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)), 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.

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

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