scratch_buffer: Document the exported API

[Bruno Haible]

It looks like the 'scratch_buffer' could be useful to some programs
outside of glibc. But you cannot expect that a developer will dig
into lib/malloc/scratch_buffer.h, to read definitions full of __libc_
prefixed symbols and libc_hidden_proto declarations, and understand
what function names they can actually use.

So, I'm adding readable documentation for this module.

Paul: Note that the 'dynarray' documentation has the same problem.
Plus, it seems that some functions have a 'gl_' prefix and some
don't. Does this matter? Maybe it does not matter because
the contents of lib/malloc/dynarray.h is irrelevant and only
lib/malloc/dynarray-skeleton.c matters?


2021-02-21  Bruno Haible  <bruno@clisp.org>

        scratch_buffer: Document the exported API.
        * lib/scratch_buffer.h: Add comments, taken from
        lib/malloc/scratch_buffer.h.

diff --git a/lib/scratch_buffer.h b/lib/scratch_buffer.h
index 603b0d6..f4b5f9e 100644
--- a/lib/scratch_buffer.h
+++ b/lib/scratch_buffer.h
@@ -19,6 +19,97 @@
 #ifndef _GL_SCRATCH_BUFFER_H
 #define _GL_SCRATCH_BUFFER_H
 
+/* Scratch buffers with a default stack allocation and fallback to
+   heap allocation.  It is expected that this function is used in this
+   way:
+
+     struct scratch_buffer tmpbuf;
+     scratch_buffer_init (&tmpbuf);
+
+     while (!function_that_uses_buffer (tmpbuf.data, tmpbuf.length))
+       if (!scratch_buffer_grow (&tmpbuf))
+         return -1;
+
+     scratch_buffer_free (&tmpbuf);
+     return 0;
+
+   The allocation functions (scratch_buffer_grow,
+   scratch_buffer_grow_preserve, scratch_buffer_set_array_size) make
+   sure that the heap allocation, if any, is freed, so that the code
+   above does not have a memory leak.  The buffer still remains in a
+   state that can be deallocated using scratch_buffer_free, so a loop
+   like this is valid as well:
+
+     struct scratch_buffer tmpbuf;
+     scratch_buffer_init (&tmpbuf);
+
+     while (!function_that_uses_buffer (tmpbuf.data, tmpbuf.length))
+       if (!scratch_buffer_grow (&tmpbuf))
+         break;
+
+     scratch_buffer_free (&tmpbuf);
+
+   scratch_buffer_grow and scratch_buffer_grow_preserve are guaranteed
+   to grow the buffer by at least 512 bytes.  This means that when
+   using the scratch buffer as a backing store for a non-character
+   array whose element size, in bytes, is 512 or smaller, the scratch
+   buffer only has to grow once to make room for at least one more
+   element.
+*/
+
+/* Scratch buffer.  Must be initialized with scratch_buffer_init
+   before its use.  */
+struct scratch_buffer;
+
+/* Initializes *BUFFER so that BUFFER->data points to BUFFER->__space
+   and BUFFER->length reflects the available space.  */
+#if 0
+extern void scratch_buffer_init (struct scratch_buffer *buffer);
+#endif
+
+/* Deallocates *BUFFER (if it was heap-allocated).  */
+#if 0
+extern void scratch_buffer_free (struct scratch_buffer *buffer);
+#endif
+
+/* Grow *BUFFER by some arbitrary amount.  The buffer contents is NOT
+   preserved.  Return true on success, false on allocation failure (in
+   which case the old buffer is freed).  On success, the new buffer is
+   larger than the previous size.  On failure, *BUFFER is deallocated,
+   but remains in a free-able state, and errno is set.  */
+#if 0
+extern bool scratch_buffer_grow (struct scratch_buffer *buffer);
+#endif
+
+/* Like scratch_buffer_grow, but preserve the old buffer
+   contents on success, as a prefix of the new buffer.  */
+#if 0
+extern bool scratch_buffer_grow_preserve (struct scratch_buffer *buffer);
+#endif
+
+/* Grow *BUFFER so that it can store at least NELEM elements of SIZE
+   bytes.  The buffer contents are NOT preserved.  Both NELEM and SIZE
+   can be zero.  Return true on success, false on allocation failure
+   (in which case the old buffer is freed, but *BUFFER remains in a
+   free-able state, and errno is set).  It is unspecified whether this
+   function can reduce the array size.  */
+#if 0
+extern bool scratch_buffer_set_array_size (struct scratch_buffer *buffer,
+                                           size_t nelem, size_t size);
+#endif
+
+/* Return a copy of *BUFFER's first SIZE bytes as a heap-allocated block,
+   deallocating *BUFFER if it was heap-allocated.  SIZE must be at
+   most *BUFFER's size.  Return NULL (setting errno) on memory
+   exhaustion.  */
+#if 0
+extern void *scratch_buffer_dupfree (struct scratch_buffer *buffer,
+                                     size_t size);
+#endif
+
+
+/* The implementation is imported from glibc.  */
+
 #include <libc-config.h>
 
 #define __libc_scratch_buffer_dupfree gl_scratch_buffer_dupfree