SETPGID(P) POSIX Programmer's Manual SETPGID(P)
setpgid - set process group ID for job control
int setpgid(pid_t pid, pid_t pgid);
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.
Upon successful completion, setpgid() shall return 0; otherwise, -1 shall be returned and
errno shall be set to indicate the error.
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
EINVAL The value of the pgid argument is less than 0, or is not a value supported by the
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
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
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.
The setpgid() function shall group processes together for the purpose of signaling, place-
ment 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 synchro-
nization property of vfork(); that is, the shell is suspended until the child has com-
pleted 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 par-
ent 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.)
exec() , getpgrp() , setsid() , tcsetpgrp() , the Base Definitions volume of
IEEE Std 1003.1-2001, <sys/types.h>, <unistd.h>
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 orig-
inal Standard can be obtained online at http://www.opengroup.org/unix/online.html .
IEEE/The Open Group 2003 SETPGID(P)