.\" Copyright (c) 2001-2003 The Open Group, All Rights Reserved 
.TH "SETPGID" 3P 2003 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\" setpgid 
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.SH NAME
setpgid \- set process group ID for job control
.SH SYNOPSIS
.LP
\fB#include <unistd.h>
.br
.sp
int setpgid(pid_t\fP \fIpid\fP\fB, pid_t\fP \fIpgid\fP\fB);
.br
\fP
.SH DESCRIPTION
.LP
The \fIsetpgid\fP() function shall either join an existing process
group or create a new process group within the session of
the calling process. The process group ID of a session leader shall
not change. Upon successful completion, the process group ID of
the process with a process ID that matches \fIpid\fP shall be set
to \fIpgid\fP. As a special case, if \fIpid\fP is 0, the
process ID of the calling process shall be used. Also, if \fIpgid\fP
is 0, the process ID of the indicated process shall be
used.
.SH RETURN VALUE
.LP
Upon successful completion, \fIsetpgid\fP() shall return 0; otherwise,
-1 shall be returned and \fIerrno\fP shall be set to
indicate the error.
.SH ERRORS
.LP
The \fIsetpgid\fP() function shall fail if:
.TP 7
.B EACCES
The value of the \fIpid\fP argument matches the process ID of a child
process of the calling process and the child process has
successfully executed one of the \fIexec\fP functions.
.TP 7
.B EINVAL
The value of the \fIpgid\fP argument is less than 0, or is not a value
supported by the implementation.
.TP 7
.B EPERM
The process indicated by the \fIpid\fP argument is a session leader.
.TP 7
.B EPERM
The value of the \fIpid\fP argument matches the process ID of a child
process of the calling process and the child process is
not in the same session as the calling process.
.TP 7
.B EPERM
The value of the \fIpgid\fP argument is valid but does not match the
process ID of the process indicated by the \fIpid\fP
argument and there is no process with a process group ID that matches
the value of the \fIpgid\fP argument in the same session as
the calling process.
.TP 7
.B ESRCH
The value of the \fIpid\fP argument does not match the process ID
of the calling process or of a child process of the calling
process.
.sp
.LP
\fIThe following sections are informative.\fP
.SH EXAMPLES
.LP
None.
.SH APPLICATION USAGE
.LP
None.
.SH RATIONALE
.LP
The \fIsetpgid\fP() function shall group processes together for the
purpose of signaling, placement in foreground or
background, and other job control actions.
.LP
The \fIsetpgid\fP() function is similar to the \fIsetpgrp\fP() function
of 4.2 BSD,
except that 4.2 BSD allowed the specified new process group to assume
any value. This presents certain security problems and is
more flexible than necessary to support job control.
.LP
To provide tighter security, \fIsetpgid\fP() only allows the calling
process to join a process group already in use inside its
session or create a new process group whose process group ID was equal
to its process ID.
.LP
When a job control shell spawns a new job, the processes in the job
must be placed into a new process group via
\fIsetpgid\fP(). There are two timing constraints involved in this
action:
.IP " 1." 4
The new process must be placed in the new process group before the
appropriate program is launched via one of the \fIexec\fP functions.
.LP
.IP " 2." 4
The new process must be placed in the new process group before the
shell can correctly send signals to the new process
group.
.LP
.LP
To address these constraints, the following actions are performed.
The new processes call \fIsetpgid\fP() to alter their own
process groups after \fIfork\fP() but before \fIexec\fP. This satisfies
the first constraint. Under 4.3 BSD, the second constraint is satisfied
by
the synchronization property of \fIvfork\fP(); that is, the shell
is suspended until the
child has completed the \fIexec\fP, thus ensuring that the child has
completed the
\fIsetpgid\fP(). A new version of \fIfork\fP() with this same synchronization
property was
considered, but it was decided instead to merely allow the parent
shell process to adjust the process group of its child processes
via \fIsetpgid\fP(). Both timing constraints are now satisfied by
having both the parent shell and the child attempt to adjust the
process group of the child process; it does not matter which succeeds
first.
.LP
Since it would be confusing to an application to have its process
group change after it began executing (that is, after \fIexec\fP),
and because the child process would already have adjusted its process
group before
this, the [EACCES] error was added to disallow this.
.LP
One non-obvious use of \fIsetpgid\fP() is to allow a job control shell
to return itself to its original process group (the one
in effect when the job control shell was executed). A job control
shell does this before returning control back to its parent when
it is terminating or suspending itself as a way of restoring its job
control "state" back to what its parent would expect. (Note
that the original process group of the job control shell typically
matches the process group of its parent, but this is not
necessarily always the case.)
.SH FUTURE DIRECTIONS
.LP
None.
.SH SEE ALSO
.LP
\fIexec\fP(), \fIgetpgrp\fP(), \fIsetsid\fP(), \fItcsetpgrp\fP(),
the Base Definitions volume of
IEEE\ Std\ 1003.1-2001, \fI<sys/types.h>\fP, \fI<unistd.h>\fP
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group. In the
event of any discrepancy between this version and the original IEEE and
The Open Group Standard, the original IEEE and The Open Group Standard
is the referee document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .