ei_connect

C library

ei_connect

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 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 ERL_EPMD_PORT 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 _tmo 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 erl_errno is set to ETIMEDOUT. With communication primitive is meant an operation on the socket, like connect, accept, recv, or send.

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

A time-out value of 0 (zero) means that time-outs are disabled. Calling a _tmo function with the last argument as 0 is therefore the same thing as calling the function without the _tmo 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 _tmo functions inherit all the return values and the semantics from the functions without the _tmo suffix.

Exports

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)

Convenience functions for some common name lookup functions.

int ei_accept(ei_cnode *ec, int listensock, ErlConnect *conp)

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)

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

int ei_connect(ei_cnode* ec, char *nodename)int ei_xconnect(ei_cnode* ec, Erl_IpAddr adr, char *alivename)

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

EHOSTUNREACH
The remote host node is unreachable.
ENOMEM
No more memory is available.
EIO
I/O error.

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

Example:

#define NODE   "[email protected]"
#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.s_addr = inet_addr(IP_ADDR);
fd = ei_xconnect(&ec, &addr, ALIVE);

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)

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

  • thisalivename is the registered name of the process.

  • thisnodename is the full name of the node, that is, einode@durin.

  • thispaddr if the IP address of the host.

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

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:

int n = 0;
struct in_addr addr;
ei_cnode ec;
addr.s_addr = inet_addr("150.236.14.75");
if (ei_connect_xinit(&ec,
                     "chivas",
                     "madonna",
                     "[email protected]",
                     &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(stderr,"ERROR when initializing: %d",erl_errno);
    exit(-1);
}

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)

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

int ei_get_tracelevel(void)void ei_set_tracelevel(int level)

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.

int ei_publish(ei_cnode *ec, int port)

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 is to 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. Do not use ei_unpublish(), which is deprecated anyway.

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

Also, 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)

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

int ei_receive(int fd, unsigned char* bufp, int bufsize)

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, 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 ERL_TICK and no message is placed in the buffer. Also, erl_errno is 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 sets erl_errno to one of the following:

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

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 is to point to a memory area allocated by malloc. Argument bufsz 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 *bufsz and update *mbufp.

Returns either ERL_TICK or the msgtype field of the erlang_msg *msg. The length of the message is put in *msglen. On error a value < 0 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.

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

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

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)

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 larger than the pre-allocated 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 functions return ERL_MSG and the msg struct is 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 the following:

ERL_SEND

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

ERL_REG_SEND

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

ERL_LINK or ERL_UNLINK

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

ERL_EXIT

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

The return value is the same as for ei_receive.

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)

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.

int ei_receive_tmo(int fd, unsigned char* bufp, int bufsize, unsigned timeout_ms)

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

int ei_reg_send(ei_cnode* ec, int fd, char* server_name, char* buf, int len)

Sends an Erlang term to a registered 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.

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

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

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)

Supports 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 in Kernel.

  • 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 milliseconds) to wait for results. Specify ERL_NO_TIMEOUT to wait forever. ei_rpc() waits infinitely for the answer, that is, 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 is structure of type erlang_msg and contains information on the message received. For a description of the erlang_msg format, see ei_receive_msg.

  • x points to the dynamic buffer that receives the result. For ei_rpc() this is the result without the version magic number. For ei_rpc_from() the result returns 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 the number of bytes, otherwise one of ERL_TICK, ERL_TIMEOUT, and ERL_ERROR. When failing, all three functions set erl_errno to one of the following:

EIO
I/O error.
ETIMEDOUT
Time-out 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();

erlang_pid *ei_self(ei_cnode *ec)

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.

int ei_send(int fd, erlang_pid* to, char* buf, int len)

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.

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

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

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

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

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

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 ei_cnode as first argument, it takes a second argument, an erlang_pid, which is to 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)

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

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

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)

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

These function simply fetch 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.

int ei_unpublish(ei_cnode *ec)

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 ei_publish().

Warning

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

ec 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 erl_errno is set to EIO.

int ei_unpublish_tmo(ei_cnode *ec, unsigned timeout_ms)

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:

  • erl_errno.
  • 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 ei library
  • That environment variable ERL_EPMD_PORT is set correctly

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

© 2010–2017 Ericsson AB
Licensed under the Apache License, Version 2.0.