This is pty/pty_master_open_bsd.c (Listing 64-4, page 1396), an example from the book, The Linux Programming Interface.

The source code file is copyright 2022, Michael Kerrisk, and is licensed under the GNU General Public License, version 3.

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/* pty_master_open_bsd.c

   Implement our ptyMasterOpen() function, based on BSD pseudoterminals.
   See comments below.

   Note: BSD pseudoterminals are not specified in SUSv3, and are considered
   obsolete on Linux.

   See also pty_master_open.c.
#include <fcntl.h>
#include "pty_master_open.h"            /* Declares ptyMasterOpen() */
#include "tlpi_hdr.h"

#define PTYM_PREFIX     "/dev/pty"
#define PTYS_PREFIX     "/dev/tty"
#define PTY_PREFIX_LEN  (sizeof(PTYM_PREFIX) - 1)
#define PTY_NAME_LEN    (PTY_PREFIX_LEN + sizeof("XY"))
#define X_RANGE         "pqrstuvwxyzabcde"
#define Y_RANGE         "0123456789abcdef"
/* Open a BSD pty master, returning file descriptor, or -1 on error.
   On successful completion, the name of the corresponding pty slave
   is returned in 'slaveName'. 'snLen' should be set to indicate the
   size of the buffer pointed to by 'slaveName'. */

ptyMasterOpen(char *slaveName, size_t snLen)
    int masterFd, n;
    char *x, *y;
    char masterName[PTY_NAME_LEN];

    if (PTY_NAME_LEN > snLen) {
        errno = EOVERFLOW;
        return -1;

    memset(masterName, 0, PTY_NAME_LEN);
    strncpy(masterName, PTYM_PREFIX, PTY_PREFIX_LEN);

    /* Scan through all possible BSD pseudoterminal master names until
       we find one that we can open. */

    for (x = X_RANGE; *x != '\0'; x++) {
        masterName[PTY_PREFIX_LEN] = *x;

        for (y = Y_RANGE; *y != '\0'; y++) {
            masterName[PTY_PREFIX_LEN + 1] = *y;

            masterFd = open(masterName, O_RDWR);

            if (masterFd == -1) {
                if (errno == ENOENT)    /* No such file */
                    return -1;          /* Probably no more pty devices */
                else                    /* Other error (e.g., pty busy) */

            } else {            /* Return slave name corresponding to master */
                n = snprintf(slaveName, snLen, "%s%c%c", PTYS_PREFIX, *x, *y);
                if (n >= snLen) {
                    errno = EOVERFLOW;
                    return -1;
                } else if (n == -1) {
                    return -1;

                return masterFd;

    return -1;                  /* Tried all ptys without success */


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