setsid(2) creates a session and sets the process group ID


#include <unistd.h>

pid_t setsid(void);


setsid() creates a new session if the calling process is not a process group leader. The calling process is the leader of the new session (i.e., its session ID is made the same as its process ID). The calling process also becomes the process group leader of a new process group in the session (i.e., its process group ID is made the same as its process ID).

The calling process will be the only process in the new process group and in the new session. The new session has no controlling terminal.


On success, the (new) session ID of the calling process is returned. On error, (pid_t) -1 is returned, and errno is set to indicate the error.


The process group ID of any process equals the PID of the calling process. Thus, in particular, setsid() fails if the calling process is already a process group leader.


POSIX.1-2001, POSIX.1-2008, SVr4.


A child created via fork(2) inherits its parent's session ID. The session ID is preserved across an execve(2).

A process group leader is a process whose process group ID equals its PID. Disallowing a process group leader from calling setsid() prevents the possibility that a process group leader places itself in a new session while other processes in the process group remain in the original session; such a scenario would break the strict two-level hierarchy of sessions and process groups. In order to be sure that setsid() will succeed, fork(2) and _exit(2), and have the child do setsid().


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