19962009 Ericsson AB. All Rights Reserved. The contents of this file are subject to the Erlang Public License, Version 1.1, (the "License"); you may not use this file except in compliance with the License. You should have received a copy of the Erlang Public License along with this software. If not, it can be retrieved online at http://www.erlang.org/. Software distributed under the License is distributed on an "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License for the specific language governing rights and limitations under the License. erl_connect Torbjörn Törnkvist Torbjörn Törnkvist Bjarne Däcker Torbjörn Törnkvist 980703 A erl_connect.sgml
erl_connect Communicate with Distributed Erlang

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 .

 interl_connect_init(number, cookie, creation) interl_connect_xinit(host, alive, node, addr, cookie, creation) Initialize communication int number; char *cookie; short creation; char *host,*alive,*node; struct in_addr *addr;

These functions initialize the 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.

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

If you use 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 instead.

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

is the alivename of the node.

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

is the 32-bit IP address of .

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

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 .

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

Example 1:

when initializing !"); ]]>

Example 2:

when initializing !"); ]]> interl_connect(node) interl_xconnect(addr, alive) Establishe a connection to an Erlang node char *node, *alive; struct in_addr *addr;

These functions set up a connection to an Erlang node.

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

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

is the alivename of the remote 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 to one of:

The remote host is unreachable No more memory available. I/O error.

Additionally, values from (2) and (2) system calls may be propagated into .

 interl_close_connection(fd) Close a connection to an Erlang node int fd;

This function closes an open connection to an Erlang node.

is a file descriptor obtained from or .

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.

 interl_receive(fd, bufp, bufsize) Receive a message int fd; char *bufp; int bufsize;

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

is an open descriptor to an Erlang connection.

is a buffer large enough to hold the expected message.

indicates the size of .

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 and no message will be placed in the buffer. Also, will be set to .

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 to one of:

Temporary error: Try again. Buffer too small. I/O error. interl_receive_msg(fd, bufp, bufsize, emsg) Receive and decodes a message int fd; unsigned char *bufp; int bufsize; ErlMessage *emsg;

This function receives the message into the specified buffer, and decodes into the .

is an open descriptor to an Erlang connection.

is a buffer large enough to hold the expected message.

indicates the size of .

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

The definition of has changed since earlier versions of Erl_Interface.

identifies the type of message, one of , , , and .

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

If contains one of or , then to]]> and from]]> contain the pids of the sender and recipient of the link or unlink. msg]]> is not used in these cases.

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

It is the caller's responsibility to release the memory pointed to by msg]]>, to]]> and 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 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 again.

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

Buffer too small. No more memory available. I/O error. interl_xreceive_msg(fd, bufpp, bufsizep, emsg) Receive and decodes a message int fd; unsigned char **bufpp; int *bufsizep; ErlMessage *emsg;

This function is similar to . The difference is that expects the buffer to have been allocated by , 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 and the struct will be initialized as described above, or , in which case no message is returned. On failure, the function returns and will set to one of:

Buffer too small. No more memory available. I/O error. interl_send(fd, to, msg) Send a message int fd; ETERM *to, *msg;

This function sends an Erlang term to a process.

is an open descriptor to an Erlang connection.

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

is the Erlang term to be sent.

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

Invalid argument: is not a valid Erlang pid. No more memory available. I/O error. interl_reg_send(fd, to, msg) Send a message to a registered name int fd; char *to; ETERM *msg;

This function sends an Erlang term to a registered process.

is an open descriptor to an Erlang connection.

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

is the Erlang term to be sent.

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

No more memory available. I/O error. ETERM *erl_rpc(fd, mod, fun, args) interl_rpc_to(fd, mod, fun, args) interl_rpc_from(fd, timeout, emsg) Remote Procedure Call int fd, timeout; char *mod, *fun; ETERM *args; ErlMessage *emsg;

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

is an open descriptor to an Erlang connection.

is the maximum time (in ms) to wait for results. Specify to wait forever. When erl_rpc() calls erl_rpc_from(), the call will never timeout.

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

is the name of the function to run.

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

is a message containing the result of the function call.

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

It is the caller's responsibility to free the returned structure as well as the memory pointed to by msg]]> and to]]>.

returns the remote function's return value (or if it failed). returns 0 on success, and a negative number on failure. returns when successful (with now containing the reply tuple), and one of , and otherwise. When failing, all three functions set to one of:

No more memory available. I/O error. Timeout expired. Temporary error: Try again. interl_publish(port) Publish a node name 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 and on an open socket.

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 .

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

I/O error

Additionally, values from (2) and (2) system calls may be propagated into .

interl_accept(listensock, conp) Accept a connection int listensock; ErlConnect *conp;

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

is an open socket descriptor on which has previously been called.

is a pointer to an struct, described as follows:

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

 const char *erl_thiscookie() const char *erl_thisnodename() const char *erl_thishostname() const char *erl_thisalivename() shorterl_thiscreation() Retrieve some values

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

 interl_unpublish(alive) Unpublish a node name 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 .

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

is the name of the node to unregister, i.e., the first component of the nodename, without the .

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

 struct hostent*erl_gethostbyname(name) struct hostent*erl_gethostbyaddr(addr, length, type) struct hostent*erl_gethostbyname_r(name, hostp, buffer, buflen, h_errnop) struct hostent*erl_gethostbyaddr_r(addr, length, type, hostp, buffer, buflen, h_errnop) Name lookup functions const char *name; const char *addr; int length; int type; struct hostent *hostp; char *buffer; int buflen; int *h_errnop;

These are convenience functions for some common name lookup functions.
Debug Information

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

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 library.