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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
     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


     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_names-
     paces(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.


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


     This  page  is  part of release 4.16 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

Linux 2017-11-26 GETPID(2)

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