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>

stdio-common/printf_buffer_as_file.h

@@ -0,0 +1,87 @@
+/* FILE * interface to a struct __*printf_buffer.
+   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/>.  */
+
+/* Registered printf format specifier callbacks produce data via a
+   FILE * stream.  struct __printf_buffer_as_file enables vfprintf to
+   create a suitable stdio stream.  Likewise struct
+   __wprintf_buffer_as_file for vfwprintf.  */
+
+#ifndef PRINTF_BUFFER_AS_FILE_H
+#define PRINTF_BUFFER_AS_FILE_H
+
+#include <libio/libioP.h>
+
+struct __printf_buffer;
+
+struct __printf_buffer_as_file
+{
+  /* Interface to libio.  */
+  FILE stream;
+  const struct _IO_jump_t *vtable;
+
+  /* Pointer to the underlying buffer.  */
+  struct __printf_buffer *next;
+};
+
+/* Initialization *FP so that data written to its FILE * stream ends
+   up in NEXT.  */
+void __printf_buffer_as_file_init (struct __printf_buffer_as_file *fp,
+                                   struct __printf_buffer *next)
+  attribute_hidden;
+
+/* Returns the FILE * that can be used to write data to the
+   buffer.  */
+static inline FILE *
+__printf_buffer_as_file_get (struct __printf_buffer_as_file *file)
+{
+  return &file->stream;
+}
+
+/* Transfers all pending data from the FILE * to the underlying
+   buffer.  Returns true if there have been no errors.  */
+bool __printf_buffer_as_file_terminate (struct __printf_buffer_as_file *)
+  attribute_hidden;
+
+/* Wide variant follows.  */
+
+struct __wprintf_buffer;
+struct __wprintf_buffer_as_file
+{
+  /* Interface to libio.  */
+  FILE stream;
+  const struct _IO_jump_t *vtable;
+  struct _IO_wide_data wide_stream;
+
+  /* Pointer to the underlying buffer.  */
+  struct __wprintf_buffer *next;
+};
+
+void __wprintf_buffer_as_file_init (struct __wprintf_buffer_as_file *fp,
+                                    struct __wprintf_buffer *next)
+  attribute_hidden;
+
+static inline FILE *
+__wprintf_buffer_as_file_get (struct __wprintf_buffer_as_file *file)
+{
+  return &file->stream;
+}
+
+bool __wprintf_buffer_as_file_terminate (struct __wprintf_buffer_as_file *)
+  attribute_hidden;
+
+#endif /* PRINTF_BUFFER_AS_FILE_H */