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GETPID(2)                  Linux Programmer's ManualGETPID(2)NAME       getpid, getppid - get process identificationSYNOPSIS       #include <sys/types.h>       #include <unistd.h>       pid_t getpid(void);       pid_t getppid(void);DESCRIPTION       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 (eitherinit(1) or a       "subreaper" process defined via theprctl(2) PR_SET_CHILD_SUBREAPER op-       eration).ERRORS       These functions are always successful.CONFORMING TO       POSIX.1-2001, POSIX.1-2008, 4.3BSD, SVr4.NOTES       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 inclone(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 forfork(2),vfork(2),  andclone(2):       if an application bypassed the glibc wrappers for these system calls by       usingsyscall(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 invokingclone(2) via the glibc wrapper func-       tion.  (For a discussion of one such case, see BUGS inclone(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.   Seesyscall(2) for details regarding       register mapping.SEE ALSOclone(2),fork(2),gettid(2),kill(2),exec(3),mkstemp(3),tempnam(3),tmpfile(3),tmpnam(3),credentials(7),pid_namespaces(7)COLOPHON       This  page  is  part of release 5.05 of the Linux man-pages project.  A       description of the project, information about reporting bugs,  and  the       latest     version     of     this    page,    can    be    found    at       https://www.kernel.org/doc/man-pages/.Linux                             2019-03-06GETPID(2)