ptypair.c

#include <errno.h>
#include <fcntl.h>
#include <grp.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <unistd.h>


/* get_master_pty() takes a double-indirect character pointer in which
 * to put a slave name, and returns an integer file descriptor.
 * If it returns < 0, an error has occurred.
 * Otherwise, it has returned the master pty file descriptor, and fills
 * in *name with the name of the corresponding slave pty.
 * Once the slave pty has been opened, you are responsible to free *name.
 */

int get_master_pty(char **name) { 
   int i, j;
   /* default to returning error */
   int master = -1;

   /* create a dummy name to fill in */
   *name = strdup("/dev/ptyXX");

   /* search for an unused pty */
   for (i=0; i<16 && master <= 0; i++) {
      for (j=0; j<16 && master <= 0; j++) {
         (*name)[8] = "pqrstuvwxyzPQRST"[i];
         (*name)[9] = "0123456789abcdef"[j];
         /* open the master pty */
         if ((master = open(*name, O_RDWR)) < 0) {
            if (errno == ENOENT) {
               /* we are out of pty devices */
               free (*name);
               return (master);
            }
         }
      }
   }
   if ((master < 0) && (i == 16) && (j == 16)) {
      /* must have tried every pty unsuccessfully */
      free (*name);
      return (master);
   }

   /* By substituting a letter, we change the master pty
    * name into the slave pty name.
    */
   (*name)[5] = 't';

   return (master);
}

/* get_slave_pty() returns an integer file descriptor.
 * If it returns < 0, an error has occurred.
 * Otherwise, it has returned the slave file descriptor.
 */

int get_slave_pty(char *name) { 
   struct group *gptr;
   gid_t gid;
   int slave = -1;

   /* chown/chmod the corresponding pty, if possible.
    * This will only work if the process has root permissions.
    * Alternatively, write and exec a small setuid program that
    * does just this.
    */
   if ((gptr = getgrnam("tty")) != 0) {
      gid = gptr->gr_gid;
   } else {
      /* if the tty group does not exist, don't change the
       * group on the slave pty, only the owner
       */
      gid = -1;
   }

   /* Note that we do not check for errors here.  If this is code
    * where these actions are critical, check for errors!
    */
   chown(name, getuid(), gid);
   /* This code only makes the slave read/writeable for the user.
    * If this is for an interactive shell that will want to
    * receive "write" and "wall" messages, OR S_IWGRP into the
    * second argument below.
    */
   chmod(name, S_IRUSR|S_IWUSR);

   /* open the corresponding slave pty */
   slave = open(name, O_RDWR);

   return (slave);
}