[Bug stdio/31137] Regression vfprintf + setbuf (no buffering)

["fweimer at redhat dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=31137

--- 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:

“
no setbuf
=========

Calling fprintf
idx=0 size=511

Calling vfprintf
idx=0 size=511


setbuf _IONBF
=============

Calling fprintf
idx=0 size=128
idx=1 size=128
idx=2 size=128
idx=3 size=127

Calling vfprintf
idx=0 size=128
idx=1 size=128
idx=2 size=128
idx=3 size=127
”

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 large
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 fopencookie
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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