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Describe getpid()'s PID caching and its consequences. 

getpid() caches the PID after the first call.  This relies
on support in the glibc wrappers for fork()/vfork()/clone().
However, if syscall() is used to directly invoke fork()/vfork()/clone(),
the cache is not updated, and getpid() in the child procudes the wrong
result.
This commit is contained in:
Michael Kerrisk 2008-09-23 04:12:27 +02:00
parent 59bf85dffc
commit 67f0f50d07
2 changed files with 58 additions and 1 deletions
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@ -9,6 +9,7 @@ Contributors
The following people contributed notes, ideas, or patches that have
been incorporated in changes in this release:
John Reiser <jreiser@bitwagon.com>
Apologies if I missed anyone!
@ -26,3 +27,6 @@ Global changes
Changes to individual pages
---------------------------
getpid.2
mtk, after a comment by John Reiser
Describe getpid()'s PID caching and its consequences.

55
man2/getpid.2
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@ -20,7 +20,7 @@
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.TH GETPID 2 1993-07-23 "Linux" "Linux Programmer's Manual"
.TH GETPID 2 2008-09-23 "Linux" "Linux Programmer's Manual"
.SH NAME
getpid, getppid \- get process identification
.SH SYNOPSIS
@ -43,7 +43,60 @@ returns the process ID of the parent of the calling process.
These functions are always successful.
.SH "CONFORMING TO"
POSIX.1-2001, 4.3BSD, SVr4.
.SH NOTES
Since glibc version 2.3.4,
the glibc wrapper function for
.BR getpid ()
caches PIDs,
so as to avoid additional system calls when a process calls
.BR getpid ()
repeatedly.
Normally this caching is invisible,
but its correct operation relies on support in the wrapper functions for
.BR fork (2),
.BR vfork (2),
and
.BR clone (2):
if an application bypasses the glibc wrappers for these system calls by using
.BR syscall (2),
then a call to
.BR getpid ()
in the child will return the wrong value
(to be precise: it will 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 <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", (long) getpid());
.\"
.\" if (syscall(SYS_fork) == 0) {
.\" if (getpid() != syscall(SYS_getpid))
.\" printf("child getpid() mismatch: getpid()=%ld; "
.\" "syscall(SYS_getpid)=%ld\n",
.\" (long) getpid(), (long) syscall(SYS_getpid));
.\" exit(EXIT_SUCCESS);
.\" }
.\" wait(NULL);
.\"}
See also
.BR clone (2)
for discussion of a case where
.BR getpid ()
may return the wrong value even when invoking
.BR clone (2)
via the glibc wrapper function.
.SH "SEE ALSO"
.BR clone (2),
.BR fork (2),
.BR kill (2),
.BR exec (3),