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[thirdparty/man-pages.git] / man2 / getpid.2

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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)"
-.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)