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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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SETPGID(3P) POSIX Programmer's Manual SETPGID(3P)
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.
setpgid — set process group ID for job control
#include <unistd.h>
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 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.
None.
None.
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.)
None.
exec(1p), getpgrp(3p), setsid(3p), tcsetpgrp(3p)
The Base Definitions volume of POSIX.1‐2017, sys_types.h(0p),
unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 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 .
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 .
IEEE/The Open Group 2017 SETPGID(3P)
Pages that refer to this page: unistd.h(0p), _Exit(3p), getpgid(3p), getpgrp(3p), getpid(3p), getppid(3p), getsid(3p), posix_spawn(3p), setpgrp(3p), setsid(3p), tcgetpgrp(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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