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From: "hpa at zytor dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug malloc/31222] New: RFE: malloc_size() and/or a realloc() which
 returns the allocated size
Date: Mon, 08 Jan 2024 06:13:37 +0000
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X-Bugzilla-Product: glibc
X-Bugzilla-Component: malloc
X-Bugzilla-Version: unspecified
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X-Bugzilla-Severity: enhancement
X-Bugzilla-Who: hpa at zytor dot com
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X-Bugzilla-Priority: P2
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D31222

            Bug ID: 31222
           Summary: RFE: malloc_size() and/or a realloc() which returns
                    the allocated size
           Product: glibc
           Version: unspecified
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P2
         Component: malloc
          Assignee: unassigned at sourceware dot org
          Reporter: hpa at zytor dot com
  Target Milestone: ---

glibc currently provides malloc_usable_size(), which has the caveat:

of which the programmer should rely on. This function is intended to only be
used for diagnostics and statistics; writing to the excess memory without f=
irst
calling realloc(3) to resize the allocation is not supported.  The returned
value is only valid at the time of the call.

In contrast, BSD provides malloc_size(), which provides the permanently
allocated size of the memory area, which can be relied upon by the programm=
er.

This is really useful in a loop of this form:

void *buf =3D NULL;
size_t bufsiz =3D 0;

while (1) {
    size_t needbuf =3D bufsiz;
    function_writing_to_sized_buffer(buf, &needbuf, ....);
    if (needbuf <=3D bufsiz)
        break; /* Buffer big enough, operation successful */
    buf =3D realloc(buf, needbuf);  /* Need to handle realloc() failure */
    bufsiz =3D needbuf;
}

This form is safe against function_writing_to_sized_buffer() not producing =
the
same output on a subsequent call.

However, this also means that work is thrown away unnecessarily if the block
actually returned by realloc() was rounded to a value big enough and the
realloc() was unnecessary.

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