufp`), where the character pointer should point to a memory area allocated by `malloc`. The argument `bufsz` should be a pointer to an integer containing the exact size (in bytes) of the memory area. The function may reallocate the memory area and will in such cases put the new size in `*bufsz` and update `*mbufp`.

Furthermore the function returns either ERL_TICK or the `msgtype` field of the `erlang_msg *msg`. The actual length of the message is put in `*msglen`. On error it will return a value `< 0`.

It is recommended to use ei_xreceive_msg instead when possible, for the sake of readability. The function will however be retained in the interface for compatibility and will not be removed not be removed in future releases without notice.

`int ei_receive_encoded_tmo(int fd, char **mbufp, int *bufsz, erlang_msg *msg, int *msglen, unsigned timeout_ms)`

ei_receive_encoded with an optional timeout argument, see the description at the beginning of this document.

`int ei_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_send_tmo(int fd, erlang_pid* to, char* buf, int len, unsigned timeout_ms)`

ei_send with an optional timeout argument, see the description at the beginning of this document.

`int ei_send_encoded(int fd, erlang_pid* to, char* buf, int len)`

Works exactly as ei_send, the alternative name reained for backward compatibility. The function will not be removed without notice.

`int ei_send_encoded_tmo(int fd, erlang_pid* to, char* buf, int len, unsigned timeout_ms)`

ei_send_encoded with an optional timeout argument, see the description at the beginning of this document.

`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_reg_send_tmo(ei_cnode* ec, int fd, char* server_name, char* buf, int len, unsigned timeout_ms)`

ei_reg_send with an optional timeout argument, see the description at the beginning of this document.

`int ei_send_reg_encoded(int fd, const erlang_pid *from, const char *to, const char *buf, int len)`

This function is retained for compatibility with code generated by the interface compiler and with code following examples in the same application.

The function works as `ei_reg_send` with one exception. Instead of taking the `ei_cnode` as a first argument, it takes a secod argument, an `erlang_pid` which should be the process identifier of the sending process (in the erlang distribution protocol).

A suitable `erlang_pid` can be constructed from the `ei_cnode` structure by the following example code:

              ei_cnode ec;
              erlang_pid *self;
              int fd; /* the connection fd */
              ...
              self = ei_self(&ec);
              self->num = fd;

`int ei_send_reg_encoded_tmo(int fd, const erlang_pid *from, const char *to, const char *buf, int len)`

ei_send_reg_encoded with an optional timeout argument, see the description at the beginning of this document.

`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_publish_tmo(ei_cnode *ec, int port, unsigned timeout_ms)`

ei_publish with an optional timeout argument, see the description at the beginning of this document.

`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_accept_tmo(ei_cnode *ec, int listensock, ErlConnect *conp, unsigned timeout_ms)`

ei_accept with an optional timeout argument, see the description at the beginning of this document.

`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`.

`int ei_unpublish_tmo(ei_cnode *ec, unsigned timeout_ms)`

ei_unpublish with an optional timeout argument, see the description at the beginning of this document.

`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.

`struct hostent *ei_gethostbyname(const char *name)`
`struct hostent *ei_gethostbyaddr(const char *addr, int len, int type)`
`struct hostent *ei_gethostbyname_r(const char *name, struct hostent *hostp, char *buffer, int buflen, int *h_errnop)`
`struct hostent *ei_gethostbyaddr_r(const char *addr, int length, int type, struct hostent *hostp, char *buffer, int buflen, int *h_errnop)`

These are convinience functions for some common name lookup functions.

### 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 3.5.5.3
Copyright © 1991-2007 Ericsson AB