man_2_getpid.txt

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
       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 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)),  getp‐
       pid() 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  con‐
       trasts  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  cor‐
       rect  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 discussion of one such case, see BUGS in clone(2).)  Further‐
       more, 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  re‐
       moved:  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 get‐
       pid() and getppid() wrapper functions transparently deal with this.  See syscall(2) for
       details regarding register mapping.

SEE ALSO
       clone(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                                     2019-03-06                                 GETPID(2)