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>

doc/apiref.rst

@@ -948,6 +948,28 @@ These functions output UTF-8:
    error. *flags* is described above. The return value must be freed
    by the caller using :func:`free()`.
 
+.. function:: size_t json_dumpb(const json_t *json, char *buffer, size_t size, size_t flags)
+
+   Writes the JSON representation of *json* to the *buffer* of
+   *size* bytes. Returns the number of bytes that would be written
+   or 0 on error. *flags* is described above. *buffer* is not
+   null-terminated.
+
+   This function never writes more than *size* bytes. If the return
+   value is greater than *size*, the contents of the *buffer* are
+   undefined. This behavior enables you to specify a NULL *buffer*
+   to determine the length of the encoding. For example::
+
+       size_t size = json_dumpb(json, NULL, 0, 0);
+       if (size == 0)
+           return -1;
+
+       char *buf = alloca(size);
+
+       size = json_dumpb(json, buf, size, 0);
+
+   .. versionadded:: 2.10
+
 .. function:: int json_dumpf(const json_t *json, FILE *output, size_t flags)
 
    Write the JSON representation of *json* to the stream *output*.

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);

test/suites/api/test_dump.c

@@ -22,6 +22,9 @@ static void encode_null()
     if(json_dumps(NULL, JSON_ENCODE_ANY) != NULL)
         fail("json_dumps didn't fail for NULL");
 
+    if(json_dumpb(NULL, NULL, 0, JSON_ENCODE_ANY) != 0)
+        fail("json_dumps didn't fail for NULL");
+
     if(json_dumpf(NULL, stderr, JSON_ENCODE_ANY) != -1)
         fail("json_dumpf didn't fail for NULL");
 
@@ -212,6 +215,28 @@ static void dump_file()
     remove("json_dump_file.json");
 }
 
+static void dumpb()
+{
+    char buf[2];
+    json_t *obj;
+    size_t size;
+
+    obj = json_object();
+
+    size = json_dumpb(obj, buf, sizeof(buf), 0);
+    if(size != 2 || strncmp(buf, "{}", 2))
+      fail("json_dumpb failed");
+
+    json_decref(obj);
+    obj = json_pack("{s:s}", "foo", "bar");
+
+    size = json_dumpb(obj, buf, sizeof(buf), JSON_COMPACT);
+    if(size != 13)
+      fail("json_dumpb size check failed");
+
+    json_decref(obj);
+}
+
 static void run_tests()
 {
     encode_null();
@@ -221,4 +246,5 @@ static void run_tests()
     escape_slashes();
     encode_nul_byte();
     dump_file();
+    dumpb();
 }