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.xml
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 can communicate with it in a normal manner, but the node name is not shown in the listing provided by erlang:nodes/0 in ERTS.

The environment variable can be used to indicate which logical cluster a C-node belongs to.
Time-Out Functions

Most functions appear in a version with the suffix appended to the function name. Those functions take an extra argument, a time-out 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 returns an error and is set to . With communication primitive is meant an operation on the socket, like , , , or .

Clearly the time-outs are for implementing fault tolerance, not to keep hard real-time promises. The functions are for detecting non-responsive peers and to avoid blocking on socket operations.

A time-out value of (zero) means that time-outs are disabled. Calling a function with the last argument as is therefore the same thing as calling the function without the suffix.

As with all other functions starting with ei_, 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 time-out 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 handles 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.

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

Convenience functions for some common name lookup functions.

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

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 time-out.

Equivalent to ei_accept with an optional time-out argument, see the description at the beginning of this manual page.

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

Sets 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 the latter case they set to one of the following:

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

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

Example:

 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.

Initializes the structure, to identify the node name and cookie of the server. One of them must be called before other functions that works on the type or a file descriptor associated with a connection to another node is 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 are running on. If long names are to be used, they are to be fully qualified (that is, instead of ).

is the registered name of the process.

is the full name of the node, that is, .

if the IP address of the host.

A C-node acting as a server is assigned a creation number when it calls .

A connection is closed by simply closing the socket. For information about how to close the socket gracefully (when there are outgoing packets before close), see the relevant system documentation.

These functions return a negative value indicating that an error occurred.

Example 1:

Example 2:

 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 time-out.

Equivalent to ei_connect and ei_xconnect with an optional time-out argument, see the description at the beginning of this manual page.

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

Used to set tracing on the distribution. The levels are different verbosity levels. A higher level means more information. See also section Debug Information.

These functions are not thread safe.

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

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 is to 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 function returns a descriptor connecting the calling process to EPMD. On failure, -1 is returned and is set to .

Also, 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 time-out.

Equivalent to ei_publish with an optional time-out argument, see the description at the beginning of this manual page.

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

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, that is, the Erlang node on the other end of the connection has polled this node to see if it is still alive, the function returns and no message is placed in the buffer. Also, is 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 sets to one of the following:

Temporary error: Try again. Buffer is too small. I/O error. 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 is to point to a memory area allocated by . Argument is to 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 .

Returns either ERL_TICK or the field of the . The length of the message is put in . On error a value is returned.

It is recommended to use ei_xreceive_msg instead when possible, for the sake of readability. However, the function will be retained in the interface for compatibility and will not be removed in future releases without prior 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 time-out.

Equivalent to ei_receive_encoded with an optional time-out argument, see the description at the beginning of this manual page.

 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.

Receives a message to the buffer in . allows the buffer in to grow, but fails if the message is larger than the pre-allocated 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 functions return and the struct is initialized. is defined as follows:

identifies the type of message, and is one of the following:

Indicates that an ordinary send operation has occurred. to]]> contains the pid of the recipient (the C-node).

A registered send operation occurred. from]]> contains the pid of the sender.

 or

to]]> and from]]> contain the pids of the sender and recipient of the link or unlink.

Indicates a broken link. to]]> and from]]> contain the pids of the linked processes.

The return value is the same as for ei_receive.

 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 time-out.

Equivalent to ei_receive_msg and ei_xreceive_msg with an optional time-out argument, see the description at the beginning of this manual page.

 intei_receive_tmo(int fd, unsigned char* bufp, int bufsize, unsigned timeout_ms) Receive a message with optional time-out.

Equivalent to ei_receive with an optional time-out argument, see the description at the beginning of this manual page.

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

Sends an Erlang term to a registered 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.

Returns 0 if successful, otherwise -1. In the latter case it sets 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 time-out

Equivalent to ei_reg_send with an optional time-out argument, see the description at the beginning of this manual page.

 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.

Supports 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 rpc:call/4 in Kernel.

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 milliseconds) to wait for results. Specify to wait forever. waits infinitely for the answer, that is, 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.

is structure of type and contains information on the message received. For a description of the format, see ei_receive_msg.

points to the dynamic buffer that receives the result. For this is the result without the version magic number. For the result returns a version magic number and a 2-tuple .

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

I/O error. Time-out expired. Temporary error: Try again.

Example:

Check to see if an Erlang process is alive:

 erlang_pid *ei_self(ei_cnode *ec) Retrieve the pid of the C-node.

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.

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

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.

Returns 0 if successful, otherwise -1. In the latter case it sets to .

 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 is retained for backward compatibility. The function will not be removed without prior 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 time-out.

Equivalent to ei_send_encoded with an optional time-out argument, see the description at the beginning of this manual page.

 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 as first argument, it takes a second argument, an , which is to 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 time-out.

Equivalent to ei_send_reg_encoded with an optional time-out argument, see the description at the beginning of this manual page.

 intei_send_tmo(int fd, erlang_pid* to, char* buf, int len, unsigned timeout_ms) Send a message with optional time-out.

Equivalent to ei_send with an optional time-out argument, see the description at the beginning of this manual page.

 const char *ei_thisnodename(ei_cnode *ec) const char *ei_thishostname(ei_cnode *ec) const char *ei_thisalivename(ei_cnode *ec) Retrieve some values.

Can be used to retrieve information about the C-node. These values are initially set with or .

These function simply fetch 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.

 intei_unpublish(ei_cnode *ec) Forcefully unpublish a node name.

Can be called by a process to unregister a specified node from EPMD on the local host. This is, however, usually not allowed, unless EPMD was started with flag -relaxed_command_check, which it normally is not.

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, -1 is returned and is set to .

 intei_unpublish_tmo(ei_cnode *ec, unsigned timeout_ms) Unpublish a node name with optional time-out.

Equivalent to ei_unpublish with an optional time-out argument, see the description at the beginning of this manual page.
Debug Information

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

. That the correct cookie was used That EPMD is running That the remote Erlang node on the other side is running the same version of Erlang as the library That environment variable is set correctly

The connection attempt can be traced by setting a trace level by either using or by setting environment variable . The trace levels have 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