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, 55 insertions, 0 deletions

diff --git a/stdio-common/wprintf_buffer_to_file.c b/stdio-common/wprintf_buffer_to_file.c
new file mode 100644
index 0000000..ac936fa
--- /dev/null
+++ b/stdio-common/wprintf_buffer_to_file.c
@@ -0,0 +1,55 @@
+/* Wide printf buffers writing data to a FILE *.
+   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/>.  */
+
+/* This implementation is not optimized (unlikely the multibyte
+   implementation) and always writes to the temporary buffer first.  */
+
+#include <printf_buffer_to_file.h>
+
+#include <array_length.h>
+#include <libio/libioP.h>
+
+void
+__wprintf_buffer_flush_to_file (struct __wprintf_buffer_to_file *buf)
+{
+  size_t count = buf->base.write_ptr - buf->stage;
+  if ((size_t) _IO_sputn (buf->fp, buf->stage, count) != count)
+    {
+      __wprintf_buffer_mark_failed (&buf->base);
+      return;
+    }
+  buf->base.written += count;
+  buf->base.write_ptr = buf->stage;
+}
+
+void
+__wprintf_buffer_to_file_init (struct __wprintf_buffer_to_file *buf, FILE *fp)
+{
+  __wprintf_buffer_init (&buf->base, buf->stage, array_length (buf->stage),
+                         __wprintf_buffer_mode_to_file);
+  buf->fp = fp;
+}
+
+int
+__wprintf_buffer_to_file_done (struct __wprintf_buffer_to_file *buf)
+{
+  if (__wprintf_buffer_has_failed (&buf->base))
+    return -1;
+  __wprintf_buffer_flush_to_file (buf);
+  return __wprintf_buffer_done (&buf->base);
+}