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
.
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 in the
ei_connect
module 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 erl_publish()
or
erl_xpublish()
.
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).
Example 1:
int n = 0; struct in_addr addr; ei_cnode ec; addr = inet_addr("150.236.14.75"); if (ei_connect_xinit(&ec, "chivas", "madonna", "madonna@chivas.du.etx.ericsson.se", &addr; "cookie..."), n++) < 0) erl_err_quit("<ERROR> when initializing !");
Example 2:
if (ei_connect_init(&ec, "madonna", "cookie...", n++) < 0) erl_err_quit("<ERROR> when initializing !");
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.
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
node
is unreachableENOMEM
EIO
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 = erl_connect(&ec, NODE); /*** Variant 2 ***/ struct in_addr addr; addr = inet_addr(IP_ADDR); erl_xconnect(&ec, &addr, ALIVE);
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 erl_connect
or
erl_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
EMSGSIZE
EIO
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 receipient 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.
intei_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 erl_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_rpc(ei_cnode *ec, int fd, char *mod,
char *fun, const char* argbuf, int argbuflen, ei_x_buff* x)
int erl_rpc_to(ei_cnode *ec, int fd, char *mod,
char* fun, const char *argbu, int argbuflen)
int erl_rpc_from(ei_cnode *ec, int fd,
int timeout, erlamg_msg *msg, ei_x_buff *x)
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
.
ec
is the C-node structure.
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.
x
points to the dynamic buffer that receives the
result.
The actual message returned by the rpc server is a 2-tuple
{rex,Reply}
.
erl_rpc()
returns the number of bytes in the result
on success and -1 on failure. erl_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
ETIMEDOUT
EAGAIN
Expample, 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_with_version(&args); ei_x_encode_pid(&args, &check_pid); if (ei_rpc(&ec, fd, "erlang", "is_process_alive", args.buff, args.index, &result) < 0) error(); if (ei_decode_version(result.buff, &index) < 0 || ei_decode_bool(result.buff, &index, &is_alive) < 0) error();
int ei_publish(ei_cnode *ec, int port)
These functions are used by a server process to register
with the local name server epmd, thereby allowing
other processes to send messages by using the registered name.
Before calling either of these functions, the process should
have called bind()
and listen()
on an open socket.
ec
is the C-node structure.
port
is the local name to register, and should be the
same as the port number that was previously bound to the socket.
addr
is the 32-bit IP address of the local host.
To unregister with epmd, simply close the returned
descriptor. See also ei_unpublish()
.
On success, the functions return a descriptor connecting the
calling process to epmd. On failure, they return -1 and set
erl_errno
to EIO
.
Additionally, errno
values from socket
(2)
and connect
(2) system calls may be propagated
into erl_errno
.
int ei_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_unpublish(ei_cnode *ec)
This function can be called by a process to unregister a
specified node from epmd on the localhost. This may be
useful, for example, when epmd has not detected the failure of a
node, and will not allow the name to be reused. If you use this
function to unregister your own process, be sure to also close
the descriptor that was returned by ei_publish()
.
Careless use of this function may have unpredictable results, if the registered node is in fact still running. |
ec
is the node structure of the node to unregister.
If the node was successfully unregistered from epmd, the
function returns 0. Otherwise, it returns -1 and sets
erl_errno
is to EIO
.
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.
If a connection attempt fails, the following can be checked:
erl_errno
erl_interface
library.