Start of an implementation using doubly-linked lists and virtual

addresses.  This will have to be reworked to use physical addresses thanks
to the PXE spec.

src/core/buffer.c

@@ -0,0 +1,131 @@
+/*
+ * Routines for filling a buffer with data received piecemeal, where
+ * the size of the data is not necessarily known in advance.
+ *
+ * Some protocols do not provide a mechanism for us to know the size
+ * of the file before we happen to receive a particular block
+ * (e.g. the final block in an MTFTP transfer).  In addition, some
+ * protocols (all the multicast protocols plus any TCP-based protocol)
+ * can, in theory, provide the data in any order.
+ *
+ * Rather than requiring each protocol to implement its own equivalent
+ * of "dd" to arrange the data into well-sized pieces before handing
+ * off to the image loader, we provide these generic buffer functions
+ * which assemble a file into a single contiguous block.  The whole
+ * block is then passed to the image loader.
+ *
+ */
+
+#include "stddef.h"
+#include "string.h"
+#include "buffer.h"
+
+/*
+ * Split a free block at the specified address, to produce two
+ * consecutive free blocks.  If the address is not within the free
+ * block, do nothing and return success.  If one of the resulting free
+ * blocks would be too small to contain the free block descriptor,
+ * return failure.
+ *
+ */
+static int split_free_block ( struct buffer_free_block *block, void *split ) {
+	struct buffer_free_block *new_block = split;
+
+	if ( ( split <= ( void * ) block ) || ( split >= block->end ) ) {
+		/* Split is outside block; nothing to do */
+		return 1;
+	}
+	
+	if ( ( ( block + 1 ) > new_block ) ||
+	     ( ( ( void * ) ( new_block + 1 ) ) > block->end ) ) {
+		/* Split block would be too small; fail */
+		return 0;
+	}
+
+	/* Create new block, link into free list */
+	new_block->next		= block->next;
+	new_block->next->prev	= new_block;
+	new_block->prev		= block->prev;
+	new_block->end 		= block->end;
+	block->next		= new_block;
+	block->end		= new_block;
+	return 1;
+}
+
+/*
+ * Remove a block from the free list.
+ *
+ * Note that this leaves block->next intact.
+ *
+ */
+static inline void unfree_block ( struct buffer_free_block *block ) {
+	block->prev->next = block->next;
+	block->next->prev = block->prev;
+}
+
+/*
+ * Mark a stretch of memory within a buffer as allocated.
+ *
+ */
+static inline int mark_allocated ( struct buffer *buffer,
+				   void *start, void *end ) {
+	struct buffer_free_block *block = buffer->free_blocks.next;
+
+	while ( block != &buffer->free_blocks ) {
+		if ( ! ( split_free_block ( block, start ) &&
+			 split_free_block ( block, end ) ) ) {
+			/* Block split failure; fail */
+			return 0;
+		}
+		/* At this point, block can be entirely contained
+		 * within [start,end), but it can't overlap.
+		 */
+		if ( ( ( ( void * ) block ) >= start ) &&
+		     ( ( ( void * ) block ) < end ) ) {
+			unfree_block ( block );
+		}
+		block = block->next;
+	}
+
+	return 1;
+}
+
+/*
+ * Place data into a buffer
+ *
+ */
+int fill_buffer ( struct buffer *buffer, void *data,
+		  off_t offset, size_t len ) {
+	void *start = buffer->start + offset;
+	void *end = start + len;
+
+	if ( ! mark_allocated ( buffer, start, end ) ) {
+		/* Allocation failure; fail */
+		return 0;
+	}
+	memcpy ( start, data, len );
+	return 1;
+}
+
+/*
+ * Initialise a buffer
+ *
+ */
+static void init_buffer ( struct buffer *buffer, void *start, size_t len ) {
+	struct buffer_free_block *block;
+
+	block = start;
+	block->next = &buffer->free_blocks;
+	block->prev = &buffer->free_blocks;
+	block->end = start + len;
+
+	buffer->free_blocks.next = block;
+	buffer->free_blocks.prev = block;
+	buffer->start = start;
+	buffer->end = start + len;
+}
+
+/*
+ * Move a buffer
+ *
+ */

src/include/buffer.h

@@ -0,0 +1,16 @@
+#ifndef BUFFER_H
+#define BUFFER_H
+
+struct buffer_free_block {
+	struct buffer_free_block *next;
+	struct buffer_free_block *prev;	
+	void *end;
+};
+
+struct buffer {
+	struct buffer_free_block free_blocks;
+	void *start;
+	void *end;
+};
+
+#endif /* BUFFER_H */