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NAME

       ei_connect - 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 meant 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.s_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(stderr,"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.s_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.  Do  not use
              ei_unpublish(), which is deprecated anyway.

              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 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 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, 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