GNU C Library (libc) Programming Guide - Terminal Access Functions

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27.7.3 Functions for Controlling Terminal Access

These are the functions for reading or setting the foreground process group of a terminal. You should include the header files sys/types.h and unistd.h in your application to use these functions. Although these functions take a file descriptor argument to specify the terminal device, the foreground job is associated with the terminal file itself and not a particular open file descriptor.

— Function: pid_t tcgetpgrp (int filedes)

This function returns the process group ID of the foreground process group associated with the terminal open on descriptor filedes.
If there is no foreground process group, the return value is a number greater than 1 that does not match the process group ID of any existing process group. This can happen if all of the processes in the job that was formerly the foreground job have terminated, and no other job has yet been moved into the foreground.
In case of an error, a value of -1 is returned. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
ENOSYS
The system doesn't support job control.
ENOTTY
The terminal file associated with the filedes argument isn't the controlling terminal of the calling process.

— Function: int tcsetpgrp (int filedes, pid_t pgid)

This function is used to set a terminal's foreground process group ID. The argument filedes is a descriptor which specifies the terminal; pgid specifies the process group. The calling process must be a member of the same session as pgid and must have the same controlling terminal.
For terminal access purposes, this function is treated as output. If it is called from a background process on its controlling terminal, normally all processes in the process group are sent a SIGTTOU signal. The exception is if the calling process itself is ignoring or blocking SIGTTOU signals, in which case the operation is performed and no signal is sent.
If successful, tcsetpgrp returns 0. A return value of -1 indicates an error. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
EINVAL
The pgid argument is not valid.
ENOSYS
The system doesn't support job control.
ENOTTY
The filedes isn't the controlling terminal of the calling process.
EPERM
The pgid isn't a process group in the same session as the calling process.

— Function: pid_t tcgetsid (int fildes)

This function is used to obtain the process group ID of the session for which the terminal specified by fildes is the controlling terminal. If the call is successful the group ID is returned. Otherwise the return value is (pid_t) -1 and the global variable errno is set to the following value:

EBADF
The filedes argument is not a valid file descriptor.
ENOTTY
The calling process does not have a controlling terminal, or the file is not the controlling terminal.