ei_connect(3)

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 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.

  User Supplied Socket Implementation

By default ei supplies a TCP/IPv4 socket interface that is used when communicating. The user can however plug in his/her own IPv4 socket implementation. This, for example, in order to communicate over TLS. A user supplied socket implementation is plugged in by passing a callback structure to either ei_connect_init_ussi() or ei_connect_xinit_ussi().

All callbacks in the ei_socket_callbacks structure should return zero on success; and a posix error code on failure.

The addr argument of the listen, accept, and connect callbacks refer to appropriate address structure for currently used protocol. Currently ei only supports IPv4. That is, at this time addr always points to a struct sockaddr_in structure.

The ei_socket_callbacks structure may be enlarged in the future. All fields not set, needs to be zeroed out. Currently the following fields exist: flags
    Flags informing ei about the behaviour of the callbacks. Flags should be bitwise or:ed together. If no flag, is set, the flags field should contain 0. Currently, supported flags: EI_SCLBK_FLG_FULL_IMPL
        If set, the accept(), connect(), writev(), write(), and read() callbacks implements timeouts. The timeout is passed in the tmo argument and is given in milli seconds. Note that the tmo argument to these callbacks differ from the timeout arguments in the ei API. Zero means a zero timeout. That is, poll and timeout immediately unless the operation is successful. EI_SCLBK_INF_TMO (max unsigned) means infinite timeout. The file descriptor is in blocking mode when a callback is called, and it must be in blocking mode when the callback returns.

        If not set, ei will implement the timeout using select() in order to determine when to call the callbacks and when to time out. The tmo arguments of the accept(), connect(), writev(), write(), and read() callbacks should be ignored. The callbacks may be called in non-blocking mode. The callbacks are not allowed to change between blocking and non-blocking mode. In order for this to work, select() needs to interact with the socket primitives used the same way as it interacts with the ordinary socket primitives. If this is not the case, the callbacks need to implement timeouts and this flag should be set.

    More flags may be introduced in the future.
int (*socket)(void **ctx, void *setup_ctx)
    Create a socket and a context for the socket.

    On success it should set *ctx to point to a context for the created socket. This context will be passed to all other socket callbacks. This function will be passed the same setup_context as passed to the preceding ei_connect_init_ussi() or ei_connect_xinit_ussi() call.

    NOTE: During the lifetime of a socket, the pointer *ctx has to remain the same. That is, it cannot later be relocated.

    This callback is mandatory.
int (*close)(void *ctx)
    Close the socket identified by ctx and destroy the context.

    This callback is mandatory.
int (*listen)(void *ctx, void *addr, int *len, int backlog)
    Bind the socket identified by ctx to a local interface and then listen on it.

    The addr and len arguments are both input and output arguments. When called addr points to an address structure of length *len containing information on how to bind the socket. Upon return this callback should have updated the structure referred by addr with information on how the socket actually was bound. *len should be updated to reflect the size of *addr updated. backlog identifies the size of the backlog for the listen socket.

    This callback is mandatory.
int (*accept)(void **ctx, void *addr, int *len, unsigned tmo)
    Accept connections on the listen socket identified by *ctx.

    When a connection is accepted, a new context for the accepted connection should be created and *ctx should be updated to point to the new context for the accepted connection. When called addr points to an uninitialized address structure of length *len. Upon return this callback should have updated this structure with information about the client address. *len should be updated to reflect the size of *addr updated.

    If the EI_SCLBK_FLG_FULL_IMPL flag has been set, tmo contains timeout time in milliseconds.

    NOTE: During the lifetime of a socket, the pointer *ctx has to remain the same. That is, it cannot later be relocated.

    This callback is mandatory.
int (*connect)(void *ctx, void *addr, int len, unsigned tmo)
    Connect the socket identified by ctx to the address identified by addr.

    When called addr points to an address structure of length len containing information on where to connect.

    If the EI_SCLBK_FLG_FULL_IMPL flag has been set, tmo contains timeout time in milliseconds.

    This callback is mandatory.
int (*writev)(void *ctx, const void *iov, long iovcnt, ssize_t *len, unsigned tmo)
    Write data on the connected socket identified by ctx.

    iov points to an array of struct iovec structures of length iovcnt containing data to write to the socket. On success, this callback should set *len to the amount of bytes successfully written on the socket.

    If the EI_SCLBK_FLG_FULL_IMPL flag has been set, tmo contains timeout time in milliseconds.

    This callback is optional. Set the writev field in the the ei_socket_callbacks structure to NULL if not implemented.
int (*write)(void *ctx, const char *buf, ssize_t *len, unsigned tmo)
    Write data on the connected socket identified by ctx.

    When called buf points to a buffer of length *len containing the data to write on the socket. On success, this callback should set *len to the amount of bytes successfully written on the socket.

    If the EI_SCLBK_FLG_FULL_IMPL flag has been set, tmo contains timeout time in milliseconds.

    This callback is mandatory.
int (*read)(void *ctx, char *buf, ssize_t *len, unsigned tmo)
    Read data on the connected socket identified by ctx.

    buf points to a buffer of length *len where the read data should be placed. On success, this callback should update *len to the amount of bytes successfully read on the socket.

    If the EI_SCLBK_FLG_FULL_IMPL flag has been set, tmo contains timeout time in milliseconds.

    This callback is mandatory.
int (*handshake_packet_header_size)(void *ctx, int *sz)
    Inform about handshake packet header size to use during the Erlang distribution handshake.

    On success, *sz should be set to the handshake packet header size to use. Valid values are 2 and 4. Erlang TCP distribution use a handshake packet size of 2 and Erlang TLS distribution use a handshake packet size of 4.

    This callback is mandatory.
int (*connect_handshake_complete)(void *ctx)
    Called when a locally started handshake has completed successfully.

    This callback is optional. Set the connect_handshake_complete field in the ei_socket_callbacks structure to NULL if not implemented.
int (*accept_handshake_complete)(void *ctx)
    Called when a remotely started handshake has completed successfully.

    This callback is optional. Set the accept_handshake_complete field in the ei_socket_callbacks structure to NULL if not implemented.
int (*get_fd)(void *ctx, int *fd)
    Inform about file descriptor used by the socket which is identified by ctx.

    NOTE: During the lifetime of a socket, the file descriptor has to remain the same. That is, repeated calls to this callback with the same context should always report the same file descriptor. The file descriptor has to be a real file descriptor. That is, no other operation should be able to get the same file descriptor until it has been released by the close() callback.

    This callback is mandatory.

  Data Types
ei_cnode
    Opaque data type representing a C-node. A ei_cnode structure is initialized by calling ei_connect_init() or friends.

    • ei_socket_callbacks

        typedef struct {
            int flags;
            int (*socket)(void **ctx, void *setup_ctx);
            int	(*close)(void *ctx);
            int (*listen)(void *ctx, void *addr, int *len, int backlog);
            int (*accept)(void **ctx, void *addr, int *len, unsigned tmo);
            int (*connect)(void *ctx, void *addr, int len, unsigned tmo);
            int (*writev)(void *ctx, const void *iov, int iovcnt, ssize_t *len, unsigned tmo);
            int (*write)(void *ctx, const char *buf, ssize_t *len, unsigned tmo);
            int (*read)(void *ctx, char *buf, ssize_t *len, unsigned tmo);
            int (*handshake_packet_header_size)(void *ctx, int *sz);
            int (*connect_handshake_complete)(void *ctx);
            int (*accept_handshake_complete)(void *ctx);
            int (*get_fd)(void *ctx, int *fd);
        } ei_socket_callbacks;

    Callbacks functions for a User Supplied Socket Implementation. Documentation of each field can be found in the User Supplied Socket Implementation section above.

    • ErlConnect

        typedef struct {
            char ipadr[4]; /* Ip v4 address in network byte order */
            char nodename[MAXNODELEN];
        } ErlConnect;

    IP v4 address and nodename.

    • Erl_IpAddr

        typedef struct {
            unsigned s_addr; /* Ip v4 address in network byte order */
        } Erl_IpAddr;

    IP v4 address.

    • erlang_msg

        typedef struct {
            long msgtype;
            erlang_pid from;
            erlang_pid to;
            char toname[MAXATOMLEN+1];
            char cookie[MAXATOMLEN+1];
            erlang_trace token;
        } erlang_msg;

    Information about a message received via ei_receive_msg() or friends.

  ei_gethostbyaddr()

  ei_gethostbyaddr_r()

  ei_gethostbyname()

  ei_gethostbyname_r()

    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.

  ei_accept()

    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.

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.

  ei_accept_tmo()

    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.

  ei_close_connection()

    int ei_close_connection(int fd);

Closes a previously opened connection or listen socket.

Available since OTP 21.3

  ei_connect()

  ei_xconnect()

  ei_connect_host_port()

Available since OTP 23.0

  ei_xconnect_host_port()

    int ei_connect(ei_cnode* ec, char *nodename);

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

    int ei_connect_host_port(ei_cnode* ec, char *hostname, int port);

    int ei_xconnect_host_port(ei_cnode* ec, Erl_IpAddr adr, int port);

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. The ei_xconnect_host_port() function provides yet another alternative that will work even if there is no EPMD instance on the host where the remote node is running. The ei_xconnect_host_port() function requires the IP address and port of the remote node to be specified. The ei_connect_host_port() function is an alternative to ei_xconnect_host_port() that lets the user specify a hostname instead of an IP address.

    • adr 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.

    • port is the port number 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   "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);

Available since OTP 23.0

  ei_connect_init()

  ei_connect_init_ussi()

Available since OTP 21.3

  ei_connect_xinit()

  ei_connect_xinit_ussi()

    int ei_connect_init(ei_cnode* ec, const char* this_node_name, const char *cookie, unsigned creation);

    int ei_connect_init_ussi(ei_cnode* ec, const char* this_node_name, const char *cookie,
      unsigned creation, ei_socket_callbacks *cbs, int cbs_sz, void *setup_context);

    int ei_connect_xinit(ei_cnode* ec, const char *thishostname, const char *thisalivename,
      const char *thisnodename, Erl_IpAddr thisipaddr, const char *cookie, unsigned creation);

    int ei_connect_xinit_ussi(ei_cnode* ec, const char *thishostname, const char *thisalivename,
      const char *thisnodename, Erl_IpAddr thisipaddr, const char *cookie, unsigned creation,
      ei_socket_callbacks *cbs, int cbs_sz, void *setup_context);

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 name of the C-node (the name before '@' in the full node name).

    • 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.

    NOTE: The type of the creation argument was changed from short (16 bit) to unsigned int (32 bit) in OTP 25. This should cause no practical problem other than maybe a compiler warning.

    • 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 name of the local C-node (the name before '@' in the full node name). Can be NULL (from OTP 23) to get a dynamically assigned name from the peer node.

    • thisnodename is the full name of the local C-node, that is, mynode@myhost. Can be NULL if thisalivename is NULL.

    • thispaddr if the IP address of the host.

    • cbs is a pointer to a callback structure implementing and alternative socket interface.

    • cbs_sz is the size of the structure pointed to by cbs.

    • setup_context is a pointer to a structure that will be passed as second argument to the socket callback in the cbs structure.

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:

    unsigned 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);
    }

Available since OTP 21.3

  ei_connect_tmo()

  ei_xconnect_tmo()

  ei_connect_host_port_tmo()

Available since OTP 23.0

  ei_xconnect_host_port_tmo()

    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);

    int ei_connect_host_port_tmo(ei_cnode* ec, char *hostname, int port, unsigned ms);

    int ei_xconnect_host_port_tmo(ei_cnode* ec, Erl_IpAddr adr, int port, unsigned ms);

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

Available since OTP 23.0

  ei_get_tracelevel()

Available since OTP R13B04

  ei_set_tracelevel()

    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.

Available since OTP R13B04

  ei_listen()

Available since OTP 21.3

  ei_xlisten()

    int ei_listen(ei_cnode *ec, int *port, int backlog);

    int ei_xlisten(ei_cnode *ec, Erl_IpAddr adr, int *port, int backlog);

Used by a server process to setup a listen socket which later can be used for accepting connections from client processes.

    • ec is the C-node structure.

    • adr is local interface to bind to.

    • port is a pointer to an integer containing the port number to bind to. If *port equals 0 when calling ei_listen(), the socket will be bound to an ephemeral port. On success, ei_listen() will update the value of *port to the port actually bound to.

    • backlog is maximum backlog of pending connections.

ei_listen will create a socket, bind to a port on the local interface identified by adr (or all local interfaces if ei_listen() is called), and mark the socket as a passive socket (that is, a socket that will be used for accepting incoming connections).

On success, a file descriptor is returned which can be used in a call to ei_accept(). On failure, ERL_ERROR is returned and erl_errno is set to EIO.

Available since OTP 21.3

  ei_make_pid()

    int ei_make_pid(ei_cnode *ec, erlang_pid *pid);

Creates a new process identifier in the argument pid. This process identifier refers to a conseptual process residing on the C-node identified by the argument ec. On success 0 is returned. On failure ERL_ERROR is returned and erl_errno is set.

The C-node identified by ec must have been initialized and must have received a name prior to the call to ei_make_pid(). Initialization of the C-node is done by a call to ei_connect_init() or friends. If the name is dynamically assigned from the peer node, the C-node also has to be connected.

Available since OTP 23.0

  ei_make_ref()

    int ei_make_ref(ei_cnode *ec, erlang_ref *ref);

Creates a new reference in the argument ref. This reference originates from the C-node identified by the argument ec. On success 0 is returned. On failure ERL_ERROR is returned and erl_errno is set.

The C-node identified by ec must have been initialized and must have received a name prior to the call to ei_make_ref(). Initialization of the C-node is done by a call to ei_connect_init() or friends. If the name is dynamically assigned from the peer node, the C-node also has to be connected.

Available since OTP 23.0

  ei_publish()

    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.

  ei_publish_tmo()

    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.

  ei_receive()

    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.

  ei_receive_encoded()

    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.

  ei_receive_encoded_tmo()

    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.

  ei_receive_msg()

  ei_xreceive_msg()

    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.

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.

  ei_receive_msg_tmo()

  ei_xreceive_msg_tmo()

    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.

  ei_receive_tmo()

    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.

  ei_reg_send()

    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();

  ei_reg_send_tmo()

    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.

  ei_rpc()

  ei_rpc_to()

  ei_xrpc_to()

Available since OTP 24.0

  ei_rpc_from()

    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_xrpc_to(ei_cnode *ec, int fd, char *mod, char *fun, const char *argbuf,
      int argbuflen, int flags);

    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.

The ei_xrpc_to() function is equivalent to ei_rpc_to() when its flags parameter is set to 0. When the flags parameter of ei_xrpc_to() is set to EI_RPC_FETCH_STDOUT, stdout (standard output) data are forwarded. See the documentation for the flags parameter for more information about the EI_RPC_FETCH_STDOUT flag.

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 an ei_rpc_from() call the result consists of a version magic number and a 2-tuple. The 2-tuple can be in one of the following two forms: {rex,Reply}
        This response value means that the RPC has completed. The result value is the Reply term. This is the only type of response that one can get from an RPC triggered by a call to ei_rpc_to() or ei_xrpc_to() without the EI_RPC_FETCH_STDOUT flag. If the RPC was triggered by a call to ei_xrpc_to() with the EI_RPC_FETCH_STDOUT flag set, then all forwarded stdout data has been received.
    {rex_stdout,StdOutUTF8Binary}
        This response value can only be obtained if the RPC call was triggered by a call to ei_xrpc_to() with the EI_RPC_FETCH_STDOUT flag set. This response value means that forwarded stdout data has been received. The stdout data is stored in a binary and is UTF-8 encoded. One may need to call ei_rpc_from() multiple times to read all the stdout data. The stdout data is received in the same order as it was written. All forwarded stdout data have been received when a {rex,Reply} tuple has been obtained from an ei_rpc_from() call.

    • flags The flag EI_RPC_FETCH_STDOUT is currently the only flag that is supported by ei_xrpc_to(). When EI_RPC_FETCH_STDOUT is set, the called function is executed in a new process with a group leader that forwards all stdout data. This means that stdout data that are written during the execution of the called function, by the called function and by descendant processes, will be forwarded (given that the group leader has not been changed by a call to erlang:group_leader/2). The forwarded stdout data need to be collected by a sequence of calls to ei_rpc_from(). See the description of the x parameter for how ei_rpc_from() is used to receive stdout data. See the documentation of the the I/O protocol, for more information about the group leader concept.

    NOTE: The flag EI_RPC_FETCH_STDOUT only works when interacting with a node with a version greater or equal to OTP-24.

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. The functions ei_rpc_to() and ei_xrpc_to() returns 0 if successful, otherwise -1. When failing, all four 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();

  ei_self()

    erlang_pid * ei_self(ei_cnode *ec);

Retrieves a generic 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. Do not modify this structure.

On success a pointer to the process identifier is returned. On failure NULL is returned and erl_errno is set.

The C-node identified by ec must have been initialized and must have received a name prior to the call to ei_self(). Initialization of the C-node is done by a call to ei_connect_init() or friends. If the name is dynamically assigned from the peer node, the C-node also has to be connected.

  ei_send()

    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.

  ei_send_encoded()

    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.

  ei_send_encoded_tmo()

    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.

  ei_send_reg_encoded()

    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 retrieved from the ei_cnode structure by calling ei_self(cnode_pointer).

  ei_send_reg_encoded_tmo()

    int ei_send_reg_encoded_tmo(int fd, const erlang_pid *from, const char *to, const char *buf,
      int len, unsigned timeout_ms);

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

  ei_send_tmo()

    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.

  ei_thisnodename()

  ei_thishostname()

  ei_thisalivename()

    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.

  ei_unpublish()

    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.

  ei_unpublish_tmo()

    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