Makefile/user-macro comments.

libraries/liblmdb/Makefile

@@ -1,3 +1,34 @@
+# Makefile for liblmdb (Lightning memory-mapped database library).
+
+########################################################################
+# Configuration. The compiler options must enable threaded compilation.
+#
+# Preprocessor macros (for CPPFLAGS) of interest:
+#
+# To compile successfully if the default does not:
+# - MDB_USE_POSIX_SEM	(enabled by default on BSD, Apple)
+#	Define if shared mutexes are unsupported.  Note that Posix
+#	semaphores and shared mutexes have different behaviors and
+#	different problems, see the Caveats section in lmdb.h.
+#
+# For best performence or to compile successfully:
+# - MDB_DSYNC = "O_DSYNC" (default) or "O_SYNC" (less efficient)
+#	If O_DSYNC is undefined but exists in /usr/include,
+#	preferably set some compiler flag to get the definition.
+# - MDB_FDATASYNC = "fdatasync" or "fsync"
+#	Function for flushing the data of a file. Define this to
+#	"fsync" if fdatasync() is not supported. fdatasync is
+#	default except on BSD, Apple, Android which use fsync.
+# - MDB_USE_PWRITEV
+#	Define if the pwritev() function is supported.
+#
+# Data format:
+# - MDB_MAXKEYSIZE
+#	Controls data packing and limits, see mdb.c.
+#
+# Debugging:
+# - MDB_DEBUG, MDB_PARANOID.
+#
 CC	= gcc
 W	= -W -Wall -Wno-unused-parameter -Wbad-function-cast
 OPT = -O2 -g
@@ -6,6 +37,8 @@ LDLIBS	=
 SOLIBS	=
 prefix	= /usr/local
 
+########################################################################
+
 IHDRS	= lmdb.h
 ILIBS	= liblmdb.a liblmdb.so
 IPROGS	= mdb_stat mdb_copy

libraries/liblmdb/mdb.c

@@ -348,6 +348,9 @@ static txnid_t mdb_debug_start;
 #define MDB_VERSION	 1
 
 	/**	@brief The maximum size of a key in the database.
+	 *
+	 *	The library rejects bigger keys, and cannot deal with records
+	 *	with bigger keys stored by a library with bigger max keysize.
 	 *
 	 *	We require that keys all fit onto a regular page. This limit
 	 *	could be raised a bit further if needed; to something just