getpid(2)

1getpid(2)                     System Calls Manual                    getpid(2)
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NAME

6       getpid, getppid - get process identification
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LIBRARY

9       Standard C library (libc, -lc)
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SYNOPSIS

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

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

29       These functions are always successful.
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VERSIONS

32       On  Alpha,  instead of a pair of getpid() and getppid() system calls, a
33       single getxpid() system call is provided, which returns a pair  of  PID
34       and  parent  PID.   The  glibc getpid() and getppid() wrapper functions
35       transparently deal with this.  See  syscall(2)  for  details  regarding
36       register mapping.
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STANDARDS

39       POSIX.1-2008.
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HISTORY

42       POSIX.1-2001, 4.3BSD, SVr4.
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44   C library/kernel differences
45       From  glibc  2.3.4  up  to  and including glibc 2.24, the glibc wrapper
46       function for getpid() cached PIDs, with the goal of avoiding additional
47       system  calls  when a process calls getpid() repeatedly.  Normally this
48       caching was invisible, but its correct operation relied on  support  in
49       the wrapper functions for fork(2), vfork(2), and clone(2): if an appli‐
50       cation bypassed the glibc wrappers for  these  system  calls  by  using
51       syscall(2), then a call to getpid() in the child would return the wrong
52       value (to be precise: it would return the PID of the  parent  process).
53       In  addition,  there  were  cases where getpid() could return the wrong
54       value even when invoking clone(2) via the glibc wrapper function.  (For
55       a discussion of one such case, see BUGS in clone(2).)  Furthermore, the
56       complexity of the caching code had been the source of a few bugs within
57       glibc over the years.
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59       Because of the aforementioned problems, since glibc 2.25, the PID cache
60       is removed: calls to getpid() always invoke  the  actual  system  call,
61       rather than returning a cached value.
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NOTES

64       If  the  caller's parent is in a different PID namespace (see pid_name‐
65       spaces(7)), getppid() returns 0.
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67       From a kernel perspective, the PID (which  is  shared  by  all  of  the
68       threads  in  a  multithreaded  process)  is sometimes also known as the
69       thread group ID (TGID).  This  contrasts  with  the  kernel  thread  ID
70       (TID),  which is unique for each thread.  For further details, see get‐
71       tid(2) and the discussion of the CLONE_THREAD flag in clone(2).
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