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Erlang Interface
Reference Manual
Version 3.6.5

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# 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 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.

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

### Timeout functions

Most functions appear in a version with the suffix _tmo 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 erl_errno will be set to ETIMEDOUT. With communication primitive is ment an operation on the socket, like connect, accept, recv or send.

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

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

### EXPORTS

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)

These function initializes the ec 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 ei_cnode or a file descriptor associated with a connection to another node are 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're running on. If long names are to be used, it should be fully qualified (i.e. durin.erix.ericsson.se instead of durin).

thisalivename is the registered name of the process.

thisnodename is the full name of the node, i.e. einode@durin.

thispaddr if the IP address of the host.

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

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:

    int n = 0;
    struct in_addr addr;
    ei_cnode ec;
    addr = inet_addr("150.236.14.75");
    if (ei_connect_xinit(&ec,
                         "chivas",
                         "madonna",
                         "madonna@chivas.du.etx.ericsson.se",
                         &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("ERROR when initializing: %d",erl_errno);
        exit(-1);
    }

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

These functions set 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 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.

Example:

    #define NODE   "madonna@chivas.du.etx.ericsson.se"
    #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 = inet_addr(IP_ADDR);
    fd = ei_xconnect(&ec, &addr, ALIVE);

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)

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

int ei_receive(int fd, unsigned 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. 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, 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 ERL_ERROR and will set erl_errno to one of:

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

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

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

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)

These functions 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 bigger than the preallocated 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 function returns ERL_MSG and the msg struct will be 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 ERL_SEND, ERL_REG_SEND, ERL_LINK, ERL_UNLINK and ERL_EXIT.

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

If msgtype is ERL_LINK or ERL_UNLINK, then msg->to and msg->from contain the pids of the sender and recipient of the link or unlink.

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

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

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)

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

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 should point to a memory area allocated by malloc. The argument bufsz 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 *bufsz and update *mbufp.

Furthermore the function returns either ERL_TICK or the msgtype field of the erlang_msg *msg. The actual length of the message is put in *msglen. On error it will return a value < 0.

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.

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

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

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

This function 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.

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

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

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

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

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

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

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

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

This function sends an Erlang term to a registered process.

This function sends an Erlang term to a 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.

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

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

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 the ei_cnode as a first argument, it takes a second argument, an erlang_pid which should 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)

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

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)

These functions support 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.

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 ms) to wait for results. Specify ERL_NO_TIMEOUT to wait forever. ei_rpc() will wait infinitely for the answer, i.e. 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 structure of type erlang_msg and contains information on the message received. See ei_receive_msg() for a description of the erlang_msg format.

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

EIO
  I/O error.
ETIMEDOUT
  Timeout 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();

int ei_publish(ei_cnode *ec, 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.

ec is the C-node structure.

port is the local name to register, and should 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. See also ei_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.

Additionally, 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)

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

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

This function is 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)

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

int ei_unpublish(ei_cnode *ec)

This function can be called by a process to unregister a specified node 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 ei_publish().

Note

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

ec 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 erl_errno is to EIO.

int ei_unpublish_tmo(ei_cnode *ec, unsigned timeout_ms)

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)

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

They simply fetches 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.

erlang_pid * ei_self(ei_cnode *ec)

This function 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.

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)

These are convenience functions for some common name lookup functions.

int ei_get_tracelevel(void)
void ei_set_tracelevel(int level)

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 EI_TRACELEVEL below.

ei_set_tracelevel and ei_get_tracelevel are not thread safe.

### 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 ei library.
- the environment variable ERL_EPMD_PORT is set correctly.

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