[PATCH v4 2/2] libio: Ignore doallocate for open_memstream and open_wmemstream [BZ #34019]
Gao Xiang
gaoxiang@kylinos.cn
Thu May 7 09:40:00 GMT 2026
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 | 2 +
libio/libioP.h | 3 ++
libio/memstream.c | 10 ++++
libio/tst-memstream-setvbuf-doallocate.c | 61 +++++++++++++++++++++++
libio/tst-wmemstream-setvbuf-doallocate.c | 20 ++++++++
libio/vtables.c | 6 ++-
libio/wmemstream.c | 10 ++++
7 files changed, 110 insertions(+), 2 deletions(-)
create mode 100644 libio/tst-memstream-setvbuf-doallocate.c
create mode 100644 libio/tst-wmemstream-setvbuf-doallocate.c
diff --git a/libio/Makefile b/libio/Makefile
index 6b56567d44..e165a8b4cb 100644
--- a/libio/Makefile
+++ b/libio/Makefile
@@ -117,6 +117,7 @@ tests = \
tst-fwrite-error \
tst-getdelim \
tst-memstream-setbuf \
+ tst-memstream-setvbuf-doallocate \
tst-memstream1 \
tst-memstream2 \
tst-memstream3 \
@@ -141,6 +142,7 @@ tests = \
tst-wfiledoallocate-static \
tst-widetext \
tst-wmemstream-setbuf \
+ tst-wmemstream-setvbuf-doallocate \
tst-wmemstream1 \
tst-wmemstream2 \
tst-wmemstream3 \
diff --git a/libio/libioP.h b/libio/libioP.h
index 17c0b6e76d..fa547a4a29 100644
--- a/libio/libioP.h
+++ b/libio/libioP.h
@@ -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_;
diff --git a/libio/memstream.c b/libio/memstream.c
index 739080e96a..3ba7bcb28b 100644
--- a/libio/memstream.c
+++ b/libio/memstream.c
@@ -122,3 +122,13 @@ _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;
+}
diff --git a/libio/tst-memstream-setvbuf-doallocate.c b/libio/tst-memstream-setvbuf-doallocate.c
new file mode 100644
index 0000000000..4910f02d5c
--- /dev/null
+++ b/libio/tst-memstream-setvbuf-doallocate.c
@@ -0,0 +1,61 @@
+/* Test setvbuf on open_memstream, 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 <libio/tst-memstream.h>
+
+static int
+do_test (void)
+{
+ /* Regression test for setvbuf doallocate on open_memstream and
+ open_wmemstream. This test covers the _IO_setvbuf path for:
+
+ setvbuf (stream, NULL, _IOFBF, 0)
+
+ This path may call _IO_DOALLOCATE and return without invoking
+ the stream setbuf hook. The generic doallocate hook must not
+ replace the growable result buffer. */
+
+ CHAR_T *buf = NULL;
+ size_t len = 0;
+ FILE *fp = OPEN_MEMSTREAM (&buf, &len);
+
+ TEST_VERIFY_EXIT (fp != NULL);
+
+ TEST_COMPARE (setvbuf (fp, NULL, _IOFBF, 0), 0);
+
+ TEST_COMPARE (FPUTC (W('A'), fp), W('A'));
+ TEST_COMPARE (fflush (fp), 0);
+ TEST_COMPARE (len, 1);
+ TEST_VERIFY_EXIT (buf != NULL);
+ TEST_VERIFY (buf[0] == W('A'));
+ TEST_VERIFY (buf[1] == W('\0'));
+
+ TEST_COMPARE (FPUTC (W('B'), fp), W('B'));
+ TEST_COMPARE (fclose (fp), 0);
+ TEST_COMPARE (len, 2);
+ TEST_VERIFY_EXIT (buf != NULL);
+ TEST_VERIFY (buf[0] == W('A'));
+ TEST_VERIFY (buf[1] == W('B'));
+ TEST_VERIFY (buf[2] == W('\0'));
+
+ free (buf);
+
+ return 0;
+}
+
+#include <support/test-driver.c>
diff --git a/libio/tst-wmemstream-setvbuf-doallocate.c b/libio/tst-wmemstream-setvbuf-doallocate.c
new file mode 100644
index 0000000000..857239aa85
--- /dev/null
+++ b/libio/tst-wmemstream-setvbuf-doallocate.c
@@ -0,0 +1,20 @@
+/* Test setvbuf on open_wmemstream, 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/>. */
+
+#define TEST_WCHAR
+#include <libio/tst-memstream-setvbuf-doallocate.c>
diff --git a/libio/vtables.c b/libio/vtables.c
index ca9f1b2dc4..ba3f9566f8 100644
--- a/libio/vtables.c
+++ b/libio/vtables.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),
diff --git a/libio/wmemstream.c b/libio/wmemstream.c
index 168232314a..c449e482c8 100644
--- a/libio/wmemstream.c
+++ b/libio/wmemstream.c
@@ -127,3 +127,13 @@ _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;
+}
--
2.53.0
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