From: Xiang Gao <gaoxiang@kylinos.cn>
setvbuf (stream, NULL, _IOFBF, 0) takes a special path in
_IO_setvbuf: if the byte-oriented buffer base is NULL, it calls
_IO_DOALLOCATE and returns without invoking the stream setbuf hook.
For open_wmemstream, the byte-oriented buffer base is NULL although
the wide result buffer has already been initialized in _wide_data.
As a result, this path calls _IO_wdefault_doallocate, which may
replace the wide buffer managed by open_wmemstream.
Install an open_wmemstream-specific doallocate hook that leaves
the growable result buffer unchanged. Add a regression test for this
path.
Install a narrow memstream doallocate hook as well. It keeps both
memstream vtables consistent (generic stdio allocation must not
replace the growable result buffer).
Signed-off-by: Xiang Gao <gaoxiang@kylinos.cn>
---
libio/Makefile | 1 +
libio/libioP.h | 3 ++
libio/memstream.c | 9 ++++
libio/tst-wmemstream-setvbuf-doallocate.c | 64 +++++++++++++++++++++++
libio/vtables.c | 6 ++-
libio/wmemstream.c | 9 ++++
6 files changed, 90 insertions(+), 2 deletions(-)
create mode 100644 libio/tst-wmemstream-setvbuf-doallocate.c
On Mai 06 2026, Gao Xiang wrote:
> diff --git a/libio/tst-wmemstream-setvbuf-doallocate.c b/libio/tst-wmemstream-setvbuf-doallocate.c
> new file mode 100644
> index 0000000000..595a716db5
> --- /dev/null
> +++ b/libio/tst-wmemstream-setvbuf-doallocate.c
> @@ -0,0 +1,64 @@
> +/* Test setvbuf on open_wmenstream, BZ #34019.
> + Copyright (C) 2026 Free Software Foundation, Inc.
> + This file is part of the GNU C Library.
> +
> + The GNU C Library is free software; you can redistribute it and/or
> + modify it under the terms of the GNU Lesser General Public
> + License as published by the Free Software Foundation; either
> + version 2.1 of the License, or (at your option) any later version.
> +
> + The GNU C Library is distributed in the hope that it will be useful,
> + but WITHOUT ANY WARRANTY; without even the implied warranty of
> + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
> + Lesser General Public License for more details.
> +
> + You should have received a copy of the GNU Lesser General Public
> + License along with the GNU C Library; if not, see
> + <https://www.gnu.org/licenses/>. */
> +
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <wchar.h>
> +#include <support/check.h>
> +
> +static int
> +do_test (void)
> +{
> + /* Regression test for setvbuf doallocate on open_wmemstream.
> + This test cover the _IO_setvbuf path for:
> +
> + setvbuf (stream, NULL, _IOFBF, 0)
> +
> + This path may call _IO_DOALLOCATE and return without invoking
> + the stream setbuf hook. For open_wmemstream, the generic wide
> + doallocate hook must not replace the growable result buffer. */
> +
> + wchar_t *wbuf = NULL;
> + size_t wlen = 0;
> + FILE *fp = open_wmemstream (&wbuf, &wlen);
If you base the test on libio/tst-memstream.h you can test both narrow
and wide streams with the same test.
@@ -140,6 +140,7 @@ tests = \
tst-wfile-sync \
tst-wfiledoallocate-static \
tst-widetext \
+ tst-wmemstream-setvbuf-doallocate \
tst-wmemstream1 \
tst-wmemstream2 \
tst-wmemstream3 \
@@ -742,10 +742,13 @@ extern int _IO_mem_sync (FILE *fp) __THROW attribute_hidden;
extern void _IO_mem_finish (FILE *fp, int) __THROW attribute_hidden;
extern FILE *_IO_mem_setbuf (FILE *fp, char *buf, ssize_t size)
__THROW attribute_hidden;
+extern int _IO_mem_doallocate (FILE *fp) __THROW attribute_hidden;
+
extern int _IO_wmem_sync (FILE *fp) __THROW attribute_hidden;
extern void _IO_wmem_finish (FILE *fp, int) __THROW attribute_hidden;
extern FILE *_IO_wmem_setbuf (FILE *fp, char *buf, ssize_t size)
__THROW attribute_hidden;
+extern int _IO_wmem_doallocate (FILE *fp) __THROW attribute_hidden;
/* Other strfile functions */
struct _IO_strfile_;
@@ -121,3 +121,12 @@ _IO_mem_setbuf (FILE *fp, char *p, ssize_t len)
(void) len;
return fp;
}
+
+int
+_IO_mem_doallocate (FILE *fp)
+{
+ /* memstream manage a growable buffer internally. The doallocate
+ hook must not replace it with a generic stdio buffer. */
+ (void) fp;
+ return 1;
+}
new file mode 100644
@@ -0,0 +1,64 @@
+/* Test setvbuf on open_wmenstream, BZ #34019.
+ Copyright (C) 2026 Free Software Foundation, Inc.
+ This file is part of the GNU C Library.
+
+ The GNU C Library is free software; you can redistribute it and/or
+ modify it under the terms of the GNU Lesser General Public
+ License as published by the Free Software Foundation; either
+ version 2.1 of the License, or (at your option) any later version.
+
+ The GNU C Library is distributed in the hope that it will be useful,
+ but WITHOUT ANY WARRANTY; without even the implied warranty of
+ MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
+ Lesser General Public License for more details.
+
+ You should have received a copy of the GNU Lesser General Public
+ License along with the GNU C Library; if not, see
+ <https://www.gnu.org/licenses/>. */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <wchar.h>
+#include <support/check.h>
+
+static int
+do_test (void)
+{
+ /* Regression test for setvbuf doallocate on open_wmemstream.
+ This test cover the _IO_setvbuf path for:
+
+ setvbuf (stream, NULL, _IOFBF, 0)
+
+ This path may call _IO_DOALLOCATE and return without invoking
+ the stream setbuf hook. For open_wmemstream, the generic wide
+ doallocate hook must not replace the growable result buffer. */
+
+ wchar_t *wbuf = NULL;
+ size_t wlen = 0;
+ FILE *fp = open_wmemstream (&wbuf, &wlen);
+
+ TEST_VERIFY_EXIT (fp != NULL);
+
+ TEST_COMPARE (setvbuf (fp, NULL, _IOFBF, 0), 0);
+
+ TEST_COMPARE (fputwc (L'A', fp), L'A');
+ TEST_COMPARE (fflush (fp), 0);
+ TEST_COMPARE (wlen, 1);
+ TEST_VERIFY (wbuf != NULL);
+ TEST_VERIFY (wbuf[0] == L'A');
+ TEST_VERIFY (wbuf[1] == L'\0');
+
+ TEST_COMPARE (fputwc (L'B', fp), L'B');
+ TEST_COMPARE (fclose (fp), 0);
+ TEST_COMPARE (wlen, 2);
+ TEST_VERIFY (wbuf != NULL);
+ TEST_VERIFY (wbuf[0] == L'A');
+ TEST_VERIFY (wbuf[1] == L'B');
+ TEST_VERIFY (wbuf[2] == L'\0');
+
+ free (wbuf);
+
+ return 0;
+}
+
+#include <support/test-driver.c>
@@ -79,10 +79,12 @@
# pragma weak _IO_mem_finish
# pragma weak _IO_mem_setbuf
# pragma weak _IO_mem_sync
+# pragma weak _IO_mem_doallocate
# pragma weak _IO_wmem_finish
# pragma weak _IO_wmem_setbuf
# pragma weak _IO_wmem_sync
+# pragma weak _IO_wmem_doallocate
# pragma weak __printf_buffer_as_file_overflow
# pragma weak __printf_buffer_as_file_xsputn
@@ -338,7 +340,7 @@ const struct _IO_jump_t __io_vtables[] attribute_relro =
JUMP_INIT (seekpos, _IO_default_seekpos),
JUMP_INIT (setbuf, _IO_mem_setbuf),
JUMP_INIT (sync, _IO_mem_sync),
- JUMP_INIT (doallocate, _IO_default_doallocate),
+ JUMP_INIT (doallocate, _IO_mem_doallocate),
JUMP_INIT (read, _IO_default_read),
JUMP_INIT (write, _IO_default_write),
JUMP_INIT (seek, _IO_default_seek),
@@ -361,7 +363,7 @@ const struct _IO_jump_t __io_vtables[] attribute_relro =
JUMP_INIT (seekpos, _IO_default_seekpos),
JUMP_INIT (setbuf, _IO_wmem_setbuf),
JUMP_INIT (sync, _IO_wmem_sync),
- JUMP_INIT (doallocate, _IO_wdefault_doallocate),
+ JUMP_INIT (doallocate, _IO_wmem_doallocate),
JUMP_INIT (read, _IO_default_read),
JUMP_INIT (write, _IO_default_write),
JUMP_INIT (seek, _IO_default_seek),
@@ -127,3 +127,12 @@ _IO_wmem_setbuf (FILE *fp, char *p, ssize_t len)
(void) len;
return fp;
}
+
+int
+_IO_wmem_doallocate (FILE *fp)
+{
+ /* wmemstreams manage a growable buffer internally. The doallocate
+ hook must not replace it with a generic stdio buffer. */
+ (void) fp;
+ return 1;
+}