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From: "vries at gcc dot gnu.org" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug manual/26959] New: __malloc_hook example does not adhere to
 documented usage
Date: Fri, 27 Nov 2020 08:38:08 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D26959

            Bug ID: 26959
           Summary: __malloc_hook example does not adhere to documented
                    usage
           Product: glibc
           Version: unspecified
            Status: NEW
          Severity: minor
          Priority: P2
         Component: manual
          Assignee: unassigned at sourceware dot org
          Reporter: vries at gcc dot gnu.org
                CC: mtk.manpages at gmail dot com
  Target Milestone: ---

Consider
https://www.gnu.org/software/libc/manual/html_node/Hooks-for-Malloc.html .

It states:
...
An issue to look out for is the time at which the malloc hook functions can=
 be
safely installed. If the hook functions call the malloc-related functions
recursively, it is necessary that malloc has already properly initialized
itself at the time when __malloc_hook etc. is assigned to.
...

Then an example follows, which does call malloc recursively, but does nothi=
ng
specific before __malloc_hook is assigned to for the first time.

[ My guess is that the doc list two solutions for the same problem: the fir=
st
call to malloc may change __malloc_hook from malloc_hook_ini to malloc_chec=
k,
say if you run with GLIBC_TUNABLES=3Dglibc.malloc.check=3D3.

The first solution is to ensure that malloc is called once before saving and
then assigning __malloc_hook, such that the saved __malloc_hook value can be
assumed to remain constant, and does not need to be saved again after every
recursive call to malloc.

The second solution is to make sure the __malloc_hook value is saved again
after every recursive call to malloc. ]

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