stdio-common: Introduce buffers for implementing printf

These buffers will eventually be used instead of FILE * objects
to implement printf functions.  The multibyte buffer is struct
__printf_buffer, the wide buffer is struct __wprintf_buffer.

To enable writing type-generic code, the header files
printf_buffer-char.h and printf_buffer-wchar_t.h define the
Xprintf macro differently, enabling Xprintf (buffer) to stand
for __printf_buffer and __wprintf_buffer as appropriate.  For
common cases, macros like Xprintf_buffer are provided as a more
syntactically convenient shortcut.

Buffer-specific flush callbacks are implemented with a switch
statement instead of a function pointer, to avoid hardening issues
similar to those of libio vtables.  struct __printf_buffer_as_file
is needed to support custom printf specifiers because the public
interface for that requires passing a FILE *, which is why there
is a trapdoor back from these buffers to FILE * streams.

Since the immediate user of these interfaces knows when processing
has finished, there is no flush callback for the end of processing,
only a flush callback for the intermediate buffer flush.

Reviewed-by: Adhemerval Zanella  <adhemerval.zanella@linaro.org>

1 files changed, 42 insertions, 0 deletions

diff --git a/stdio-common/printf_buffer_flush.c b/stdio-common/printf_buffer_flush.c
new file mode 100644
index 0000000..9b25c0f
--- /dev/null
+++ b/stdio-common/printf_buffer_flush.c
@@ -0,0 +1,42 @@
+/* Flush a struct __printf_buffer.  Multibyte version.
+   Copyright (C) 2022 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 <printf_buffer.h>
+
+#include "printf_buffer-char.h"
+#include "Xprintf_buffer_flush.c"
+
+/* The __printf_buffer_flush_* functions are defined together with
+   functions that are pulled in by strong references.  */
+#ifndef SHARED
+# pragma weak __printf_buffer_flush_to_file
+#endif /* !SHARED */
+
+static void
+__printf_buffer_do_flush (struct __printf_buffer *buf)
+{
+  switch (buf->mode)
+    {
+    case __printf_buffer_mode_failed:
+      return;
+    case __printf_buffer_mode_to_file:
+      __printf_buffer_flush_to_file ((struct __printf_buffer_to_file *) buf);
+      return;
+    }
+  __builtin_trap ();
+}