GETPID(2)                 (2020-11-01)                  GETPID(2)

     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 pro-
          cess.  (This is often used by routines that generate unique
          temporary filenames.)

          getppid() returns the process ID of the parent of the call-
          ing 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 pro-
          cess 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 invis-
          ible, 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

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     GETPID(2)                 (2020-11-01)                  GETPID(2)

          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).)  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.

          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 map-
          ping.

     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
          This page is part of release 5.10 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 https://www.kernel.org/doc/man-pages/.

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