telnet中的终端操作.

telnet中的终端操作.
telnet使用make_new_session来创建一个新的session.
在make_new_session中调用xgetpty来创建一个伪终端会话。
int FAST_FUNC xgetpty(char *line)
{
    int p;

#if ENABLE_FEATURE_DEVPTS
    p = open("/dev/ptmx", O_RDWR);
    if (p > 0) {
        grantpt(p); /* chmod+chown corresponding slave pty */
        unlockpt(p); /* (what does this do?) */
#if 0 /* if ptsname_r is not available... */
        const char *name;
        name = ptsname(p); /* find out the name of slave pty */
        if (!name) {
            bb_perror_msg_and_die("ptsname error (is /dev/pts mounted?)");
        }
        safe_strncpy(line, name, GETPTY_BUFSIZE);
#else
        /* find out the name of slave pty */
        if (ptsname_r(p, line, GETPTY_BUFSIZE-1) != 0) {
            bb_perror_msg_and_die("ptsname error (is /dev/pts mounted?)");
        }
        line[GETPTY_BUFSIZE-1] = '\0';
#endif
        return p;
    }
#else
    struct stat stb;
    int i;
    int j;

    strcpy(line, "/dev/ptyXX");

    for (i = 0; i < 16; i++) {
        line[8] = "pqrstuvwxyzabcde"[i];
        line[9] = '0';
        if (stat(line, &stb) < 0) {
            continue;
        }
        for (j = 0; j < 16; j++) {
            line[9] = j < 10 ? j + '0' : j - 10 + 'a';
            if (DEBUG)
                fprintf(stderr, "Trying to open device: %s\n", line);
            p = open(line, O_RDWR | O_NOCTTY);
            if (p >= 0) {
                line[5] = 't';
                return p;
            }
        }
    }
#endif /* FEATURE_DEVPTS */
    bb_error_msg_and_die("can't find free pty");
}
好像现在的linux操作系统都是会进入ENABLE_FEATURE_DEVPTS中。
即通过打开/dev/ptmx来创建一个伪终端会话。
/dev/ptmx是一个用于创建伪终端对。当一个进程调用open打开/dev/ptmx的时候，它会返回主伪终端的fd，并且在会在/dev/pts目录下创建一个对应的slave伪终端。
man说明如下。

NAME
       ptmx and pts - pseudo-terminal master and slave

DESCRIPTION
       The  file  /dev/ptmx  is a character file with major number 5 and minor
       number 2, usually of mode 0666 and owner.group  of  root.root.   It  is
       used to create a pseudo-terminal master and slave pair.

       When a process opens /dev/ptmx, it gets a file descriptor for a pseudo-
       terminal master (PTM), and a pseudo-terminal slave (PTS) device is cre-
       ated in the /dev/pts directory.  Each file descriptor obtained by open-
       ing /dev/ptmx is an independent PTM with its own associated PTS,  whose
       path can be found by passing the descriptor to ptsname(3).

       Before  opening  the  pseudo-terminal slave, you must pass the master’s
       file descriptor to grantpt(3) and unlockpt(3).

       Once both the pseudo-terminal master and slave are open, the slave pro-
       vides  processes  with an interface that is identical to that of a real
       terminal.

       Data written to the slave is presented  on  the  master  descriptor  as
       input.   Data written to the master is presented to the slave as input.

       In practice, pseudo-terminals are used for implementing terminal emula-
       tors such as xterm(1), in which data read from the pseudo-terminal mas-
       ter is interpreted by the application in the same way a  real  terminal
       would  interpret  the  data, and for implementing remote-login programs
       such as sshd(8), in which data read from the pseudo-terminal master  is
       sent across the network to a client program that is connected to a ter-
       minal or terminal emulator.

       Pseudo-terminals can also be used to send input to programs  that  nor-
       mally refuse to read input from pipes (such as su(1), and passwd(1)).

FILES
       /dev/ptmx, /dev/pts/*



NAME
       ptsname, ptsname_r - get the name of the slave pseudo-terminal

SYNOPSIS
       #define _XOPEN_SOURCE
       #include

       char *ptsname(int fd);

       #define _GNU_SOURCE
       #include

       int ptsname_r(int fd, char *buf, size_t buflen);

DESCRIPTION
       The ptsname() function returns the name of the slave pseudo-terminal (pty) device corresponding to the master referred to by fd.

       The  ptsname_r()  function is the reentrant equivalent of ptsname().  It returns the name of the slave pseudo-terminal device as a null-terminated string
       in the buffer pointed to by buf.  The buflen argument specifies the number of bytes available in buf.