271.htm

unix高级编程原吗

    for (;;) {                          /* loop for phone calls */ <br>
      if ((t= get_connection(s)) < 0) { /* get a connection */ <br>
        if (errno == EINTR)             /* EINTR might happen on accept(), * <br>
/ <br>
          continue;                     /* try again */ <br>
        perror("accept");               /* bad */ <br>
        exit(1); <br>
      } <br>
      switch(fork()) {                  /* try to handle connection */ <br>
      case -1 :                         /* bad news.  scream and die */ <br>
        perror("fork"); <br>
        close(s); <br>
        close(t); <br>
        exit(1); <br>
      case 0 :                          /* we're the child, do something */ <br>
        do_something(t); <br>
        exit(0); <br>

      default :                         /* we're the parent so look for */ <br>
        close(t);                       /* another connection */ <br>
        continue; <br>
      } <br>
    } <br>
  } <br>
  /* as children die we should get catch their returns or else we get <br>
   * zombies, A Bad Thing.  fireman() catches falling children. <br>
   */ <br>
  void fireman() <br>
  { union wait wstatus; <br>
    while(wait3(&wstatus,WNOHANG,NULL) > 0) <br>
        ; <br>
  } <br>
  /* this is the function that plays with the socket.  it will be called <br>
   * after getting a connection. <br>
   */ <br>
  void do_something(s) <br>
  int s; <br>
  { <br>
    /* do your thing with the socket here <br>
        : <br>
        : <br>
        : <br>
     */ <br>
  } <br>
Dialing (or: How to call a socket) <br>
You now know how to create a socket that will accept incoming calls. So how <br>
do y <br>
u call it? As with <br>
the telephone, you must first have the phone before using it to call. You us <br>
e th <br>
 socket() function to do <br>
this, exactly as you establish a socket to listen to. <br>
After getting a socket to make the call with, and giving it an address, you <br>
use <br>
he connect() function <br>
to try to connect to a listening socket. The following function calls a part <br>
icul <br>
r port number on a <br>
particular host: <br>
  int call_socket(hostname, portnum) <br>
  char *hostname; <br>
  { struct sockaddr_in sa; <br>
    struct hostent     *hp; <br>

    int a, s; <br>
    if ((hp= gethostbyname(hostname)) == NULL) { /* do we know the host's */ <br>
  <br>
      errno= ECONNREFUSED;                       /* address? */ <br>
      return(-1);                                /* no */ <br>
    } <br>
    bzero(&sa,sizeof(sa)); <br>
    bcopy(hp->h_addr,(char *)&sa.sin_addr,hp->h_length); /* set address */ <br>
    sa.sin_family= hp->h_addrtype; <br>
    sa.sin_port= htons((u_short)portnum); <br>
    if ((s= socket(hp->h_addrtype,SOCK_STREAM,0)) < 0)   /* get socket */ <br>
      return(-1); <br>
    if (connect(s,&sa,sizeof sa) < 0) {                  /* connect */ <br>
      close(s); <br>
      return(-1); <br>
    } <br>
    return(s); <br>
  } <br>
This function returns a connected socket through which data can flow. <br>
Conversation (or: How to talk between sockets) <br>
Now that you have a connection between sockets you want to send data between <br>
 the <br>
 the <br>
. The read() <br>
and write() functions are used to do this, just as they are for normal files <br>
. Th <br>
re is only one major <br>
difference between socket reading and writing and file reading and writing: <br>
you <br>
on't usually get back <br>
the same number of characters that you asked for, so you usually loop until <br>
you <br>
ave read the number <br>
of characters that you want. A simple func- tion to read a given number of c <br>
hara <br>
ters into a buffer is: <br>
  int read_data(s,buf,n) <br>
  int  s;                /* connected socket */ <br>
  char *buf;             /* pointer to the buffer */ <br>
  int  n;                /* number of characters (bytes) we want */ <br>
  { int bcount,          /* counts bytes read */ <br>
        br;              /* bytes read this pass */ <br>
    bcount= 0; <br>
    br= 0; <br>
    while (bcount < n) {             /* loop until full buffer */ <br>

      if ((br= read(s,buf,n-bcount)) > 0) { <br>
        bcount += br;                /* increment byte counter */ <br>
        buf += br;                   /* move buffer ptr for next read */ <br>
      } <br>
      if (br < 0)                    /* signal an error to the caller */ <br>
        return(-1); <br>
    } <br>
    return(bcount); <br>
  } <br>
A very similar function should be used to write data; we leave that function <br>
 as <br>
n exercise to the <br>
reader. <br>
Hanging Up (or: What to do when you're done with a socket) <br>
Just as you hang up when you're through speaking to someone over the telepho <br>
ne, <br>
o must you close <br>
a connection between sockets. The normal close() function is used to close e <br>
ach <br>
nd of a socket <br>
connection. If one end of a socket is closed and the other tries to write to <br>
 its <br>
 its <br>
end, the write will return <br>
an error. <br>
Speaking The Language (or: Byte order is important) <br>
Now that you can talk between machines, you have to be careful what you say. <br>
 Man <br>
 machines use <br>
differing dialects, such as ASCII versus (yech) EBCDIC. More commonly there <br>
are <br>
yte-order <br>
problems. Unless you always pass text, you'll run up against the byte-order <br>
prob <br>
em. Luckily people <br>
have already figured out what to do about it. <br>
Once upon a time in the dark ages someone decided which byte ord- er was "ri <br>
ght" <br>
 Now there exist <br>
functions that convert one to the other if necessary. Some of these function <br>
s ar <br>
 htons() (host to <br>
network short integer), ntohs() (network to host short integer), htoni() (ho <br>
st t <br>
 network integer), <br>

ntohi() (network to host integer), htonl() (host to network long integer), a <br>
nd n <br>
ohl() (network to host <br>
long integer). Before sending an in- teger through a socket, you should firs <br>
t ma <br>
sage it with the <br>
htoni() function: <br>
  i= htoni(i); <br>
  write_data(s, &i, sizeof(i)); <br>
and after reading data you should convert it back with ntohi(): <br>
  read_data(s, &i, sizeof(i)); <br>
  i= ntohi(i); <br>
If you keep in the habit of using these functions you'll be less likely to g <br>
oof <br>
t up in those <br>
circumstances where it is neces- sary. <br>
The Future Is In Your Hands (or: What to do now) <br>
Using just what's been discussed here you should be able to build your own p <br>
rogr <br>
ms that <br>
communicate with sockets. As with all new things, however, it would be a goo <br>
d id <br>

a to look at what's <br>
al- ready been done. Many public domain programs exist which make use of the <br>
 soc <br>
et concept, and <br>
many books exist which go into much more depth than I have here. In addition <br>
 I'v <br>
 deliberately left <br>
out a lot of details such as what kinds of things can go wrong; the manual p <br>
ages <br>
for each of the <br>
functions should be con- sulted for this information. <br>
If you have further questions about sockets or this primer, please feel free <br>
 to <br>
sk me at email address <br>
[e-mail] <br>
http://www.ecst.csuchico.edu/~chafey/prog/sockets/sinfo2.html <br>
-- <br>
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