Implement json_dumpb()

This function encodes the json_t object to a pre-allocated buffer.
It compliments the already existing json_loadb() function and is
useful for parsing JSON-RPC (among other protocols) when sent over
datagram sockets.

Signed-off-by: Nathaniel McCallum <npmccallum@redhat.com>

src/dump.c

@@ -25,11 +25,28 @@
 #define FLAGS_TO_INDENT(f)      ((f) & 0x1F)
 #define FLAGS_TO_PRECISION(f)   (((f) >> 11) & 0x1F)
 
+struct buffer {
+    const size_t size;
+    size_t used;
+    char *data;
+};
+
 static int dump_to_strbuffer(const char *buffer, size_t size, void *data)
 {
     return strbuffer_append_bytes((strbuffer_t *)data, buffer, size);
 }
 
+static int dump_to_buffer(const char *buffer, size_t size, void *data)
+{
+    struct buffer *buf = (struct buffer *)data;
+
+    if(buf->used + size <= buf->size)
+        memcpy(&buf->data[buf->used], buffer, size);
+
+    buf->used += size;
+    return 0;
+}
+
 static int dump_to_file(const char *buffer, size_t size, void *data)
 {
     FILE *dest = (FILE *)data;
@@ -416,6 +433,16 @@ char *json_dumps(const json_t *json, size_t flags)
     return result;
 }
 
+size_t json_dumpb(const json_t *json, char *buffer, size_t size, size_t flags)
+{
+    struct buffer buf = { size, 0, buffer };
+
+    if(json_dump_callback(json, dump_to_buffer, (void *)&buf, flags))
+        return 0;
+
+    return buf.used;
+}
+
 int json_dumpf(const json_t *json, FILE *output, size_t flags)
 {
     return json_dump_callback(json, dump_to_file, (void *)output, flags);

src/jansson.def

@@ -48,6 +48,7 @@ EXPORTS
     json_object_key_to_iter
     json_object_seed
     json_dumps
+    json_dumpb
     json_dumpf
     json_dump_file
     json_dump_callback

src/jansson.h

@@ -292,6 +292,7 @@ json_t *json_load_callback(json_load_callback_t callback, void *data, size_t fla
 typedef int (*json_dump_callback_t)(const char *buffer, size_t size, void *data);
 
 char *json_dumps(const json_t *json, size_t flags);
+size_t json_dumpb(const json_t *json, char *buffer, size_t size, size_t flags);
 int json_dumpf(const json_t *json, FILE *output, size_t flags);
 int json_dump_file(const json_t *json, const char *path, size_t flags);
 int json_dump_callback(const json_t *json, json_dump_callback_t callback, void *data, size_t flags);