NAME

       getpid, getppid - get process identification

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #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.

VERSIONS

       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.

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001, 4.3BSD, SVr4.

   C library/kernel differences
       From  glibc  2.3.4 up to and including glibc 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 2.25, the PID cache is removed:  calls
       to getpid() always invoke the actual system call, rather than returning a cached value.

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
       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).

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)