20012016 Ericsson AB. All Rights Reserved. Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at http://www.apache.org/licenses/LICENSE-2.0 Unless required by applicable law or agreed to in writing, software distributed under the License is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the License for the specific language governing permissions and limitations under the License. ei_connect Jakob Cederlund ? ? 2001-09-01 A ei_connect.sgml
ei_connect Communicate with distributed erlang

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 .

The environment variable can be used to indicate which logical cluster a C node belongs to.
Timeout functions

Most functions appear in a version with the suffix 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 will be set to . With communication primitive is meant an operation on the socket, like , , or .

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

A timeout value of (zero), means that timeouts are disabled. Calling a -function with the last argument as is therefore exactly the same thing as calling the function without the 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 functions inherit all the return values and the semantics from the functions without the suffix.

intei_connect_init(ei_cnode* ec, const char* this_node_name, const char *cookie, short creation) intei_connect_xinit(ei_cnode* ec, const char *thishostname, const char *thisalivename, const char *thisnodename, Erl_IpAddr thisipaddr, const char *cookie, short creation) Initialize for a connection.

These function initializes the 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 or a file descriptor associated with a connection to another node are used.

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

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

is the cookie for the node.

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.

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

is the registered name of the process.

is the full name of the node, i.e. .

if the IP address of the host.

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

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:

Example 2:

intei_connect(ei_cnode* ec, char *nodename) intei_xconnect(ei_cnode* ec, Erl_IpAddr adr, char *alivename) Establishe a connection to an Erlang node

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 .

Example:

 intei_connect_tmo(ei_cnode* ec, char *nodename, unsigned timeout_ms) intei_xconnect_tmo(ei_cnode* ec, Erl_IpAddr adr, char *alivename, unsigned timeout_ms) Establish a connection to an Erlang node with optional timeout

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

 intei_receive(int fd, unsigned char* bufp, int bufsize) Receive a message

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

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

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 and will set to one of:

Temporary error: Try again. Buffer too small. I/O error. intei_receive_tmo(int fd, unsigned char* bufp, int bufsize, unsigned timeout_ms) Receive a message with optional timeout

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

 intei_receive_msg(int fd, erlang_msg* msg, ei_x_buff* x) intei_xreceive_msg(int fd, erlang_msg* msg, ei_x_buff* x) Receive a message

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

is an open descriptor to an Erlang connection.

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

is buffer obtained from .

On success, the function returns and the struct will be initialized. is defined as follows:

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

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

If is or , then to]]> and from]]> contain the pids of the sender and recipient of the link or unlink.

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

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

 intei_receive_msg_tmo(int fd, erlang_msg* msg, ei_x_buff* x, unsigned imeout_ms) intei_xreceive_msg_tmo(int fd, erlang_msg* msg, ei_x_buff* x, unsigned timeout_ms) Receive a message with optional timeout

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

 intei_receive_encoded(int fd, char **mbufp, int *bufsz, erlang_msg *msg, int *msglen) Obsolete function for receiving a message

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 , but instead of using an ei_x_buff, the function expects a pointer to a character pointer (), where the character pointer should point to a memory area allocated by . The argument 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 and update .

Furthermore the function returns either ERL_TICK or the field of the . The actual length of the message is put in . On error it will return a value .

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.

 intei_receive_encoded_tmo(int fd, char **mbufp, int *bufsz, erlang_msg *msg, int *msglen, unsigned timeout_ms) Obsolete function for receiving a message with timeout

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

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

This function sends an Erlang term to a process.

is an open descriptor to an Erlang connection.

is the Pid of the intended recipient of the message.

is the buffer containing the term in binary format.

is the length of the message in bytes.

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

 intei_send_tmo(int fd, erlang_pid* to, char* buf, int len, unsigned timeout_ms) Send a message with optional timeout

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

 intei_send_encoded(int fd, erlang_pid* to, char* buf, int len) Obsolete function to send a message

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

 intei_send_encoded_tmo(int fd, erlang_pid* to, char* buf, int len, unsigned timeout_ms) Obsolete function to send a message with optional timeout

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

 intei_reg_send(ei_cnode* ec, int fd, char* server_name, char* buf, int len) Send a message to a registered name

This function sends an Erlang term to a registered process.

This function sends an Erlang term to a process.

is an open descriptor to an Erlang connection.

is the registered name of the intended recipient.

is the buffer containing the term in binary format.

is the length of the message in bytes.

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

Example, send the atom "ok" to the process "worker":

 intei_reg_send_tmo(ei_cnode* ec, int fd, char* server_name, char* buf, int len, unsigned timeout_ms) Send a message to a registered name with optional timeout

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

 intei_send_reg_encoded(int fd, const erlang_pid *from, const char *to, const char *buf, int len) Obsolete function to send a message to a registered name

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 with one exception. Instead of taking the as a first argument, it takes a second argument, an which should be the process identifier of the sending process (in the erlang distribution protocol).

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

num = fd; ]]> intei_send_reg_encoded_tmo(int fd, const erlang_pid *from, const char *to, const char *buf, int len) Obsolete function to send a message to a registered name with timeout

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

 intei_rpc(ei_cnode *ec, int fd, char *mod, char *fun, const char *argbuf, int argbuflen, ei_x_buff *x) intei_rpc_to(ei_cnode *ec, int fd, char *mod, char *fun, const char *argbuf, int argbuflen) intei_rpc_from(ei_cnode *ec, int fd, int timeout, erlang_msg *msg, ei_x_buff *x) Remote Procedure Call from C to Erlang

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 the C-node structure previously initiated by a call to or

is an open descriptor to an Erlang connection.

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

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

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

structure of type and contains information on the message received. See for a description of the format.

points to the dynamic buffer that receives the result. For for this will be the result without the version magic number. For the result will return a version magic number and a 2-tuple .

returns the number of bytes in the result on success and -1 on failure. returns number of bytes or one of , and otherwise. When failing, all three functions set to one of:

I/O error. Timeout expired. Temporary error: Try again.

Example, check to see if an erlang process is alive:

 intei_publish(ei_cnode *ec, int port) Publish a node name

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 C-node structure.

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

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

To unregister with epmd, simply close the returned descriptor. Do not use , which is deprecated anyway.

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

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

 intei_publish_tmo(ei_cnode *ec, int port, unsigned timeout_ms) Publish a node name with optional timeout

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

 intei_accept(ei_cnode *ec, int listensock, ErlConnect *conp) Accept a connection from another node

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

is the C-node structure.

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 .

 intei_accept_tmo(ei_cnode *ec, int listensock, ErlConnect *conp, unsigned timeout_ms) Accept a connection from another node with optional timeout

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

 intei_unpublish(ei_cnode *ec) Forcefully unpublish a node name

This function can be called by a process to unregister a specified node from epmd on the localhost. This is however usually not allowed, unless epmd was started with the -relaxed_command_check flag, which it normally isn't.

To unregister a node you have published, you should close the descriptor that was returned by .

This function is deprecated and will be removed in a future release.

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 is to .

 intei_unpublish_tmo(ei_cnode *ec, unsigned timeout_ms) Unpublish a node name with optional timeout

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) Retrieve some values

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

They simply fetches the appropriate field from the 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) Retrieve the Pid of the C-node

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

These are convenience functions for some common name lookup functions.

 intei_get_tracelevel(void) voidei_set_tracelevel(int level) Get and set functions for tracing.

These functions are used to set tracing on the distribution. The levels are different verbosity levels. A higher level means more information. See also Debug Information and below.

and are not thread safe.

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. the environment variable is set correctly.

The connection attempt can be traced by setting a tracelevel by either using or by setting the environment variable . The different tracelevels has the following messages:

1: Verbose error messages 2: Above messages and verbose warning messages 3: Above messages and progress reports for connection handling 4: Above messages and progress reports for communication 5: Above messages and progress reports for data conversion