[Ericsson AB]

3 How to implement an alternative carrier for the erlang distribution

This document describes how one can implement ones own carrier protocol for the erlang distibution. The distribution is normally carried by the TCP/IP protocol. Whats explained here is the method for replacing TCP/IP whith another protocol.

The document is a step by step explanation of the uds_dist example application (seated in the kernel applications examples directory). The uds_dist application implements distribution over Unix domain sockets and is written for the Sun Solaris 2 operating environment. The mechanisms are however general and applies to any operating system erlang runs on. The reason the C code is not made portable, is simply readability.

3.1 Introduction

To implement a new carrier for the erlang distribution, one must first make the protocol available to the erlang machine, which involves writing an erlang driver. There is no way one can use a port program, there has to be an erlang driver. Erlang drivers can either be statically linked to the emulator, which can be an alternative when using the open source distribution of erlang, or dynamically loaded into the erlang machines address space, which is the only alternative if a precompiled version of erlang is to be used.

Writing an erlang driver is by no means easy. The driver is written as a couple of callback functions called by the erlang emulator when data is sent to the driver or the driver has any data available on a file descriptor. As the driver callback routines execute in the main thread of the erlang machine, the callback functions can perform no blocking activity whatsoever. The callbacks should only set up file descriptors for waiting and/or read/write available data. All I/O has to be non blocking. Driver callbacks are however executed in sequence, why a global state can safely be updated within the routines.

When the driver is implemented, one would preferrably write an erlang interface for the driver to be able to test the functionality of the driver separately. This interface can then be used by the distribution module which will cover the details of the protocol from the net_kernel. The easiest path is to mimic the inet and gen_tcp interfaces, but a lot of functionality in those modules need not be implemented. In the example application, only a few of the usual interfaces are implemented, and they are much simplified.

When the protocol is available to erlang throug a driver and an erlang interface module, a distribution module can be written. The distribution module is a module with well defined callbacks, much like a gen_server (there is no compiler support for checking the callbacks though). The details of finding other nodes (i.e. talking to epmd or something similar), creating a listen port (or similar), connecting to other nodes and performing the handshakes/cookie verification are all implemented by this module. There is however a utility module, dist_util, that will do most of the hard work of handling handshakes, cookies, timers and ticking. Using dist_util makes implementing a distribution module much easier and that's what we are doing in the example application.

The last step is to create boot scripts to make the protocol implementation available at boot time. The implementation can be debugged by starting the distribution when all of the system is running, but in a real system the distribution should start very early, why a bootscript and some command line parameters are necessary. This last step also implies that the erlang code in the interface and distribution modules is written in such a way that it can be run in the startup phase. Most notably there can be no calls to the application module or to any modules not loaded at boottime (i.e. only kernel, stdlib and the application itself can be used).

3.2 The driver

Although erlang drivers in general may be beyond the scope of this document, a brief introduction seems to be in place.

3.2.1 Drivers in general

An erlang driver is a native code module written in C (or assembler) which serves as an interface for some special operating system service. This is a general mechanism that is used throughout the erlang emulator for all kinds of I/O. An erlang driver can be dynamically linked (or loaded) to the erlang emulator at runtime by using the erl_ddll erlang module. Some of the drivers in OTP are however statically linked to the runtime system, but that's more an optimization than a necessity.

The driver datatypes and the functions available to the driver writer are defined in the header file erl_driver.h (there is also an deprecated version called driver.h, dont use that one.) seated in erlang's include directory (and in $ERL_TOP/erts/emulator/beam in the source code distribution). Refer to that file for function prototypes etc.

When writing a driver to make a communications protocol avalable to erlang, one should know just about everything worth knowing about that particular protocol. All operation has to be non blocking and all possible situations should be accounted for in the driver. A non stable driver will affect and/or crash the whole erlang runtime system, which is seldom what's wanted.

The emulator calls the driver in the following situations:

3.2.2 The distribution driver's data structures

The driver used for erlang distribution should implement a reliable, order mainataining, variable length packet oriented protocol. All error correction, resending and such need to be implemented in the driver or by the underlying communications protocol. If the protocol is stream oriented (as is the case with both TCP/IP and our streamed Unix domain sockets), some mechanism for packaging is needed. We will use the simple method of having a header of four bytes containing the length of the package in a big endian 32 bit integer (as Unix domain sockets only can be used between processes on the same machine, we actually dont need to code the integer in some special endianess, but i'll do it anyway brcause in most situation you do need to do it. Unix domain sockets are reliable and order maintaining, so we dont need to implement resends and such in our driver.

Lets start writing our example Unix domain sockets driver by declaring prototypes and filling in a static ErlDrvEntry structure.

( 1) #include <stdio.h>
( 2) #include <stdlib.h>
( 3) #include <string.h>
( 4) #include <unistd.h>
( 5) #include <errno.h>
( 6) #include <sys/types.h>
( 7) #include <sys/stat.h>
( 8) #include <sys/socket.h>
( 9) #include <sys/un.h>
(10) #include <fcntl.h>

(11) #define HAVE_UIO_H
(12) #include "erl_driver.h"

(13) /*
(14) ** Interface routines
(15) */
(16) static ErlDrvData uds_start(ErlDrvPort port, char *buff);
(17) static void uds_stop(ErlDrvData handle);
(18) static void uds_command(ErlDrvData handle, char *buff, int bufflen);
(19) static void uds_input(ErlDrvData handle, ErlDrvEvent event);
(20) static void uds_output(ErlDrvData handle, ErlDrvEvent event);
(21) static void uds_finish(void);
(22) static int uds_control(ErlDrvData handle, unsigned int command, 
(23)                        char* buf, int count, char** res, int res_size);

(24) /* The driver entry */
(25) static ErlDrvEntry uds_driver_entry = {
(26)     NULL,                  /* init, N/A */
(27)     uds_start,             /* start, called when port is opened */
(28)     uds_stop,              /* stop, called when port is closed */
(29)     uds_command,           /* output, called when erlang has sent */
(30)     uds_input,             /* ready_input, called when input descriptor 
(31)                               ready */
(32)     uds_output,            /* ready_output, called when output 
(33)                               descriptor ready */
(34)     "uds_drv",             /* char *driver_name, the argument 
(35)                               to open_port */
(36)     uds_finish,            /* finish, called when unloaded */
(37)     NULL,                  /* void * that is not used (BC) */
(38)     uds_control,           /* control, port_control callback */
(39)     NULL,                  /* timeout, called on timeouts */
(40)     NULL                   /* outputv, vector output interface */
(41) };    

On line 1 to 10 we have included the OS headers needed for our driver. As this driver is written for Solaris, we know that the header uio.h exists, why we can define the preprocessor variable HAVE_UIO_H before we include erl_driver.h at line 12. The definition of HAVE_UIO_H will make the I/O vectors used in erlangs driver queues to correspond to the operating systems dito, which is very convenient.

The different callback functions are declared ("forward declarations") on line 16 to 23.

The driver structure is similar for statically linked in drivers an dynamically loaded. However some of the fields should be left empty (i.e. initialized to NULL) in the different types of drivers. The first field (the init function pointer) is always left blank in a dynamically loaded driver, which can be seen on line 26. The NULL on line 37 should always be there, the field is no longer used and is retained for backward compatibility. We use no timers in this driver, why no callback for timers is needed. The last field (line 40) can be used to implement an interface similar to Unix writev for output. There is no need for such interface in a distribution driver, so we leave it with a NULL value (We will however use scatter/gather I/O internally in the driver).

Our defined callbacks thus are:

The ports implemented by this driver will operate in two major modes, which i will call the command and data modes. In command mode, only passive reading and writing (like gen_tcp:recv/gen_tcp:send) can be done, and this is the mode the port will be in during the distribution handshake. When the connection is up, the port will be switched to data mode and all data will be immediately read and passed further to the erlang emulator. In data mode, no data arriving to the uds_command will be interpreted, but just packaged and sent out on the socket. The uds_control callback will do the switching between those two modes.

While the net_kernel informs different subsystems that the connection is coming up, the port should accept data to send, but not receive any data, to avoid that data arrives from another node before every kernel subsystem is prepared to handle it. We have a third mode for this intermediate stage, lets call it the intermediate mode.

Lets define an enum for the differnt types of ports we have:

( 1) typedef enum { 
( 2)     portTypeUnknown,      /* An uninitialized port */
( 3)     portTypeListener,     /* A listening port/socket */
( 4)     portTypeAcceptor,     /* An intermidiate stage when accepting 
( 5)                              on a listen port */
( 6)     portTypeConnector,    /* An intermediate stage when connecting */
( 7)     portTypeCommand,      /* A connected open port in command mode */
( 8)     portTypeIntermediate, /* A connected open port in special
( 9)                              half active mode */
(10)     portTypeData          /* A connectec open port in data mode */ 
(11) } PortType;

Lets look at the different types:

Now lets look at the state we'll need for our ports. One can note that not all fields are used for all types of ports and that one could save some space by using unions, but that would clutter the code with multiple indirections, so i simply use one struct for all types of ports, for readability.

( 1) typedef unsigned char Byte;
( 2) typedef unsigned int Word;

( 3) typedef struct uds_data {
( 4)     int fd;                   /* File descriptor */
( 5)     ErlDrvPort port;          /* The port identifier */
( 6)     int lockfd;               /* The file descriptor for a lock file in 
( 7)                                  case of listen sockets */
( 8)     Byte creation;            /* The creation serial derived from the 
( 9)                                  lockfile */
(10)     PortType type;            /* Type of port */
(11)     char *name;               /* Short name of socket for unlink */
(12)     Word sent;                /* Bytes sent */
(13)     Word received;            /* Bytes received */
(14)     struct uds_data *partner; /* The partner in an accept/listen pair */
(15)     struct uds_data *next;    /* Next structure in list */
(16)     /* The input buffer and it's data */
(17)     int buffer_size;          /* The allocated size of the input buffer */
(18)     int buffer_pos;           /* Current position in input buffer */
(19)     int header_pos;           /* Where the current header is in the 
(20)                                  input buffer */
(21)     Byte *buffer;             /* The actual input buffer */
(22) } UdsData;

This structure is used for all types of ports although some fields are useless for some types. The least memory consuming solution would be to arrange this structure as a union of structures, but the multiple indirections in the code to access a field in such a structure will clutter the code to much for an example.

Let's look at the fields in our structure:

3.2.3 Selected parts of the distribution driver implementation

The distribution drivers implementation is not completely covered in this text, details about buffering and other things unrelated to driver writing are not explained. Likewise are some peculiarities of the UDS protocol not explained in detail. The chosen protocol is not important.

Prototypes for the driver callback routines can be found in the erl_driver.h header file.

The driver initialization routine is (usually) declared with a macro to make the driver easier to port between different operating systems (and flavours of systems). This is the only routine that has to have a well defined name. All other callbacks are reached through the driver structure. The macro to use is named DRIVER_INIT and takes the driver name as parameter.

(1) /* Beginning of linked list of ports */
(2) static UdsData *first_data;


(3) DRIVER_INIT(uds_drv)
(4) {
(5)     first_data = NULL;
(6)     return &uds_driver_entry;
(7) }

The routine initializes the single global data structure and returns a pointer to the driver entry. The routine will be called when erl_ddll:load_driver is called from erlang.

The uds_start routine is called when a port is opened from erlang. In our case, we only allocate a structure and initialize it. Creating the actual socket is left to the uds_command routine.

( 1) static ErlDrvData uds_start(ErlDrvPort port, char *buff)
( 2) {
( 3)     UdsData *ud;
( 4)     
( 5)     ud = ALLOC(sizeof(UdsData));
( 6)     ud->fd = -1;
( 7)     ud->lockfd = -1;
( 8)     ud->creation = 0;
( 9)     ud->port = port;
(10)     ud->type = portTypeUnknown;
(11)     ud->name = NULL;
(12)     ud->buffer_size = 0;
(13)     ud->buffer_pos = 0;
(14)     ud->header_pos = 0;
(15)     ud->buffer = NULL;
(16)     ud->sent = 0;
(17)     ud->received = 0;
(18)     ud->partner = NULL;
(19)     ud->next = first_data;
(20)     first_data = ud;
(21)     
(22)     return((ErlDrvData) ud);
(23) }

Every data item is initialized, so that no problems will arise when a newly created port is closed (without there being any corresponding socket). This routine is called when open_port({spawn, "uds_drv"},[]) is called from erlang.

The uds_command routine is the routine called when an erlang process sends data to the port. All asyncronous commands when the port is in command mode as well as the sending of all data when the port is in data mode is handeled in thi9s routine. Let's have a look at it:

( 1) static void uds_command(ErlDrvData handle, char *buff, int bufflen)
( 2) {
( 3)     UdsData *ud = (UdsData *) handle;

( 4)     if (ud->type == portTypeData || ud->type == portTypeIntermediate) {
( 5)         DEBUGF(("Passive do_send %d",bufflen));
( 6)         do_send(ud, buff + 1, bufflen - 1); /* XXX */
( 7)         return;
( 8)     } 
( 9)     if (bufflen == 0) {
(10)         return;
(11)     }
(12)     switch (*buff) {
(13)     case 'L':
(14)         if (ud->type != portTypeUnknown) {
(15)             driver_failure_posix(ud->port, ENOTSUP);
(16)             return;
(17)         }
(18)         uds_command_listen(ud,buff,bufflen);
(19)         return;
(20)     case 'A':
(21)         if (ud->type != portTypeUnknown) {
(22)             driver_failure_posix(ud->port, ENOTSUP);
(23)             return;
(24)         }
(25)         uds_command_accept(ud,buff,bufflen);
(26)         return;
(27)     case 'C':
(28)         if (ud->type != portTypeUnknown) {
(29)             driver_failure_posix(ud->port, ENOTSUP);
(30)             return;
(31)         }
(32)         uds_command_connect(ud,buff,bufflen);
(33)         return;
(34)     case 'S':
(35)         if (ud->type != portTypeCommand) {
(36)             driver_failure_posix(ud->port, ENOTSUP);
(37)             return;
(38)         }
(39)         do_send(ud, buff + 1, bufflen - 1);
(40)         return;
(41)     case 'R':
(42)         if (ud->type != portTypeCommand) {
(43)             driver_failure_posix(ud->port, ENOTSUP);
(44)             return;
(45)         }
(46)         do_recv(ud);
(47)         return;
(48)     default:
(49)         return;
(50)     }
(51) }

The command routine takes three parameters; the handle returned for the port by uds_start, which is a pointer to the internal port structure, the data buffer and the length of the data buffer. The buffer is the data sent from erlang (a list of bytes) converted to an C array (of bytes).

If Erlang sends i.e. the list [$a,$b,$c] to the port, the bufflen variable will be 3 ant the buff veriable will contain {'a','b','c'} (no null termination). Usually the first byte is used as an opcode, which is the case in our driver to (at least when the port is in command mode). The opcodes are defined as:

One may wonder what is meant by "one packet of data" in the 'R' command. This driver always sends data packeted with a 4 byte header containing a big endian 32 bit integer that represents the length of the data in the packet. There is no need for different packet sizes or soime kind of streamed mode, as this driver is for the distribuion only. One may wonder why the header word is coded explicitly in big endian when an UDS socket is local to the host. The answer simply is that I see it as a good practice when writing a distribution driver, as distribution in practice usually cross the host boundaries.

On line 4-8 we handle the case where the port is in data or intermediate mode, the rest of the routine handles the different commands. We see (first on line 15) that the routine uses the driver_failure_posix() routine to report errors. One important thing to remember is that the failure routines make a call to our uds_stop routine, which will remove the internal port data. The handle (and the casted handle ud) is therefore invalid pointers after a driver_failure call and we should immediately return. The runtime system will send exit signals to all linked processes.

The uds_input routine gets called when data is available on a file descriptor previously passed to the driver_select routine. Typically this happens when a read command is issued and no data is available. Lets look at the do_recv routine:

( 1) static void do_recv(UdsData *ud)
( 2) {
( 3)     int res;
( 4)     char *ibuf;
( 5)     for(;;) {
( 6)         if ((res = buffered_read_package(ud,&ibuf)) < 0) {
( 7)             if (res == NORMAL_READ_FAILURE) {
( 8)                 driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_READ, 1);
( 9)             } else {
(10)                 driver_failure_eof(ud->port);
(11)             }
(12)             return;
(13)         }
(14)         /* Got a package */
(15)         if (ud->type == portTypeCommand) {
(16)             ibuf[-1] = 'R'; /* There is always room for a single byte 
(17)                                opcode before the actual buffer 
(18)                                (where the packet header was) */
(19)             driver_output(ud->port,ibuf - 1, res + 1);
(20)             driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_READ,0);
(21)             return;
(22)         } else {
(23)             ibuf[-1] = DIST_MAGIC_RECV_TAG; /* XXX */
(24)             driver_output(ud->port,ibuf - 1, res + 1);
(25)             driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_READ,1);
(26)         }
(27)     }
(28) }

The routine tries to read data until a packet is read or the buffered_read_package routine returns a NORMAL_READ_FAILURE (an internally defined constant for the module that means that the read operation resulted in an EWOULDBLOCK). If the port is in command mode, the reading stops when one package is read, but if it is in data mode, the reading continues until the socket buffer is empty (read failure). If no more data can be read and more is wanted (always the case when socket is in data mode) driver_select is called to make the uds_input callback be called when more data is available for reading.

When the port is in data mode, all data is sent to erlang in a format that suits the distribution, in fact the raw data will never reach any erlang process, but will be translated/interpreted by the emulator itself and then delivered in the correct format to the correct processes. In the current emulator version, received data should be tagged with a single byte of 100. Thats what the macro DIST_MAGIC_RECV_TAG is defined to. The tagging of data in the distribution will possibly change in the future.

The uds_input routine will handle other input events (like nonblocking accept), but most importantly handle data arriving at the socket by calling do_recv:

( 1) static void uds_input(ErlDrvData handle, ErlDrvEvent event)
( 2) {
( 3)     UdsData *ud = (UdsData *) handle;

( 4)     if (ud->type == portTypeListener) {
( 5)         UdsData *ad = ud->partner;
( 6)         struct sockaddr_un peer;
( 7)         int pl = sizeof(struct sockaddr_un);
( 8)         int fd;

( 9)         if ((fd = accept(ud->fd, (struct sockaddr *) &peer, &pl)) < 0) {
(10)             if (errno != EWOULDBLOCK) {
(11)                 driver_failure_posix(ud->port, errno);
(12)                 return;
(13)             }
(14)             return;
(15)         }
(16)         SET_NONBLOCKING(fd);
(17)         ad->fd = fd;
(18)         ad->partner = NULL;
(19)         ad->type = portTypeCommand;
(20)         ud->partner = NULL;
(21)         driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_READ, 0);
(22)         driver_output(ad->port, "Aok",3);
(23)         return;
(24)     }
(25)     do_recv(ud);
(26) }

The important line here is the last line in the function, the do_read routine is called to handle new input. The rest of the function handles input on a listen socket, whinc means that there should be possible to do an accept on the socket, which is also recognized as a read event.

The output mechanisms are similar to the input. Lets first look at the do_send routine:

( 1) static void do_send(UdsData *ud, char *buff, int bufflen) 
( 2) {
( 3)     char header[4];
( 4)     int written;
( 5)     SysIOVec iov[2];
( 6)     ErlIOVec eio;
( 7)     ErlDrvBinary *binv[] = {NULL,NULL};

( 8)     put_packet_length(header, bufflen);
( 9)     iov[0].iov_base = (char *) header;
(10)     iov[0].iov_len = 4;
(11)     iov[1].iov_base = buff;
(12)     iov[1].iov_len = bufflen;
(13)     eio.iov = iov;
(14)     eio.binv = binv;
(15)     eio.vsize = 2;
(16)     eio.size = bufflen + 4;
(17)     written = 0;
(18)     if (driver_sizeq(ud->port) == 0) {
(19)         if ((written = writev(ud->fd, iov, 2)) == eio.size) {
(20)             ud->sent += written;
(21)             if (ud->type == portTypeCommand) {
(22)                 driver_output(ud->port, "Sok", 3);
(23)             }
(24)             return;
(25)         } else if (written < 0) {
(26)             if (errno != EWOULDBLOCK) {
(27)                 driver_failure_eof(ud->port);
(28)                 return;
(29)             } else {
(30)                 written = 0;
(31)             }
(32)         } else {
(33)             ud->sent += written;
(34)         }
(35)         /* Enqueue remaining */
(36)     }
(37)     driver_enqv(ud->port, &eio, written);
(38)     send_out_queue(ud);
(39) }

This driver uses the writev system call to send data onto the socket. A combination of writev and the driver output queues is very convenient. An ErlIOVec structure contains a SysIOVec (which is equivalent to the struct iovec structure defined in uio.h. The ErlIOVec also contains an array of ErlDrvBinary pointers, of the same length as the number of buffers in the I/O vector itself. One can use this to allocate the binaries for the queue "manually" in the driver, but we'll just fill the binary array with NULL values (line 7) , which will make the runtime system allocate it's own buffers when we call driver_enqv (line 37).

The routine builds an I/O vector containing the header bytes and the buffer (the opcode has been removed and the buffer length decreased by the output routine). If the queue is empty, we'll write the data directly to the socket (or at least try to). If any data is left, it is stored in the que and then we try to send the queue (line 38). An ack is sent when the message is delivered completely (line 22). The send_out_queue will send acks if the sending is completed there. If the port is in command mode, the erlang code serializes the send operations so that only one packet can be waiting for delivery at a time. Therefore the ack can be sent simply whenever the queue is empty.

A short look at the send_out_queue routine:

( 1) static int send_out_queue(UdsData *ud)
( 2) {
( 3)     for(;;) {
( 4)         int vlen;
( 5)         SysIOVec *tmp = driver_peekq(ud->port, &vlen);
( 6)         int wrote;
( 7)         if (tmp == NULL) {
( 8)             driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_WRITE, 0);
( 9)             if (ud->type == portTypeCommand) {
(10)                 driver_output(ud->port, "Sok", 3);
(11)             }
(12)             return 0;
(13)         }
(14)         if (vlen > IO_VECTOR_MAX) {
(15)             vlen = IO_VECTOR_MAX;
(16)         } 
(17)         if ((wrote = writev(ud->fd, tmp, vlen)) < 0) {
(18)             if (errno == EWOULDBLOCK) {
(19)                 driver_select(ud->port, (ErlDrvEvent) ud->fd, 
(20)                               DO_WRITE, 1);
(21)                 return 0;
(22)             } else {
(23)                 driver_failure_eof(ud->port);
(24)                 return -1;
(25)             }
(26)         }
(27)         driver_deq(ud->port, wrote);
(28)         ud->sent += wrote;
(29)     }
(30) }

What we do is simply to pick out an I/O vector from the queue (which is the whole queue as an SysIOVec). If the I/O vector is to long (IO_VECTOR_MAX is defined to 16), the vector length is decreased (line 15), otherwise the writev (line 17) call will fail. Writing is tried and anything written is dequeued (line 27). If the write fails with EWOULDBLOCK (note that all sockets are in nonblocking mode), driver_select is called to make the uds_output routine be called when there is space to write again.

We will continue trying to write until the queue is empty or the writing would block.

The routine above are called from the uds_output routine, which looks like this:

( 1) static void uds_output(ErlDrvData handle, ErlDrvEvent event)
( 2) {
( 3)    UdsData *ud = (UdsData *) handle;
( 4)    if (ud->type == portTypeConnector) {
( 5)        ud->type = portTypeCommand;
( 6)        driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_WRITE, 0);
( 7)        driver_output(ud->port, "Cok",3);
( 8)        return;
( 9)    }
(10)    send_out_queue(ud);
(11) }

The routine is simple, it first handles the fact that the output select will concern a socket in the buissiness of connectiong (and the connecting blocked). If the socket is in a connected state it simply sends the output queue, this routine is called when there is possible to write to a socket where we have an output queue, so there is no question what to do.

The driver implements a control interface, which is a syncronous interface called when erlang calls erlang:driver_control/3. This is the only interface that can control the driver when it is in data mode and it may be called with the following opcodes:

The control interface gets a buffer to return its value in, but is free to allocate it's own buffer is the provided one is to small. Here is the code for uds_control:

( 1) static int uds_control(ErlDrvData handle, unsigned int command, 
( 2)                        char* buf, int count, char** res, int res_size)
( 3) {
( 4) /* Local macro to ensure large enough buffer. */
( 5) #define ENSURE(N)                               \
( 6)    do {                                         \
( 7)        if (res_size < N) {                      \
( 8)            *res = ALLOC(N);                     \
( 9)        }                                        \
(10)    } while(0)

(11)    UdsData *ud = (UdsData *) handle;

(12)    switch (command) {
(13)    case 'S':
(14)        {
(15)            ENSURE(13);
(16)            **res = 0;
(17)            put_packet_length((*res) + 1, ud->received);
(18)            put_packet_length((*res) + 5, ud->sent);
(19)            put_packet_length((*res) + 9, driver_sizeq(ud->port));
(20)            return 13;
(21)        }
(22)    case 'C':
(23)        if (ud->type < portTypeCommand) {
(24)            return report_control_error(res, res_size, "einval");
(25)        }
(26)        ud->type = portTypeCommand;
(27)        driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_READ, 0);
(28)        ENSURE(1);
(29)        **res = 0;
(30)        return 1;
(31)    case 'I':
(32)        if (ud->type < portTypeCommand) {
(33)            return report_control_error(res, res_size, "einval");
(34)        }
(35)        ud->type = portTypeIntermediate;
(36)        driver_select(ud->port, (ErlDrvEvent) ud->fd, DO_READ, 0);
(37)        ENSURE(1);
(38)        **res = 0;
(39)        return 1;
(40)    case 'D':
(41)        if (ud->type < portTypeCommand) {
(42)            return report_control_error(res, res_size, "einval");
(43)        }
(44)        ud->type = portTypeData;
(45)        do_recv(ud);
(46)        ENSURE(1);
(47)        **res = 0;
(48)        return 1;
(49)    case 'N':
(50)        if (ud->type != portTypeListener) {
(51)            return report_control_error(res, res_size, "einval");
(52)        }
(53)        ENSURE(5);
(54)        (*res)[0] = 0;
(55)        put_packet_length((*res) + 1, ud->fd);
(56)        return 5;
(57)    case 'T': /* tick */
(58)        if (ud->type != portTypeData) {
(59)            return report_control_error(res, res_size, "einval");
(60)        }
(61)        do_send(ud,"",0);
(62)        ENSURE(1);
(63)        **res = 0;
(64)        return 1;
(65)    case 'R':
(66)        if (ud->type != portTypeListener) {
(67)            return report_control_error(res, res_size, "einval");
(68)        }
(69)        ENSURE(2);
(70)        (*res)[0] = 0;
(71)        (*res)[1] = ud->creation;
(72)        return 2;
(73)    default:
(74)        return report_control_error(res, res_size, "einval");
(75)    }
(76) #undef ENSURE
(77) }

The macro ENSURE (line 5 to 10) is used to ensure that the buffer is large enough for our answer. We switch on the command and take actions, there is not much to say about this routine. Worth noting is that we always has read select active on a port in data mode (achieved by calling do_recv on lin 45), but turn off read selection in intermediate and command modes (line 27 and 36).

The rest of the driver is more or less UDS specific and not of general interest.

3.3 Putting it all together

To test the distribution, one can use the net_kernel:start/1 function, which is useful as it starts the distribution on a running system, where tracing/debugging can be performed. The net_kernel:start/1 routine takes a list as it's single argument. The lists first element should be the node name (without the "@hostname") as an atom, and the second (and last) element should be one of the atoms shortnames or longnames. In the example case shortnames is preferred.

For net kernel to find out which distribution module to use, the command line argument -proto_dist is used. The argument is followed bu one or more distribution module names, with the "_dist" suffix removed, i.e. uds_dist as a distribution module is specified as -proto_dist uds.

If no epmd (TCP port mapper daemon) is used, one should also specify the command line option -no_epmd, which will make erlang skip the epmd startup, both as a OS process and as an erlang dito.

The path to the directory where the distribution modules reside must be known at boot, which can either be achieved by specifying -pa <path> on the command line or by building a boot script containing the applications used for your distribution protocol (in the uds_dist protocol, it's only the uds_dist application that needs to be added to the script).

The distribution will be started at boot if all the above is specified and an -sname <name> flag is present at the command line, here follows two examples:

$ erl -pa $ERL_TOP/lib/kernel/examples/uds_dist/ebin -proto_dist uds -no_epmd
Erlang (BEAM) emulator version 5.0 
 
Eshell V5.0  (abort with ^G)
1> net_kernel:start([bing,shortnames]).
{ok,<0.30.0>}
(bing@hador)2> 

...

$ erl -pa $ERL_TOP/lib/kernel/examples/uds_dist/ebin -proto_dist uds \ 
      -no_epmd -sname bong
Erlang (BEAM) emulator version 5.0 
 
Eshell V5.0  (abort with ^G)
(bong@hador)1> 

One can utilize the ERL_FLAGS environment variable to store the complicated parameters in:

$ ERL_FLAGS=-pa $ERL_TOP/lib/kernel/examples/uds_dist/ebin \ 
      -proto_dist uds -no_epmd
$ export ERL_FLAGS
$ erl -sname bang
Erlang (BEAM) emulator version 5.0 
 
Eshell V5.0  (abort with ^G)
(bang@hador)1> 

The ERL_FLAGS shuld preferrably not include the name of the node.


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