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From: "fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>
To: glibc-bugs@sourceware.org
Subject: [Bug stdio/31137] Regression vfprintf + setbuf (no buffering)
Date: Mon, 11 Dec 2023 13:48:16 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D31137

--- Comment #2 from Florian Weimer <fweimer at redhat dot com> ---
The reproducer is misleading because GCC translates the fprintf call into an
fputs call. With -fno-builtin, I get this instead:

=E2=80=9C
no setbuf
=3D=3D=3D=3D=3D=3D=3D=3D=3D

Calling fprintf
idx=3D0 size=3D511

Calling vfprintf
idx=3D0 size=3D511


setbuf _IONBF
=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D

Calling fprintf
idx=3D0 size=3D128
idx=3D1 size=3D128
idx=3D2 size=3D128
idx=3D3 size=3D127

Calling vfprintf
idx=3D0 size=3D128
idx=3D1 size=3D128
idx=3D2 size=3D128
idx=3D3 size=3D127
=E2=80=9D

So at least it's consistent.

Echoing what Andreas said, I think you are just observing the effect of a
reduced buffer size. It was always there, except that we used to have a lar=
ge
on-stack buffer in vfprintf (8192 bytes, I believe), which probably papered
over it.

The 128 bytes fallback buffer size is arbitrary. We can probably increase it
somewhat if it reduces the compatibility impact of this change. But fopenco=
okie
is inherently a byte stream interface, so there is no guarantee that one
fprintf call will be translated into one cookie write call.

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