xmlSetAttributes

doc = xmlReadStr("<root><a><b><c></c></b></a><b></b></root>");

// Retrieve all the nodes
xp = xmlXPath(doc, "//*");

// Add the attributes hello="world" and bonjour="monde" to all the nodes
xmlSetAttributes(xp, ["hello" "world" ; "bonjour" "monde"]);
xmlDump(doc)

// Add the attribute foo="bar" to the first child of root
xmlSetAttributes(doc.root.children(1), ["foo" "bar"]);
xmlDump(doc)

// Add the attribute bar="foo" to all the children of root
xmlSetAttributes(doc.root.children, ["bar" "foo"]);

// Add the attribute truc="machin" to the list of the attributes of the second <b>
xmlSetAttributes(doc.root.children(2).attributes, ["truc" "machin"]);
xmlDump(doc)

xmlDelete(doc);