getppid(2)

1GETPID(2)                  Linux Programmer's Manual                 GETPID(2)
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NAME

6       getpid, getppid - get process identification
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SYNOPSIS

9       #include <unistd.h>
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11       pid_t getpid(void);
12       pid_t getppid(void);
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DESCRIPTION

15       getpid() returns the process ID (PID) of the calling process.  (This is
16       often used by routines that generate unique temporary filenames.)
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18       getppid() returns the process ID of the parent of the calling  process.
19       This will be either the ID of the process that created this process us‐
20       ing fork(), or, if that process has already terminated, the ID  of  the
21       process  to which this process has been reparented (either init(1) or a
22       "subreaper" process defined via the prctl(2) PR_SET_CHILD_SUBREAPER op‐
23       eration).
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ERRORS

26       These functions are always successful.
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CONFORMING TO

29       POSIX.1-2001, POSIX.1-2008, 4.3BSD, SVr4.
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NOTES

32       If  the  caller's parent is in a different PID namespace (see pid_name‐
33       spaces(7)), getppid() returns 0.
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35       From a kernel perspective, the PID (which  is  shared  by  all  of  the
36       threads  in  a  multithreaded  process)  is sometimes also known as the
37       thread group ID (TGID).  This  contrasts  with  the  kernel  thread  ID
38       (TID),  which is unique for each thread.  For further details, see get‐
39       tid(2) and the discussion of the CLONE_THREAD flag in clone(2).
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41   C library/kernel differences
42       From glibc version 2.3.4 up to and including version  2.24,  the  glibc
43       wrapper  function  for  getpid() cached PIDs, with the goal of avoiding
44       additional system calls when a process calls getpid() repeatedly.  Nor‐
45       mally  this  caching was invisible, but its correct operation relied on
46       support in the wrapper functions for fork(2), vfork(2),  and  clone(2):
47       if an application bypassed the glibc wrappers for these system calls by
48       using syscall(2), then a call to getpid() in the child would return the
49       wrong  value  (to  be  precise:  it  would return the PID of the parent
50       process).  In addition, there were cases where  getpid()  could  return
51       the wrong value even when invoking clone(2) via the glibc wrapper func‐
52       tion.  (For a discussion of one such case, see BUGS in clone(2).)  Fur‐
53       thermore,  the  complexity of the caching code had been the source of a
54       few bugs within glibc over the years.
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56       Because of the aforementioned problems, since glibc version  2.25,  the
57       PID cache is removed: calls to getpid() always invoke the actual system
58       call, rather than returning a cached value.
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60       On Alpha, instead of a pair of getpid() and getppid() system  calls,  a
61       single  getxpid()  system call is provided, which returns a pair of PID
62       and parent PID.  The glibc getpid()  and  getppid()  wrapper  functions
63       transparently  deal  with  this.   See syscall(2) for details regarding
64       register mapping.
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COLOPHON

71       This  page  is  part of release 5.13 of the Linux man-pages project.  A
72       description of the project, information about reporting bugs,  and  the
73       latest     version     of     this    page,    can    be    found    at
74       https://www.kernel.org/doc/man-pages/.
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78Linux                             2021-03-22                         GETPID(2)