dorne/cursetree/_pty.c

/* WARNING: The mkpty(), forkmkpty(), & bindpty() functions are based on
 * WARNING: the glibc openpty(), forkpty(), & login_tty() functions respectively.
 * WARNING:
 * WARNING: The GNU C Library's COPYING & COPYING.LIB licenses are available at
 * WARNING: LICENSES/GLIBC-COPYING & LICENSES/GLIBC-COPYING.LIB in this repo.
 * WARNING:
 * WARNING: login_tty() maintains the original University of California license
 * WARNING: available at LICENSES/UC-LICENSE in this repo.
 */

/* _XOPEN_SOURCE unlocks <stdlib.h> pty/ptmx/pts declarations. */
#define _XOPEN_SOURCE 600
/* _GNU_SOURCE unlocks the ptsname_r declaration*/
#define _GNU_SOURCE
#include <fcntl.h>
#include <stdlib.h>
#include <asm/termbits.h>  /* TIOC* constants */
#include <sys/ioctl.h>

#include "_pty.h"

/* Allocate PTY master and slave file descriptors.
 * errno will have been set if newpty() fails.
 *
 * NOTE: This function is my alternative to GLibC's
 * NOTE: openpty() function. It exists as a learning resource.
 * REF: https://sourceware.org/git/glibc.git -> ./login/openpty.c
 */
int mkpty(int *fdmx, int *fds) {
  int _fdmx = -1, _fds = -1;
  char sname[TTYNAME_MAX];  

  // Configure PTY master (file descriptor)
  _fdmx = posix_openpt(O_RDWR | O_NOCTTY);
  if (_fdmx == -1)
    return EXIT_FAILURE;

  if (grantpt(_fdmx))
    goto fail;
  if (unlockpt(_fdmx))
    goto fail;

#ifdef TIOCGPTPEER
  /* Try to allocate slave fd solely based on PTMX fd first. */
  _fds = ioctl(_fdmx, TIOCGPTPEER, O_RDWR | O_NOCTTY);
#endif
  if (_fds == -1) {
    /* Fallback to path-based slave fd allocation
     * (if the kernel doesn't support TIOCGPTPEER, ie Linux <4.13) */
    if(ptsname_r(_fdmx, sname, sizeof(sname)))
      goto fail;

    _fds = open(sname, O_RDWR | O_NOCTTY);
    if (_fds == -1)
      goto fail;
  }

  /* Propagate file descriptors via parameters */
  *fdmx = _fdmx;
  *fds = _fds;
  return EXIT_SUCCESS;

fail:
  if (_fdmx == -1) {
    close(_fdmx);
    if (_fds == -1)
      close(_fds);
  }
  return EXIT_FAILURE;
}

/* Set fdty as the controlling terminal for the calling process.
 * Returns 0 on success, and 1 on failure.
 * NOTE: This function is my alternative to GLibC's
 * NOTE: login_tty() function. It exists as a learning resource.
 * REF: https://sourceware.org/git/glibc.git -> ./login/login_tty.c
 */
static inline int setctty(const int fdty) {  
  /* We assume any kernel compiling this defines TIOCSCTTY,
   * otherwise this implementation won't exactly work... */
  return (ioctl(fdty, TIOCSCTTY, 0) != -1);
}

/* Bind fdty (terminal fd) to stdin/stdout/stderr for the calling process. 
 * This functions blocks until the EBUSY (see `man dup2`) race condition lifts.
 * NOTE: This function is my alternative to GLibC's
 * NOTE: login_tty() function. It exists as a learning resource.
 * WARNING: This function maintains the original University of California
 * WARNING: LICENSE (1990-1993) as per glibc.git:/login/login_tty.c
 * WARNING: available at LICENSES/UC-LICENSE in this repo.
 * REF: https://sourceware.org/git/glibc.git -> ./login/login_tty.c
 */
void bindpty(const int fdty) {
  /* Adjust stdin/stdout/stderr to refer to fd*/
  BIND(fdty, STDIN_FILENO);
  BIND(fdty, STDOUT_FILENO);
  BIND(fdty, STDERR_FILENO);
  if (fdty > 2)
    close(fdty);
}

/* Allocate a PTY and fork, giving ptmx (master) to the parent
 * and binding the child's stdin/stdout/stderr to pts (slave).
 * Return value is indentical to fork(2).
 * NOTE: This function is my alternative to GLibC's
 * NOTE: forkpty() function. It exists as a learning resource.
 * REF: https://sourceware.org/git/glibc.git -> ./login/forkpty.c
 */
pid_t forkmkpty(int *fdmx) {
  int _fdmx, fds;
  pid_t pid;
  if (mkpty(&_fdmx, &fds))
    return EXIT_FAILURE;

  switch (pid = fork()) {
  case -1:
    close(_fdmx);
    close(fds);
    return -1;    
  case 0:
    /* Child Process */
    close(_fdmx);
    setctty(fds);
    bindpty(fds);
    break;
    
  default:
    /* Parent Process */
    close(fds);
    // propagate ptmx (master) fd
    *fdmx = _fdmx;
    break;
  }

  /* Both Processes */
  return pid;
}

/* Set pseudoterminal slave's window size.
 * Returns 0 on success, and fails with -1 if the kernel doesn't
 * implement this, or 1 for general errors (errno will be set).
 * NOTE: Typically this is part of a glibc openpty() call.
 */
int setptsxy(const unsigned short rows, const unsigned short cols, const int fds) {
#ifndef TIOCSWINSZ
  /* Fail if kernel doesn't support TIOCSWINSZ. */
  return -1;
#else
  struct winsize win = {
    .ws_row = rows,
    .ws_col = cols,
  };
  
  if (ioctl(fds, TIOCSWINSZ, &win) == -1)
    return EXIT_FAILURE;
  return EXIT_SUCCESS;
#endif /* TIOCSWINSZ */
}

/* Get pseudoterminal slave's window size.
 * Returns 0 on success, and fails with -1 if the kernel doesn't
 * implement this, or 1 for general errors (errno will be set).
 */
int getptsxy(unsigned short *rows, unsigned short *cols, const int fds) {
#ifndef TIOCGWINSZ
  /* Fail if kernel doesn't support TIOCGWINSZ. */
  return -1;
#else
  struct winsize win = (struct winsize){ 0 };
  if (ioctl(fds, TIOCGWINSZ, &win) == -1)
    return EXIT_FAILURE;
  *rows = win.ws_row;
  *cols = win.ws_col;
  return EXIT_SUCCESS;
#endif /* TIOCGWINSZ */
}