posix_openpt(3p) — Linux manual page


POSIX_OPENPT(3P)        POSIX Programmer's Manual       POSIX_OPENPT(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The
       Linux implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior),
       or the interface may not be implemented on Linux.

NAME         top

       posix_openpt — open a pseudo-terminal device

SYNOPSIS         top

       #include <stdlib.h>
       #include <fcntl.h>

       int posix_openpt(int oflag);

DESCRIPTION         top

       The posix_openpt() function shall establish a connection between
       a master device for a pseudo-terminal and a file descriptor. The
       file descriptor shall be allocated as described in Section 2.14,
       File Descriptor Allocation and can be used by other I/O functions
       that refer to that pseudo-terminal.

       The file status flags and file access modes of the open file
       description shall be set according to the value of oflag.

       Values for oflag are constructed by a bitwise-inclusive OR of
       flags from the following list, defined in <fcntl.h>:

       O_RDWR      Open for reading and writing.

       O_NOCTTY    If set posix_openpt() shall not cause the terminal
                   device to become the controlling terminal for the

       The behavior of other values for the oflag argument is

RETURN VALUE         top

       Upon successful completion, the posix_openpt() function shall
       open a file descriptor for a master pseudo-terminal device and
       return a non-negative integer representing the file descriptor.
       Otherwise, -1 shall be returned and errno set to indicate the

ERRORS         top

       The posix_openpt() function shall fail if:

       EMFILE All file descriptors available to the process are
              currently open.

       ENFILE The maximum allowable number of files is currently open in
              the system.

       The posix_openpt() function may fail if:

       EINVAL The value of oflag is not valid.

       EAGAIN Out of pseudo-terminal resources.

       ENOSR  Out of STREAMS resources.

       The following sections are informative.

EXAMPLES         top

   Opening a Pseudo-Terminal and Returning the Name of the Slave Device
       and a File Descriptor
           #include <fcntl.h>
           #include <stdio.h>

           int masterfd, slavefd;
           char *slavedevice;

           masterfd = posix_openpt(O_RDWR|O_NOCTTY);

           if (masterfd == -1
               || grantpt (masterfd) == -1
               || unlockpt (masterfd) == -1
               || (slavedevice = ptsname (masterfd)) == NULL)
               return -1;

           printf("slave device is: %s\n", slavedevice);

           slavefd = open(slavedevice, O_RDWR|O_NOCTTY);
           if (slavefd < 0)
               return -1;


       This function is a method for portably obtaining a file
       descriptor of a master terminal device for a pseudo-terminal. The
       grantpt() and ptsname() functions can be used to manipulate mode
       and ownership permissions, and to obtain the name of the slave
       device, respectively.

RATIONALE         top

       The standard developers considered the matter of adding a special
       device for cloning master pseudo-terminals: the /dev/ptmx device.
       However, consensus could not be reached, and it was felt that
       adding a new function would permit other implementations. The
       posix_openpt() function is designed to complement the grantpt(),
       ptsname(), and unlockpt() functions.

       On implementations supporting the /dev/ptmx clone device, opening
       the master device of a pseudo-terminal is simply:

           mfdp = open("/dev/ptmx", oflag );
           if (mfdp < 0)
               return -1;



SEE ALSO         top

       Section 2.14, File Descriptor Allocation, grantpt(3p), open(3p),
       ptsname(3p), unlockpt(3p)

       The Base Definitions volume of POSIX.1‐2017, fcntl.h(0p),

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic
       form from IEEE Std 1003.1-2017, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The
       Open Group Base Specifications Issue 7, 2018 Edition, Copyright
       (C) 2018 by the Institute of Electrical and Electronics
       Engineers, Inc and The Open Group.  In the event of any
       discrepancy between this version and the original IEEE and The
       Open Group Standard, the original IEEE and The Open Group
       Standard is the referee document. The original Standard can be
       obtained online at http://www.opengroup.org/unix/online.html .

       Any typographical or formatting errors that appear in this page
       are most likely to have been introduced during the conversion of
       the source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group               2017                  POSIX_OPENPT(3P)

Pages that refer to this page: stdlib.h(0p)grantpt(3p)ptsname(3p)unlockpt(3p)