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 IEEE Std 1003.1-2001, <sys/types.h>, <unistd.h>
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 .