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.\" Copyright 1993 Rickard E. Faith (faith@cs.unc.edu)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH GETPID 2 2021-03-22 "Linux man-pages (unreleased)" "Linux Programmer's Manual"
.SH NAME
getpid, getppid \- get process identification
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.PP
.B pid_t getpid(void);
.B pid_t getppid(void);
.fi
.SH DESCRIPTION
.BR getpid ()
returns the process ID (PID) of the calling process.
(This is often used by
routines that generate unique temporary filenames.)
.PP
.BR 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
.BR fork (),
or, if that process has already terminated,
the ID of the process to which this process has been reparented (either
.BR init (1)
or a "subreaper" process defined via the
.BR prctl (2)
.B PR_SET_CHILD_SUBREAPER
operation).
.SH ERRORS
These functions are always successful.
.SH STANDARDS
POSIX.1-2001, POSIX.1-2008, 4.3BSD, SVr4.
.SH NOTES
If the caller's parent is in a different PID namespace (see
.BR pid_namespaces (7)),
.BR getppid ()
returns 0.
.PP
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
.BR gettid (2)
and the discussion of the
.B CLONE_THREAD
flag in
.BR clone (2).
.\"
.SS C library/kernel differences
From glibc version 2.3.4 up to and including version 2.24,
the glibc wrapper function for
.BR getpid ()
cached PIDs,
with the goal of avoiding additional system calls when a process calls
.BR getpid ()
repeatedly.
Normally this caching was invisible,
but its correct operation relied on support in the wrapper functions for
.BR fork (2),
.BR vfork (2),
and
.BR clone (2):
if an application bypassed the glibc wrappers for these system calls by using
.BR syscall (2),
then a call to
.BR getpid ()
in the child would return the wrong value
(to be precise: it would return the PID of the parent process).
.\" The following program demonstrates this "feature":
.\"
.\" #define _GNU_SOURCE
.\" #include <sys/syscall.h>
.\" #include <sys/wait.h>
.\" #include <stdint.h>
.\" #include <stdio.h>
.\" #include <stdlib.h>
.\" #include <unistd.h>
.\"
.\" int
.\" main(int argc, char *argv[])
.\" {
.\"    /* The following statement fills the getpid() cache */
.\"
.\"    printf("parent PID = %ld\n", (intmax_t) getpid());
.\"
.\"    if (syscall(SYS_fork) == 0) {
.\"        if (getpid() != syscall(SYS_getpid))
.\"            printf("child getpid() mismatch: getpid()=%jd; "
.\"                    "syscall(SYS_getpid)=%ld\n",
.\"                    (intmax_t) getpid(), (long) syscall(SYS_getpid));
.\"        exit(EXIT_SUCCESS);
.\"    }
.\"    wait(NULL);
.\"}
In addition, there were cases where
.BR getpid ()
could return the wrong value even when invoking
.BR clone (2)
via the glibc wrapper function.
(For a discussion of one such case, see BUGS in
.BR clone (2).)
Furthermore, the complexity of the caching code had been
the source of a few bugs within glibc over the years.
.PP
Because of the aforementioned problems,
since glibc version 2.25, the PID cache is removed:
.\" commit c579f48edba88380635ab98cb612030e3ed8691e
.\" https://sourceware.org/glibc/wiki/Release/2.25#pid_cache_removal
calls to
.BR getpid ()
always invoke the actual system call, rather than returning a cached value.
.\" FIXME .
.\" Review progress of https://bugzilla.redhat.com/show_bug.cgi?id=1469757
.PP
On Alpha, instead of a pair of
.BR getpid ()
and
.BR getppid ()
system calls, a single
.BR getxpid ()
system call is provided, which returns a pair of PID and parent PID.
The glibc
.BR getpid ()
and
.BR getppid ()
wrapper functions transparently deal with this.
See
.BR syscall (2)
for details regarding register mapping.
.SH SEE ALSO
.BR clone (2),
.BR fork (2),
.BR gettid (2),
.BR kill (2),
.BR exec (3),
.BR mkstemp (3),
.BR tempnam (3),
.BR tmpfile (3),
.BR tmpnam (3),
.BR credentials (7),
.BR pid_namespaces (7)