After the fix from bug 17522, we do not use the remaining buffer space if the already-written buffer space is smaller than MB_LEN_MAX (16 bytes). As we start out with no written bytes (zero), we never use the full buffer, greatly reducing performance. The fix probably explains it better: - if (fp->_IO_write_ptr - fp->_IO_write_base < sizeof (mb_buf)) + if (fp->_IO_buf_end - fp->_IO_write_ptr < sizeof (mb_buf)) { /* Make sure we have room for at least one multibyte character. */ write_ptr = write_base = mb_buf; buf_end = mb_buf + sizeof (mb_buf); }
Patch posted: [PATCH] libio: Check remaining buffer size in _IO_wdo_write (bug 31183) <https://inbox.sourceware.org/libc-alpha/87bkakx3b0.fsf@oldenburg.str.redhat.com/>
Fixed for 2.39 via: commit ecc7c3deb9f347649c2078fcc0f94d4cedf92d60 Author: Florian Weimer <fweimer@redhat.com> Date: Tue Jan 2 14:36:17 2024 +0100 libio: Check remaining buffer size in _IO_wdo_write (bug 31183) The multibyte character needs to fit into the remaining buffer space, not the already-written buffer space. Without the fix, we were never moving the write pointer from the start of the buffer, always using the single-character fallback buffer. Fixes commit 04b76b5aa8b2d1d19066e42dd1 ("Don't error out writing a multibyte character to an unbuffered stream (bug 17522)").