SETPGID
Section: POSIX Programmer's Manual (3P)
Updated: 2013
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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.
NAME
setpgid
--- set process group ID for job control
SYNOPSIS
#include <unistd.h>
int setpgid(pid_t pid, pid_t pgid);
DESCRIPTION
The
setpgid()
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
pid
shall be set to
pgid.
As a special case, if
pid
is 0, the process ID of the calling process shall be used. Also, if
pgid
is 0, the process ID of the indicated process shall be used.
RETURN VALUE
Upon successful completion,
setpgid()
shall return 0; otherwise, -1 shall be returned and
errno
shall be set to indicate the error.
ERRORS
The
setpgid()
function shall fail if:
- EACCES
-
The value of the
pid
argument matches the process ID of a child process of the calling
process and the child process has successfully executed one of the
exec
functions.
- EINVAL
-
The value of the
pgid
argument is less than 0, or is not a value supported by the
implementation.
- EPERM
-
The process indicated by the
pid
argument is a session leader.
- EPERM
-
The value of the
pid
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.
- EPERM
-
The value of the
pgid
argument is valid but does not match the process ID of the process
indicated by the
pid
argument and there is no process with a process group ID that matches
the value of the
pgid
argument in the same session as the calling process.
- ESRCH
-
The value of the
pid
argument does not match the process ID of the calling process or of a
child process of the calling process.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
None.
RATIONALE
The
setpgid()
function shall group processes together for the purpose of
signaling, placement in foreground or background,
and other job control actions.
The
setpgid()
function is similar to the
setpgrp()
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.
To provide tighter security,
setpgid()
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.
When a job control shell spawns a new job, the processes in the
job must be placed into a new process group via
setpgid().
There are two timing constraints involved in this action:
- 1.
-
The new process must be placed in the new process group before the
appropriate program is launched via one of the
exec
functions.
- 2.
-
The new process must be placed in the new process group before the
shell can correctly send signals to the new process group.
To address these constraints, the following actions are performed. The
new processes call
setpgid()
to alter their own process groups after
fork()
but before
exec.
This satisfies the first constraint. Under 4.3 BSD, the second
constraint is satisfied by the synchronization property of
vfork();
that is, the shell is suspended until the child has completed the
exec,
thus ensuring that the child has completed the
setpgid().
A new version of
fork()
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
setpgid().
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.
Since it would be confusing to an application to have its process
group change after it began executing (that is, after
exec),
and because the child process would already have adjusted its process
group before this, the
[EACCES]
error was added to disallow this.
One non-obvious use of
setpgid()
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.)
FUTURE DIRECTIONS
None.
SEE ALSO
exec,
getpgrp(),
setsid(),
tcsetpgrp()
The Base Definitions volume of POSIX.1-2008,
<sys_types.h>,
<unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, Copyright (C) 2013 by the Institute of
Electrical and Electronics Engineers, Inc and The Open Group.
(This is POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) 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.unix.org/online.html .
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
Index
- PROLOG
-
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- EXAMPLES
-
- APPLICATION USAGE
-
- RATIONALE
-
- FUTURE DIRECTIONS
-
- SEE ALSO
-
- COPYRIGHT
-
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