#### 17.8.1 Allocating Pseudo-Terminals

This subsection describes functions for allocating a pseudo-terminal, and for making this pseudo-terminal available for actual use. These functions are declared in the header file `stdlib.h`.

— Function: int getpt (void)

> The `getpt` function returns a new file descriptor for the next available master pseudo-terminal. The normal return value from `getpt` is a non-negative integer file descriptor. In the case of an error, a value of -1 is returned instead. The following `errno` conditions are defined for this function:

> `ENOENT`
>   There are no free master pseudo-terminals available.

> This function is a GNU extension.

— Function: int grantpt (int filedes)

> The `grantpt` function changes the ownership and access permission of the slave pseudo-terminal device corresponding to the master pseudo-terminal device associated with the file descriptor filedes. The owner is set from the real user ID of the calling process (see Process Persona), and the group is set to a special group (typically tty) or from the real group ID of the calling process. The access permission is set such that the file is both readable and writable by the owner and only writable by the group.

> On some systems this function is implemented by invoking a special `setuid` root program (see How Change Persona). As a consequence, installing a signal handler for the `SIGCHLD` signal (see Job Control Signals) may interfere with a call to `grantpt`.

> The normal return value from `grantpt` is 0; a value of -1 is returned in case of failure. The following `errno` error conditions are defined for this function:

> `EBADF`
>   The filedes argument is not a valid file descriptor.
> `EINVAL`
>   The filedes argument is not associated with a master pseudo-terminal device.
> `EACCES`
>   The slave pseudo-terminal device corresponding to the master associated with filedes could not be accessed.

— Function: int unlockpt (int filedes)

> The `unlockpt` function unlocks the slave pseudo-terminal device corresponding to the master pseudo-terminal device associated with the file descriptor filedes. On many systems, the slave can only be opened after unlocking, so portable applications should always call `unlockpt` before trying to open the slave.

> The normal return value from `unlockpt` is 0; a value of -1 is returned in case of failure. The following `errno` error conditions are defined for this function:

> `EBADF`
>   The filedes argument is not a valid file descriptor.
> `EINVAL`
>   The filedes argument is not associated with a master pseudo-terminal device.

— Function: char * ptsname (int filedes)

> If the file descriptor filedes is associated with a master pseudo-terminal device, the `ptsname` function returns a pointer to a statically-allocated, null-terminated string containing the file name of the associated slave pseudo-terminal file. This string might be overwritten by subsequent calls to `ptsname`.

— Function: int ptsname_r (int filedes, char *buf, size_t len)

> The `ptsname_r` function is similar to the `ptsname` function except that it places its result into the user-specified buffer starting at buf with length len.

> This function is a GNU extension.

Portability Note: On System V derived systems, the file returned by the `ptsname` and `ptsname_r` functions may be STREAMS-based, and therefore require additional processing after opening before it actually behaves as a pseudo terminal.

Typical usage of these functions is illustrated by the following example:

         int
         open_pty_pair (int *amaster, int *aslave)
         {
           int master, slave;
           char *name;

           master = getpt ();
           if (master < 0)
             return 0;

           if (grantpt (master) < 0 || unlockpt (master) < 0)
             goto close_master;
           name = ptsname (master);
           if (name == NULL)
             goto close_master;

           slave = open (name, O_RDWR);
           if (slave == -1)
             goto close_master;

           if (isastream (slave))
             {
               if (ioctl (slave, I_PUSH, "ptem") < 0
                   || ioctl (slave, I_PUSH, "ldterm") < 0)
                 goto close_slave;
             }

           *amaster = master;
           *aslave = slave;
           return 1;

         close_slave:
           close (slave);

         close_master:
           close (master);
           return 0;
         }