# erl_connect

### C LIBRARY

erl_connect

### C LIBRARY SUMMARY

Communicate with Distributed Erlang

### DESCRIPTION

This module provides support for communication between distributed Erlang nodes and C nodes, in a manner that is transparent to Erlang processes.

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 erl_connect_init(number, cookie, creation)`
`int erl_connect_xinit(host, alive, node, addr, cookie, creation)`

Types:

`int number;`
`char *cookie;`
`short creation;`
`char *host,*alive,*node;`
`struct in_addr *addr;`

These functions initialize the `erl_connect` module. In particular, they are used to identify the name of the C-node from which they are called. One of these functions must be called before any of the other functions in the erl_connect module are used.

`erl_connect_xinit()` stores for later use information about the node's host name `host`, alive name `alive`, node name `node`, IP address `addr`, cookie `cookie`, and creation number `creation`. `erl_connect_init()` provides an alternative interface which does not require as much information from the caller. Instead, `erl_connect_init()` uses `gethostbyname()` to obtain default values.

If you use `erl_connect_init()` your node will have a short name, i.e., it will not be fully qualified. If you need to use fully qualified (a.k.a. long) names, use `erl_connect_xinit()` instead.

`host` is the name of the host on which the node is running.

`alive` is the alivename of the node.

`node` is the name of the node. The nodename should be of the form alivename@hostname.

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

`cookie` is the authorization string required for access to the remote node. If NULL the user HOME directory is searched for a cookie file `.erlang.cookie`. The path to the home directory is retrieved from the environment variable `HOME` on Unix and from the `HOMEDRIVE` and `HOMEPATH` variables on Windows. Refer to the `auth` module for more details.

`creation` helps identify a particular instance of a C node. In particular, it can help prevent us from receiving messages sent to an earlier process with the same registered name.

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

`number` is used by `erl_connect_init()` to construct the actual node name. In the second example shown below, "c17@a.DNS.name" will be the resulting node name.

Example 1:

    struct in_addr addr;
    addr = inet_addr("150.236.14.75");
    if (!erl_connect_xinit("chivas",
                           "madonna",
                           "madonna@chivas.du.etx.ericsson.se",
                           &addr;
                           "samplecookiestring..."),
                           0)
      erl_err_quit("<ERROR> when initializing !");

Example 2:

    if (!erl_connect_init(17, "samplecookiestring...", 0))
      erl_err_quit("<ERROR> when initializing !");

`int erl_connect(node)`
`int erl_xconnect(addr, alive)`

Types:

`char *node, *alive;`
`struct in_addr *addr;`

These functions set up a connection to an Erlang node.

`erl_xconnect()` requires the IP address of the remote host and the alive name of the remote node to be specified. `erl_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`.

    #define NODE   "madonna@chivas.du.etx.ericsson.se"
    #define ALIVE  "madonna"
    #define IP_ADDR "150.236.14.75"

    /*** Variant 1 ***/
    erl_connect( NODE );

    /*** Variant 2 ***/
    struct in_addr addr;
    addr = inet_addr(IP_ADDR);
    erl_xconnect( &addr , ALIVE );

`int erl_close_connection(fd)`

Types:

`int fd;`

This function closes an open connection to an Erlang node.

`Fd` is a file descriptor obtained from `erl_connect()` or `erl_xconnect()`.

On success, 0 is returned. If the call fails, a non-zero value is returned, and the reason for the error can be obtained with the appropriate platform-dependent call.

`int erl_receive(fd, bufp, bufsize)`

Types:

`int fd;`
`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.

`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 a negative value and will set `erl_errno` to one of:

`EAGAIN`
  Temporary error: Try again.
`EMSGSIZE`
  Buffer too small.
`EIO`
  I/O error.

`int erl_receive_msg(fd, bufp, bufsize, emsg)`

Types:

`int fd;`
`unsigned char *bufp;`
`int bufsize;`
`ErlMessage *emsg;`

This function receives the message into the specified buffer, and decodes into the `(ErlMessage *) emsg`.

`fd` is an open descriptor to an Erlang connection.

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

`bufsize` indicates the size of `bufp`.

`emsg` is a pointer to an `ErlMessage` structure, into which the message will be decoded. `ErlMessage` is defined as follows:

    typedef struct {
      int type;
      ETERM *msg;
      ETERM *to;
      ETERM *from;
      char to_name[MAXREGLEN];
    } ErlMessage;

| The definition of `ErlMessage` has changed since earlier versions of Erl_Interface.

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

If `type` contains `ERL_SEND` this indicates that an ordinary send operation has taken place, and `emsg->to` contains the Pid of the recipient. If `type` contains `ERL_REG_SEND` then a registered send operation took place, and `emsg->from` contains the Pid of the sender. In both cases, the actual message will be in `emsg->msg`.

If `type` contains one of `ERL_LINK` or `ERL_UNLINK`, then `emsg->to` and `emsg->from` contain the pids of the sender and receipient of the link or unlink. `emsg->msg` is not used in these cases.

If `type` contains `ERL_EXIT`, then this indicates that a link has been broken. In this case, `emsg->to` and `emsg->from` contain the pids of the linked processes, and `emsg->msg` contains the reason for the exit.

| It is the caller's responsibility to release the memory pointed to by `emsg->msg`, `emsg->to` and `emsg->from`.

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` indicating that the tick has been received and responded to, but no message will be placed in the buffer. In this case you should call `erl_receive_msg()` again.

On success, the function returns `ERL_MSG` and the `Emsg` struct will be initialized as described above, or `ERL_TICK`, in which case no message is returned. On failure, the function returns `ERL_ERROR` and will set `erl_errno` to one of:

`EMSGSIZE`
  Buffer too small.
`ENOMEM`
  No more memory available.
`EIO`
  I/O error.

`int erl_xreceive_msg(fd, bufpp, bufsizep, emsg)`

Types:

`int fd;`
`unsigned char **bufpp;`
`int *bufsizep;`
`ErlMessage *emsg;`

This function is similar to `erl_receive_msg`. The difference is that `erl_xreceive_msg` expects the buffer to have been allocated by `malloc`, and reallocates it if the received message does not fit into the original buffer. For that reason, both buffer and buffer length are given as pointers - their values may change by the call.

On success, the function returns `ERL_MSG` and the `Emsg` struct will be initialized as described above, or `ERL_TICK`, in which case no message is returned. On failure, the function returns `ERL_ERROR` and will set `erl_errno` to one of:

`EMSGSIZE`
  Buffer too small.
`ENOMEM`
  No more memory available.
`EIO`
  I/O error.

`int erl_send(fd, to, msg)`

Types:

`int fd;`
`ETERM *to, *msg;`

This function sends an Erlang term to a process.

`fd` is an open descriptor to an Erlang connection.

`to` is an Erlang term containing the Pid of the intended recipient of the message.

`msg` is the Erlang term to be sent.

The function returns 1 if successful, otherwise 0 --- in which case it will set `erl_errno` to one of:

`EINVAL`
  Invalid argument: `to` is not a valid Erlang pid.
`ENOMEM`
  No more memory available.
`EIO`
  I/O error.

`int erl_reg_send(fd, to, msg)`

Types:

`int fd;`
`char *to;`
`ETERM *msg;`

This function sends an Erlang term to a registered process.

`fd` is an open descriptor to an Erlang connection.

`to` is a string containing the registered name of the intended recipient of the message.

`msg` is the Erlang term to be sent.

The function returns 1 if successful, otherwise 0 --- in which case it will set `erl_errno` to one of:

`ENOMEM`
  No more memory available.
`EIO`
  I/O error.

`ETERM *erl_rpc(fd, mod, fun, args)`
`int erl_rpc_to(fd, mod, fun, args)`
`int erl_rpc_from(fd, timeout, emsg)`

Types:

`int fd, timeout;`
`char *mod, *fun;`
`ETERM *args;`
`ErlMessage *emsg;`

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

`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. When erl_rpc() calls erl_rpc_from(), the call will never timeout.

`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.

`args` is an Erlang list, containing the arguments to be passed to the function.

`emsg` is a message containing the result of the function call.

The actual message returned by the rpc server is a 2-tuple `{rex,Reply}`. If you are using `erl_rpc_from()` in your code then this is the message you will need to parse. If you are using `erl_rpc()` then the tuple itself is parsed for you, and the message returned to your program is the erlang term containing `Reply` only. Replies to rpc requests are always ERL_SEND messages.

| It is the caller's responsibility to free the returned `ETERM` structure as well as the memory pointed to by `emsg->msg` and `emsg->to`.

`erl_rpc()` returns the remote function's return value (or `NULL` if it failed). `erl_rpc_to()` returns 0 on success, and a negative number on failure. `erl_rcp_from()` returns `ERL_MSG` when successful (with `Emsg` now containing the reply tuple), and one of `ERL_TICK`, `ERL_TIMEOUT` and `ERL_ERROR` otherwise. When failing, all three functions set `erl_errno` to one of:

`ENOMEM`
  No more memory available.
`EIO`
  I/O error.
`ETIMEDOUT`
  Timeout expired.
`EAGAIN`
  Temporary error: Try again.

`int erl_publish(port)`

Types:

`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.

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

To unregister with epmd, simply close the returned descriptor. See also `erl_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`
  I/O error

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

`int erl_accept(listensock, conp)`

Types:

`int listensock;`
`ErlConnect *conp;`

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

`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`.

`const char *erl_thiscookie()`
`const char *erl_thisnodename()`
`const char *erl_thishostname()`
`const char *erl_thisalivename()`
`short erl_thiscreation()`

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

`int erl_unpublish(alive)`

Types:

`char *alive;`

This function can be called by a process to unregister a specified node name 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 `erl_publish()`.

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

`alive` is the name of the node to unregister, i.e., the first component of the nodename, without the `@hostname`.

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

### 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 `erl_interface` library.

### AUTHORS

T.Törnkvist - support@erlang.ericsson.se
Gordon Beaton - support@erlang.ericsson.se

erl_interface 3.4
Copyright © 1991-2003 Ericsson Utvecklings AB