API enhancement: permit to set initial printbuf size

The default printbuf size of 32 bytes is very conservative and
can cause a lot of malloc() action for all but trivial json objects.
The new json_global_set_printbuf_initial_size() API permits an
application to set a new default size.

printbuf.c

@@ -28,15 +28,21 @@
 #include "debug.h"
 #include "printbuf.h"
 
+static int printbuf_initial_size = 32;
 static int printbuf_extend(struct printbuf *p, int min_size);
 
+void json_global_set_printbuf_initial_size(int size)
+{
+	printbuf_initial_size = size;
+}
+
 struct printbuf* printbuf_new(void)
 {
   struct printbuf *p;
 
   p = (struct printbuf*)calloc(1, sizeof(struct printbuf));
   if(!p) return NULL;
-  p->size = 32;
+  p->size = printbuf_initial_size;
   p->bpos = 0;
   if(!(p->buf = (char*)malloc(p->size))) {
     free(p);

printbuf.h

@@ -81,6 +81,27 @@ printbuf_reset(struct printbuf *p);
 extern void
 printbuf_free(struct printbuf *p);
 
+/**
+ * Set initial size allocation for memory when creating strings,
+ * as is done for example in json_object_to_json_string(). The
+ * default size is 32, which is very conservative. If an app
+ * knows it typically deals with larger strings, performance
+ * can be improved by setting the initial size to a different
+ * number, e.g. 1k. Note that this also means that memory
+ * consumption can increase. How far entriely depens on the
+ * application and its use of json-c.
+ *
+ * Note: each time this function is called, the initial size is
+ * changed to the given value. Already existing elements are not
+ * affected. This function is usually meant to be called just once
+ * at start of an application, but there is no harm calling it more
+ * than once. Note that the function is NOT thread-safe and must not
+ * be called on different threads concurrently.
+ *
+ * @param size new initial size for printbuf (formatting buffer)
+ */
+extern void
+json_global_set_printbuf_initial_size(int size);
 #ifdef __cplusplus
 }
 #endif