Add json_sprintf and json_vsprintf

Fixes #392

test/suites/api/Makefile.am

@@ -14,6 +14,7 @@ check_PROGRAMS = \
 	test_object \
 	test_pack \
 	test_simple \
+	test_sprintf \
 	test_unpack
 
 test_array_SOURCES = test_array.c util.h
@@ -27,6 +28,7 @@ test_number_SOURCES = test_number.c util.h
 test_object_SOURCES = test_object.c util.h
 test_pack_SOURCES = test_pack.c util.h
 test_simple_SOURCES = test_simple.c util.h
+test_sprintf_SOURCES = test_sprintf.c util.h
 test_unpack_SOURCES = test_unpack.c util.h
 
 AM_CPPFLAGS = -I$(top_builddir)/src -I$(top_srcdir)/src

test/suites/api/test_sprintf.c

@@ -0,0 +1,22 @@
+#include <string.h>
+#include <jansson.h>
+#include "util.h"
+
+
+static void test_sprintf() {
+    json_t *s = json_sprintf("foo bar %d", 42);
+    if (!s)
+        fail("json_sprintf returned NULL");
+    if (!json_is_string(s))
+        fail("json_sprintf didn't return a JSON string");
+    if (strcmp(json_string_value(s), "foo bar 42"))
+        fail("json_sprintf generated an unexpected string");
+
+    json_decref(s);
+}
+
+
+static void run_tests()
+{
+    test_sprintf();
+}