GETSID(2) System Calls Manual GETSID(2)

getsid
get session ID

Standard C Library (libc, -lc)

#include <unistd.h>

pid_t
getsid(pid_t pid);

The getsid() function returns the session ID of the process specified by pid. If pid is 0, the session ID of the calling process is returned. The session ID is defined as the process group ID of the process that is the session leader.

If an error occurs, getsid() returns -1 and the global variable errno is set to indicate the error, as follows:
[]
No process can be found corresponding to that specified by pid.

getpgid(2), setsid(2), termios(4)

The getsid() function conforms to X/Open Portability Guide Issue 4, Version 2 (“XPG4.2”).
April 4, 2011 NetBSD 9.0