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

NAME
       getpid, getppid - get process identification

SYNOPSIS
       #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 rou-
       tines 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 using 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
       operation).

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_namespaces(7)),  getppid()
       returns 0.

       From  a  kernel  perspective,  the  PID (which is shared by all of the threads in a multi-
       threaded 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 gettid(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.  Normally 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 function.  (For a dis-
       cussion of one such case, see BUGS in  clone(2).)   Furthermore,  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.

SEE ALSO
       clone(2),  fork(2),  gettid(2), kill(2), exec(3), mkstemp(3), tempnam(3), tmpfile(3), tmp-
       nam(3), credentials(7), pid_namespaces(7)

COLOPHON
       This page is part of release 4.15 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                                       2017-11-26                                  GETPID(2)