Add struct scratch_buffer and its internal helper functions

These will be used from NSS modules, so they have to be exported.

malloc/scratch_buffer_set_array_size.c

@@ -0,0 +1,59 @@
+/* Variable-sized buffer with on-stack default allocation.
+   Copyright (C) 2015 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
+   <http://www.gnu.org/licenses/>.  */
+
+#include <scratch_buffer.h>
+#include <errno.h>
+
+bool
+__libc_scratch_buffer_set_array_size (struct scratch_buffer *buffer,
+				      size_t nelem, size_t size)
+{
+  size_t new_length = nelem * size;
+
+  /* Avoid overflow check if both values are small. */
+  if ((nelem | size) >> (sizeof (size_t) * CHAR_BIT / 2) != 0
+      && nelem != 0 && size != new_length / nelem)
+    {
+      /* Overflow.  Discard the old buffer, but it must remain valid
+	 to free.  */
+      scratch_buffer_free (buffer);
+      scratch_buffer_init (buffer);
+      __set_errno (ENOMEM);
+      return false;
+    }
+
+  if (new_length <= buffer->length)
+    return true;
+
+  /* Discard old buffer.  */
+  scratch_buffer_free (buffer);
+
+  char *new_ptr = malloc (new_length);
+  if (new_ptr == NULL)
+    {
+      /* Buffer must remain valid to free.  */
+      scratch_buffer_init (buffer);
+      return false;
+    }
+
+  /* Install new heap-based buffer.  */
+  buffer->data = new_ptr;
+  buffer->length = new_length;
+  return true;
+}
+libc_hidden_def (__libc_scratch_buffer_set_array_size);