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

       getpid, getppid - get process identification

       #include <sys/types.h>
       #include <unistd.h>

       pid_t getpid(void);
       pid_t getppid(void);

       getpid() returns the process ID (PID) of the calling process.  (This is
       often used by routines that generate unique temporary filenames.)

       getppid() returns the process ID of the parent of the calling  process.
       This will be either the ID of the process that created this process us-
       ing fork(), or, if that process has already terminated, the ID  of  the
       process  to which this process has been reparented (either init(1) or a
       "subreaper" process defined via the prctl(2) PR_SET_CHILD_SUBREAPER op-

       These functions are always successful.

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

       If  the  caller's parent is in a different PID namespace (see pid_name-
       spaces(7)), getppid() returns 0.

       From a kernel perspective, the PID (which  is  shared  by  all  of  the
       threads  in  a  multithreaded  process)  is sometimes also known as the
       thread group ID (TGID).  This  contrasts  with  the  kernel  thread  ID
       (TID),  which is unique for each thread.  For further details, see get-
       tid(2) and the discussion of the CLONE_THREAD flag in clone(2).

   C library/kernel differences
       From glibc version 2.3.4 up to and including version  2.24,  the  glibc
       wrapper  function  for  getpid() cached PIDs, with the goal of avoiding
       additional system calls when a process calls getpid() repeatedly.  Nor-
       mally  this  caching was invisible, but its correct operation relied on
       support in the wrapper functions for fork(2), vfork(2),  and  clone(2):
       if an application bypassed the glibc wrappers for these system calls by
       using syscall(2), then a call to getpid() in the child would return the
       wrong  value  (to  be  precise:  it  would return the PID of the parent
       process).  In addition, there were cases where  getpid()  could  return
       the wrong value even when invoking clone(2) via the glibc wrapper func-
       tion.  (For a discussion of one such case, see BUGS in clone(2).)  Fur-
       thermore,  the  complexity of the caching code had been the source of a
       few bugs within glibc over the years.

       Because of the aforementioned problems, since glibc version  2.25,  the
       PID cache is removed: calls to getpid() always invoke the actual system
       call, rather than returning a cached value.

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

       clone(2), fork(2), gettid(2), kill(2), exec(3), mkstemp(3), tempnam(3),
       tmpfile(3), tmpnam(3), credentials(7), pid_namespaces(7)

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Linux                             2019-03-06                         GETPID(2)
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