getpid(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

GETPID(2)               Linux Programmer's Manual              GETPID(2)

NAME         top

       getpid, getppid - get process identification

SYNOPSIS         top

       #include <sys/types.h>
       #include <unistd.h>

       pid_t getpid(void);
       pid_t getppid(void);

DESCRIPTION         top

       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         top

       These functions are always successful.

CONFORMING TO         top

       POSIX.1-2001, POSIX.1-2008, 4.3BSD, SVr4.

NOTES         top

       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         top

       clone(2), fork(2), gettid(2), kill(2), exec(3), mkstemp(3),
       tempnam(3), tmpfile(3), tmpnam(3), credentials(7),
       pid_namespaces(7)

COLOPHON         top

       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/.

Linux                          2020-11-01                      GETPID(2)

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