erl_connect
This module provides support for communication between distributed Erlang nodes and C nodes, in a manner that is transparent to Erlang processes.
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
.
Functions
int number;
char *cookie;
short creation;
char *host,*alive,*node;
struct in_addr *addr;
These functions initialize the erl_connect
module. In particular, they are used to identify the name of the
C-node from which they are called. One of these functions must
be called before any of the other functions in the erl_connect
module are used.
erl_connect_xinit()
stores for later use information about
the node's host name host
, alive name alive
, node
name node
, IP address addr
, cookie cookie
,
and creation number creation
. erl_connect_init()
provides an alternative interface which does not require as much
information from the caller. Instead, erl_connect_init()
uses gethostbyname()
to obtain default values.
If you use erl_connect_init()
your node will have a
short name, i.e., it will not be fully qualified. If you need to
use fully qualified (a.k.a. long) names, use
erl_connect_xinit()
instead.
host
is the name of the host on which the node is running.
alive
is the alivename of the node.
node
is the name of the node. The nodename should
be of the form alivename@hostname.
addr
is the 32-bit IP address of host
.
cookie
is the authorization string required for access
to the remote node. If NULL the user HOME directory is
searched for a cookie file .erlang.cookie
. The path to
the home directory is retrieved from the environment variable
HOME
on Unix and from the HOMEDRIVE
and
HOMEPATH
variables on Windows. Refer to the auth
module for more details.
creation
helps identify a particular instance of a C
node. In particular, it can help prevent us from receiving
messages sent to an earlier process with the same registered
name.
A C node acting as a server will be assigned a creation number
when it calls erl_publish()
.
number
is used by erl_connect_init()
to
construct the actual node name. In the second example shown
below, "c17@a.DNS.name" will be the resulting node
name.
Example 1:
struct in_addr addr; addr = inet_addr("150.236.14.75"); if (!erl_connect_xinit("chivas", "madonna", "madonna@chivas.du.etx.ericsson.se", &addr; "samplecookiestring..."), 0) erl_err_quit("<ERROR> when initializing !");
Example 2:
if (!erl_connect_init(17, "samplecookiestring...", 0)) erl_err_quit("<ERROR> when initializing !");
char *node, *alive;
struct in_addr *addr;
These functions set up a connection to an Erlang node.
erl_xconnect()
requires the IP address of the remote
host and the alive name of the remote node
to be specified. erl_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
.
#define NODE "madonna@chivas.du.etx.ericsson.se" #define ALIVE "madonna" #define IP_ADDR "150.236.14.75" /*** Variant 1 ***/ erl_connect( NODE ); /*** Variant 2 ***/ struct in_addr addr; addr = inet_addr(IP_ADDR); erl_xconnect( &addr , ALIVE );
int fd;
This function closes an open connection to an Erlang node.
Fd
is a file descriptor obtained from
erl_connect()
or erl_xconnect()
.
On success, 0 is returned. If the call fails, a non-zero value is returned, and the reason for the error can be obtained with the appropriate platform-dependent call.
int fd;
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.
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 a negative value and will set
erl_errno
to one of:
EAGAIN
- Temporary error: Try again.
EMSGSIZE
- Buffer too small.
EIO
- I/O error.
int fd;
unsigned char *bufp;
int bufsize;
ErlMessage *emsg;
This function receives the message into the specified buffer,
and decodes into the (ErlMessage *) emsg
.
fd
is an open descriptor to an Erlang connection.
bufp
is a buffer large enough to hold the expected message.
bufsize
indicates the size of bufp
.
emsg
is a pointer to an ErlMessage
structure,
into which the message will be decoded. ErlMessage
is
defined as follows:
typedef struct { int type; ETERM *msg; ETERM *to; ETERM *from; char to_name[MAXREGLEN]; } ErlMessage;
Note!
The definition of ErlMessage
has changed since
earlier versions of Erl_Interface.
type
identifies the type of message, one of
ERL_SEND
, ERL_REG_SEND
, ERL_LINK
,
ERL_UNLINK
and ERL_EXIT
.
If type
contains ERL_SEND
this indicates that an ordinary send operation has taken
place, and emsg->to
contains the Pid of the
recipient. If type
contains ERL_REG_SEND
then a
registered send operation took place, and emsg->from
contains the Pid of the sender. In both cases, the actual
message will be in emsg->msg
.
If type
contains one of ERL_LINK
or
ERL_UNLINK
, then emsg->to
and emsg->from
contain the pids of the sender and recipient of the link or unlink.
emsg->msg
is not used in these cases.
If type
contains ERL_EXIT
, then this
indicates that a link has been broken. In this case,
emsg->to
and emsg->from
contain the pids of the
linked processes, and emsg->msg
contains the reason for
the exit.
Note!
It is the caller's responsibility to release the
memory pointed to by emsg->msg
, emsg->to
and
emsg->from
.
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
indicating that the tick has been received and responded to,
but no message will be placed in the buffer. In this case you
should call erl_receive_msg()
again.
On success, the function returns ERL_MSG
and the
Emsg
struct will be initialized as described above, or
ERL_TICK
, in which case no message is returned. On
failure, the function returns ERL_ERROR
and will set
erl_errno
to one of:
EMSGSIZE
- Buffer too small.
ENOMEM
- No more memory available.
EIO
- I/O error.
int fd;
unsigned char **bufpp;
int *bufsizep;
ErlMessage *emsg;
This function is similar to erl_receive_msg
. The
difference is that erl_xreceive_msg
expects the buffer to
have been allocated by malloc
, and reallocates it if the received
message does not fit into the original buffer. For that reason,
both buffer and buffer length are given as pointers - their values
may change by the call.
On success, the function returns ERL_MSG
and the
Emsg
struct will be initialized as described above, or
ERL_TICK
, in which case no message is returned. On
failure, the function returns ERL_ERROR
and will set
erl_errno
to one of:
EMSGSIZE
- Buffer too small.
ENOMEM
- No more memory available.
EIO
- I/O error.
int fd;
ETERM *to, *msg;
This function sends an Erlang term to a process.
fd
is an open descriptor to an Erlang connection.
to
is an Erlang term containing the Pid of the
intended recipient of the message.
msg
is the Erlang term to be sent.
The function returns 1 if successful, otherwise 0 --- in
which case it will set erl_errno
to one of:
EINVAL
- Invalid argument:
to
is not a valid Erlang pid. ENOMEM
- No more memory available.
EIO
- I/O error.
int fd;
char *to;
ETERM *msg;
This function sends an Erlang term to a registered process.
fd
is an open descriptor to an Erlang connection.
to
is a string containing the registered name of
the intended recipient of the message.
msg
is the Erlang term to be sent.
The function returns 1 if successful, otherwise 0 --- in
which case it will set erl_errno
to one of:
ENOMEM
- No more memory available.
EIO
- I/O error.
int fd, timeout;
char *mod, *fun;
ETERM *args;
ErlMessage *emsg;
These functions support calling Erlang functions on remote nodes.
erl_rpc_to()
sends an rpc request to a remote node and
erl_rpc_from()
receives the results of such a call.
erl_rpc()
combines the functionality of these two functions
by sending an rpc request and waiting for the results. See also
rpc:call/4
.
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.
When erl_rpc() calls erl_rpc_from(), the call will never
timeout.
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.
args
is an Erlang list, containing the arguments to be
passed to the function.
emsg
is a message containing the result of the
function call.
The actual message returned by the rpc server
is a 2-tuple {rex,Reply}
. If you are using
erl_rpc_from()
in your code then this is the message you
will need to parse. If you are using erl_rpc()
then the
tuple itself is parsed for you, and the message returned to your
program is the erlang term containing Reply
only. Replies
to rpc requests are always ERL_SEND messages.
Note!
It is the caller's responsibility to free the returned
ETERM
structure as well as the memory pointed to by
emsg->msg
and emsg->to
.
erl_rpc()
returns the remote function's return value (or
NULL
if it failed). erl_rpc_to()
returns 0 on
success, and a negative number on failure. erl_rcp_from()
returns ERL_MSG
when successful (with Emsg
now
containing the reply tuple), and one of ERL_TICK
,
ERL_TIMEOUT
and ERL_ERROR
otherwise. When failing,
all three functions set erl_errno
to one of:
ENOMEM
- No more memory available.
EIO
- I/O error.
ETIMEDOUT
- Timeout expired.
EAGAIN
- Temporary error: Try again.
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.
port
is the local name to register, and should be the
same as the port number that was previously bound to the socket.
To unregister with epmd, simply close the returned
descriptor. See also erl_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
- I/O error
Additionally, errno
values from socket
(2)
and connect
(2) system calls may be propagated
into erl_errno
.
int listensock;
ErlConnect *conp;
This function is used by a server process to accept a connection from a client process.
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
.
These functions can be used to retrieve information about
the C Node. These values are initially set with
erl_connect_init()
or erl_connect_xinit()
.
char *alive;
This function can be called by a process to unregister a
specified node name 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 erl_publish()
.
Note!
Careless use of this function may have unpredictable results, if the registered node is in fact still running.
alive
is the name of the node to unregister, i.e., the
first component of the nodename, without the @hostname
.
If the node was successfully unregistered from epmd, the
function returns 0. Otherwise, it returns -1 and sets
erl_errno
is to EIO
.
const char *name;
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.
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
erl_interface
library.