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getpid.2: More tidy-ups in discussion of removal of getpid() PID caching 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2017-07-13 07:05:28 +02:00
parent cd065c7aec
commit 9338d37b76
1 changed files with 17 additions and 13 deletions
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30
man2/getpid.2
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@ -61,25 +61,25 @@ If the caller's parent is in a different PID namespace (see
returns 0.
.\"
.SS C library/kernel differences
Since glibc version 2.3.4,
From glibc version 2.3.4 up to and including version 2.24,
the glibc wrapper function for
.BR getpid ()
caches PIDs,
so as to avoid additional system calls when a process calls
cached PIDs,
with the goal of avoiding 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
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 bypasses the glibc wrappers for these system calls by using
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 will return the wrong value
(to be precise: it will return the PID of the parent process).
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
@ -105,16 +105,20 @@ in the child will return the wrong value
.\" }
.\" wait(NULL);
.\"}
See also
.BR clone (2)
for discussion of a case where
In addition, there were cases where
.BR getpid ()
may return the wrong value even when invoking
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
Since glibc version 2.25, the PID cache is removed:
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.