= dial: accept, announce, dial, listen, netinfo, netmkaddr, reject: make network connections     include "dial.m";    dial := load Dial Dial->PATH;     Connection: adt    {        dfd:  ref FD;  # data file        cfd:  ref FD;  # control file        dir:  string;  # pathname of line directory    };     announce:  fn(addr: string):        ref Connection;    dial:      fn(addr, local: string): ref Connection;    listen:    fn(c: ref Connection):       ref Connection;    accept:    fn(c: ref Connection): ref Sys->FD;    reject:    fn(c: ref Connection, why: string);     netmkaddr: fn(addr, defnet, defsvc: string): string;     Conninfo: adt    {        dir:   string;   # connection directory        root:  string;   # network mount point        spec:  string;   # its binding spec        lsys:  string;   # local host address        lserv: string;   # local service        rsys:  string;   # remote host address        rserv: string;   # remote service        laddr: string;   # local address in dial form        raddr: string;   # remote address in dial form    };        netinfo:   fn(c: ref Connection): ref Conninfo;    Dial establishes network connections.  The description  belowuses the following definitions: addr   is a network address in one of the following forms:        network!netaddr!service       network!netaddr       netaddr network       Any directory listed in /net (eg, tcp), or the special       token, net.  The special name net stands for any  net‐       work  that  connects  the current host and netaddr.  A       network name can be preceded by the full path name  of       a  directory  of networks, using the form /dir/network       (eg, /net.alt/tcp). netaddr       A host name, a domain name, a network  address,  or  a       meta-name of the form $attribute, which is replaced by       value from the corresponding attribute-value  pair  in       the connection server data base (see db(6)). The  functions dial and announce translate a given addr to anactual network address using the connection server cs(8).  Ifa logical name addr corresponds to several network addresses,for instance if a destination machine has several interfaces,cs  will  return  them all; dial or announce will try each inturn until one works.  In particular, if addr is net, cs willreturn  addresses  on  all networks that are common to sourceand destination.  The translation procedure accesses cs usingits  interface file cs, which is sought as follows: first, inan explicit directory /dir if one was given in network;  sec‐ond,  in  the  standard  directory  /net;  and finally in thedirectory /net.alt (dial only).   If  the  connection  servercannot be found, the addr is used as-is. If  a connection attempt is successful, the dir member of theresulting Connection will be the path name of a  line  direc‐tory  that  has files for accessing the connection.  One linedirectory exists for each possible connection.  The data filein  the  line  directory  is opened to make a connection, andread and written to communicate with  the  destination.   Thectl  file  in the line directory can be used to send commandsto the line.  See ip(3) for messages that can be  written  tothe  ctl file.  The last close of both data and ctl file willclose the connection.  The remote file in the line  directorycontains  the  address  called;  the  file local contains thelocal address assigned. The function dial calls destination  addr  on  a  multiplexednetwork.   If  the connection server returns several possiblelocations for addr, dial tries each in turn, until a  connec‐tion  is  made,  or  no  address  remains  to be tried.  Dialreturns a reference to a Connection value containing a stringdir that names the conversation directory for the connection,a file descriptor dfd open for reading and writing  the  datafile  in  that  directory, and a file descriptor cfd open forreading and writing the directory's ctl file.   If  local  isnon-empty,  and  the  network  allows the local address to beset, as is the case with UDP and TCP port numbers, the  localaddress will be set to local. Announce  and  listen  are the complements of dial.  Announceestablishes a network name to which  incoming  calls  can  bemade.   In  addr, netaddr gives the name or address of one ofthe local host's interfaces on which to listen for  calls  tothe  given  service;  it  can be * to listen for calls on anyinterface on network.  Announce returns a reference to a Con‐nection  value  in  which only the cfd descriptor is open, onthe control file representing the announcement.  Listen takesas  its  only argument a reference to the Connection returnedby a successful call to announce.  When a call  is  received,listen  returns  a  reference  to a new Connection value thatrefers to the conversation directory for the  incoming  call;only  the  cfd descriptor is open.  That call can be acceptedor rejected.  Use accept to obtain a file descriptor for  thedata  file  for  the  conversation.  Use reject to reject theincoming call; some networks will also tell  the  caller  thereason why. Netmkaddr  makes  addr  into a full network address, suitablefor dial or announce.  It adds  the  default  network  defnet(usually  "net")  and  a  default service defsvc to the givenaddr as required, including `!'  separators, and returns  theresult. Given a Connection, netinfo returns a reference to a Conninfovalue that gives details about the connection  and  its  net‐work. == examplesMake  a  call  and  return an open file descriptor to use forcommunications:        callkremvax(): ref Sys->FD       {            c := dial->dial("tcp!kremvax!80", nil);            if(c == nil)                 return nil;            return c.dfd;       } Call the local certificate signer:        dialsigner(service: string): ref Sys->FD       {            c := dial->dial("net!$SIGNER!inflogin", nil);            if(c == nil)                 return nil;            return c.dfd;       } Listen for incoming calls.        listener()       {            ac := dial->announce("tcp!*!9995");            if(ac == nil){                 sys->print("can't announce: %r\n");                 exit;            }            for(;;){                 lc := dial->listen(ac);                 if(lc == nil){                      sys->print("listen: %r\n");                      exit;                 }                 sys->print("incoming: %s\n", hd ctext(lc));                 spawn client(lc);            }       }        client(c: ref Connection)       {            dfd := dial->accept(c);            if(dfd == nil){                 sys->print("%s: can't accept: %r\n", c.dir);                 exit;            }            buf := array[Sys->ATOMICIO] of byte;            while((n := sys->read(dfd, buf, len buf)) > 0)                 sys->write(dfd, buf, n);       } == source/appl/lib/dial.b == diagnosticsThe integer valued functions return 0 on success  and  -1  onerror;  functions  returning a reference return nil on error.In those cases the system error string is set.