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ei_connect(3erl) | C Library Functions | ei_connect(3erl) |
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.EXPORTS¶
struct hostent *ei_gethostbyaddr(const char *addr, int len, int type)
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.
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)
- *
- 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;
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)
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.
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:
Also, errno values from socket(2) and
connect(2) system calls may be propagated into erl_errno.
Example:
int ei_connect_init(ei_cnode* ec, const char* this_node_name, const char
*cookie, short creation)
- *
- 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.
- EHOSTUNREACH:
- The remote host node is unreachable.
- ENOMEM:
- No more memory is available.
- EIO:
- I/O error.
#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);
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.
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 ei_connect_tmo(ei_cnode* ec, char *nodename, unsigned timeout_ms)
- *
- 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.
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); }
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)
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.
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)
- *
- 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.
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.
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:
int ei_receive_encoded(int fd, char **mbufp, int *bufsz, erlang_msg *msg, int
*msglen)
- *
- 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.
- EAGAIN:
- Temporary error: Try again.
- EMSGSIZE:
- Buffer is too small.
- EIO:
- I/O error.
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)
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.
On success, the functions return ERL_MSG and the msg struct is
initialized. erlang_msg is defined as follows:
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)
- *
- 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.
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.
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.
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":
int ei_reg_send_tmo(ei_cnode* ec, int fd, char* server_name, char* buf, int
len, unsigned timeout_ms)
- *
- 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.
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();
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)
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.
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:
Example:
Check to see if an Erlang process is alive:
erlang_pid *ei_self(ei_cnode *ec)
- *
- 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}.
- EIO:
- I/O error.
- ETIMEDOUT:
- Time-out expired.
- EAGAIN:
- Temporary error: Try again.
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();
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.
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)
- *
- 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.
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:
int ei_send_reg_encoded_tmo(int fd, const erlang_pid *from, const char *to,
const char *buf, int len)
ei_cnode ec; erlang_pid *self; int fd; /* the connection fd */ ... self = ei_self(&ec); self->num = fd;
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)
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().
int ei_unpublish_tmo(ei_cnode *ec, unsigned timeout_ms)
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.
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
- *
- 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
erl_interface 3.9.2 | Ericsson AB |