pty/pty_fork.c

+/* pty_fork.c
+
+   Implements ptyFork(), a function that creates a child process connected to
+   the parent (i.e., the calling process) via a pseudoterminal (pty). The child
+   is placed in a new session, with the pty slave as its controlling terminal,
+   and its standard input, output, and error connected to the pty slave.
+
+   In the parent, 'masterFd' is used to return the file descriptor for the
+   pty master.
+
+   If 'slaveName' is non-NULL, then it is used to return the name of the pty
+   slave. If 'slaveName' is not NULL,  then 'snLen' should be set to indicate
+   the size of the buffer pointed to by 'slaveName'.
+
+   If 'slaveTermios' and 'slaveWS' are non-NULL, then they are used respectively
+   to set the terminal attributes and window size of the pty slave.
+
+   Returns:
+        in child: 0
+        in parent: PID of child or -1 on error
+*/
 #include <fcntl.h>
 #include <termios.h>
 #include <sys/ioctl.h>
 #include "pty_master_open.h"
 #include "pty_fork.h"                   /* Declares ptyFork() */
 #include "tlpi_hdr.h"
 
-#define MAX_SNAME 1000
+#define MAX_SNAME 1000                  /* Maximum size for pty slave name */
 
 pid_t
 ptyFork(int *masterFd, char *slaveName, size_t snLen,
         const struct termios *slaveTermios, const struct winsize *slaveWS)
 {
     int mfd, slaveFd, savedErrno;
     pid_t childPid;
     char slname[MAX_SNAME];
 
     mfd = ptyMasterOpen(slname, MAX_SNAME);
     if (mfd == -1)
         return -1;
 
     if (slaveName != NULL) {            /* Return slave name to caller */
         if (strlen(slname) < snLen) {
             strncpy(slaveName, slname, snLen);
 
         } else {                        /* 'slaveName' was too small */
             close(mfd);
             errno = EOVERFLOW;
             return -1;
         }
     }
 
     childPid = fork();
 
     if (childPid == -1) {               /* fork() failed */
         savedErrno = errno;             /* close() might change 'errno' */
         close(mfd);                     /* Don't leak file descriptors */
         errno = savedErrno;
         return -1;
     }
 
     if (childPid != 0) {                /* Parent */
         *masterFd = mfd;                /* Only parent gets master fd */
         return childPid;                /* Like parent of fork() */
     }
 
     /* Child falls through to here */
 
     if (setsid() == -1)                 /* Start a new session */
         err_exit("ptyFork:setsid");
 
     close(mfd);                         /* Not needed in child */
 
     slaveFd = open(slname, O_RDWR);     /* Becomes controlling tty */
     if (slaveFd == -1)
         err_exit("ptyFork:open-slave");
 
 #ifdef TIOCSCTTY                        /* Acquire controlling tty on BSD */
     if (ioctl(slaveFd, TIOCSCTTY, 0) == -1)
         err_exit("ptyFork:ioctl-TIOCSCTTY");
 #endif
 
     if (slaveTermios != NULL)           /* Set slave tty attributes */
         if (tcsetattr(slaveFd, TCSANOW, slaveTermios) == -1)
             err_exit("ptyFork:tcsetattr");
 
     if (slaveWS != NULL)                /* Set slave tty window size */
         if (ioctl(slaveFd, TIOCSWINSZ, slaveWS) == -1)
             err_exit("ptyFork:ioctl-TIOCSWINSZ");
 
     /* Duplicate pty slave to be child's stdin, stdout, and stderr */
 
     if (dup2(slaveFd, STDIN_FILENO) != STDIN_FILENO)
         err_exit("ptyFork:dup2-STDIN_FILENO");
     if (dup2(slaveFd, STDOUT_FILENO) != STDOUT_FILENO)
         err_exit("ptyFork:dup2-STDOUT_FILENO");
     if (dup2(slaveFd, STDERR_FILENO) != STDERR_FILENO)
         err_exit("ptyFork:dup2-STDERR_FILENO");
 
     if (slaveFd > STDERR_FILENO)        /* Safety check */
         close(slaveFd);                 /* No longer need this fd */
 
     return 0;                           /* Like child of fork() */
 }