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)