The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
A newer edition of this document exists here


setsid - create session and set process group ID


#include <unistd.h>

pid_t setsid(void);


The setsid() function shall create a new session, if the calling process is not a process group leader. Upon return the calling process shall be the session leader of this new session, shall be the process group leader of a new process group, and shall have no controlling terminal. The process group ID of the calling process shall be set equal to the process ID of the calling process. The calling process shall be the only process in the new process group and the only process in the new session.


Upon successful completion, setsid() shall return the value of the new process group ID of the calling process. Otherwise, it shall return (pid_t)-1 and set errno to indicate the error.


The setsid() function shall fail if:

The calling process is already a process group leader, or the process group ID of a process other than the calling process matches the process ID of the calling process.

The following sections are informative.






The setsid() function is similar to the setpgrp() function of System V. System V, without job control, groups processes into process groups and creates new process groups via setpgrp(); only one process group may be part of a login session.

Job control allows multiple process groups within a login session. In order to limit job control actions so that they can only affect processes in the same login session, this volume of IEEE Std 1003.1-2001 adds the concept of a session that is created via setsid(). The setsid() function also creates the initial process group contained in the session. Additional process groups can be created via the setpgid() function. A System V process group would correspond to a POSIX System Interfaces session containing a single POSIX process group. Note that this function requires that the calling process not be a process group leader. The usual way to ensure this is true is to create a new process with fork() and have it call setsid(). The fork() function guarantees that the process ID of the new process does not match any existing process group ID.




getsid(), setpgid(), setpgrp(), the Base Definitions volume of IEEE Std 1003.1-2001, <sys/types.h>, <unistd.h>


First released in Issue 3. Included for alignment with the POSIX.1-1988 standard.

Issue 6

In the SYNOPSIS, the optional include of the <sys/types.h> header is removed.

The following new requirements on POSIX implementations derive from alignment with the Single UNIX Specification:

End of informative text.

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