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       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_namespaces(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 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
       discussion 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.

       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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