1SETPGID(3P)                POSIX Programmer's Manual               SETPGID(3P)
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PROLOG

6       This  manual  page is part of the POSIX Programmer's Manual.  The Linux
7       implementation of this interface may differ (consult the  corresponding
8       Linux  manual page for details of Linux behavior), or the interface may
9       not be implemented on Linux.
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11

NAME

13       setpgid — set process group ID for job control
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SYNOPSIS

16       #include <unistd.h>
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18       int setpgid(pid_t pid, pid_t pgid);
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DESCRIPTION

21       The setpgid() function shall either join an existing process  group  or
22       create a new process group within the session of the calling process.
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24       The process group ID of a session leader shall not change.
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26       Upon  successful completion, the process group ID of the process with a
27       process ID that matches pid shall be set to pgid.
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29       As a special case, if pid is 0, the process ID of the  calling  process
30       shall  be  used.  Also,  if  pgid is 0, the process ID of the indicated
31       process shall be used.
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RETURN VALUE

34       Upon successful completion, setpgid() shall  return  0;  otherwise,  −1
35       shall be returned and errno shall be set to indicate the error.
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ERRORS

38       The setpgid() function shall fail if:
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40       EACCES The  value of the pid argument matches the process ID of a child
41              process of the calling process and the child  process  has  suc‐
42              cessfully executed one of the exec functions.
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44       EINVAL The value of the pgid argument is less than 0, or is not a value
45              supported by the implementation.
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47       EPERM  The process indicated by the pid argument is a session leader.
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49       EPERM  The value of the pid argument matches the process ID of a  child
50              process  of  the calling process and the child process is not in
51              the same session as the calling process.
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53       EPERM  The value of the pgid argument is valid but does not  match  the
54              process  ID  of  the  process  indicated by the pid argument and
55              there is no process with a process group  ID  that  matches  the
56              value  of  the  pgid argument in the same session as the calling
57              process.
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59       ESRCH  The value of the pid argument does not match the process  ID  of
60              the  calling  process  or  of  a  child  process  of the calling
61              process.
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63       The following sections are informative.
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EXAMPLES

66       None.
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APPLICATION USAGE

69       None.
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RATIONALE

72       The setpgid() function shall group processes together for  the  purpose
73       of signaling, placement in foreground or background, and other job con‐
74       trol actions.
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76       The setpgid() function is similar to the setpgrp() function of 4.2 BSD,
77       except  that  4.2 BSD allowed the specified new process group to assume
78       any value. This presents certain security problems and is more flexible
79       than necessary to support job control.
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81       To  provide tighter security, setpgid() only allows the calling process
82       to join a process group already in use inside its session or  create  a
83       new process group whose process group ID was equal to its process ID.
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85       When  a  job  control  shell spawns a new job, the processes in the job
86       must be placed into a new process group via setpgid().  There  are  two
87       timing constraints involved in this action:
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89        1. The  new process must be placed in the new process group before the
90           appropriate program is launched via one of the exec functions.
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92        2. The new process must be placed in the new process group before  the
93           shell can correctly send signals to the new process group.
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95       To  address these constraints, the following actions are performed. The
96       new processes call setpgid() to alter their own  process  groups  after
97       fork() but before exec.  This satisfies the first constraint. Under 4.3
98       BSD, the second constraint is satisfied by the synchronization property
99       of  vfork();  that  is, the shell is suspended until the child has com‐
100       pleted the exec,  thus  ensuring  that  the  child  has  completed  the
101       setpgid().   A  new  version  of  fork() with this same synchronization
102       property was considered, but it was decided instead to merely allow the
103       parent shell process to adjust the process group of its child processes
104       via setpgid().  Both timing constraints are  now  satisfied  by  having
105       both the parent shell and the child attempt to adjust the process group
106       of the child process; it does not matter which succeeds first.
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108       Since it would be confusing to an application to have its process group
109       change  after it began executing (that is, after exec), and because the
110       child process would already have  adjusted  its  process  group  before
111       this, the [EACCES] error was added to disallow this.
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113       One  non-obvious  use  of  setpgid() is to allow a job control shell to
114       return itself to its original process group (the one in effect when the
115       job  control  shell was executed). A job control shell does this before
116       returning control back to its parent when it is terminating or suspend‐
117       ing itself as a way of restoring its job control ``state'' back to what
118       its parent would expect. (Note that the original process group  of  the
119       job  control  shell  typically matches the process group of its parent,
120       but this is not necessarily always the case.)
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FUTURE DIRECTIONS

123       None.
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SEE ALSO

126       exec, getpgrp(), setsid(), tcsetpgrp()
127
128       The Base Definitions volume of POSIX.1‐2008, <sys_types.h>, <unistd.h>
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131       Portions of this text are reprinted and reproduced in  electronic  form
132       from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
133       -- Portable Operating System Interface (POSIX),  The  Open  Group  Base
134       Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
135       cal and Electronics Engineers,  Inc  and  The  Open  Group.   (This  is
136       POSIX.1-2008  with  the  2013  Technical Corrigendum 1 applied.) In the
137       event of any discrepancy between this version and the original IEEE and
138       The  Open Group Standard, the original IEEE and The Open Group Standard
139       is the referee document. The original Standard can be  obtained  online
140       at http://www.unix.org/online.html .
141
142       Any  typographical  or  formatting  errors that appear in this page are
143       most likely to have been introduced during the conversion of the source
144       files  to  man page format. To report such errors, see https://www.ker
145       nel.org/doc/man-pages/reporting_bugs.html .
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149IEEE/The Open Group                  2013                          SETPGID(3P)
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