TCGETSID(3) Library Functions Manual TCGETSID(3)

tcgetsid
get session ID associated with a controlling terminal

Standard C Library (libc, -lc)

#include <sys/types.h>
#include <unistd.h>

pid_t
tcgetsid(int fd);

The tcgetsid function returns the value of the session ID associated with the specified controlling terminal device. The session ID is defined as the process group ID of the session leader.

If an error occurs, tcgetsid returns -1 and the global variable errno is set to indicate the error, as follows:
[]
The fd argument is not a valid file descriptor.
[]
The calling process does not have a controlling terminal or the underlying terminal device represented by fd is not the controlling terminal.

getsid(2), setsid(2), tcgetpgrp(3)

The tcgetsid function conforms to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”).
February 13, 1998 NetBSD 9.2