ei_connect
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.
Functions
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