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CWPack is a lightweight and yet complete implementation of the MessagePack serialization format version 5. It also supports the Timestamp extension type.

Excellent Performance

Together with MPack, CWPack is the fastest open-source messagepack implementation. Both totally outperform CMP and msgpack-c


CWPack does no memory allocations and no file handling in its basic setup. All that is done outside of CWPack. Example extensions are included.

CWPack is working against memory buffers. User defined handlers are called when buffers are filled up (packing) or needs refill (unpack).

Containers (arrays, maps) are read/written in parts, first the item containing the size and then the contained items one by one. Exception to this is the cw_skip_items function which skip whole containers.


Pack and unpack example from the MessagePack home page:

void example (void)
    cw_pack_context pc;
    char buffer[20];
    cw_pack_context_init (&pc, buffer, 20, 0);

    cw_pack_map_size (&pc, 2);
    cw_pack_str (&pc, "compact", 7);
    cw_pack_boolean (&pc, true);
    cw_pack_str (&pc, "schema", 6);
    cw_pack_unsigned (&pc, 0);

    if (pc.return_code != CWP_RC_OK)  ERROR;
    int length = pc.current - pc.start;
    if (length != 18) ERROR;

    cw_unpack_context uc;
    cw_unpack_context_init (&uc, pc.start, length, 0);

    if (cw_unpack_next_map_size(&uc) != 2) ERROR;
    if (cw_unpack_next_str_lengh(&uc) != 7) ERROR;
    if (strncmp("compact",, 7)) ERROR;
    if (cw_unpack_next_boolean(&uc) != true) ERROR;
    if (cw_unpack_next_str_lengh(&uc) != 6) ERROR;
    if (strncmp("schema",, 6)) ERROR;
    if (cw_unpack_next_signed32(&uc) != 0) ERROR;

    if (uc.return_code != CWP_RC_OK)  ERROR;
    if (uc.return_code != CWP_RC_END_OF_INPUT)  ERROR;

In the examples folder there are more examples.

Backward compatibility

CWPack may be run in compatibility mode. It affects only packing; EXT & TIMESTAMP is considered illegal, BIN are transformed to STR and generation of STR8 is supressed.

Error handling

When an error is detected in a context, the context is stopped and all future calls to that context are immediatly returned without any actions. Thus it is possible to make some calls and delay error checking until all calls are done.

CWPack does not check for illegal values (e.g. in STR for illegal unicode characters).


CWPack consists of a single src file and three header files. It is written in strict ansi C and the files are together ~ 1.4K lines. No separate build is neccesary, just include the files in your own build.

CWPack has no dependencies to other libraries.


Included in the test folder are a module test and a performance test and shell scripts to run them.


CWPack also contains an Objective-C interface. The MessagePack home page example would look like:

CWPackContext *pc = [CWPackContext newWithContext:my_cw_pack_context];
[pc packObject:@{@"compact":@YES, @"schema":@0}];

CWUnpackContext *uc = [CWUnpackContext newWithContext:my_cw_unpack_context];
NSDictionary *dict = [uc unpackNextObject];


CWPack also contains a Swift interface. The MessagePack home page example would pack like:

let packer = CWDataPacker()
packer + DictionaryHeader(2) + "compact" + true + "schema" + 0
let data =



An ANSI C encoder/decoder for the MessagePack serialization format /[C / Objective-C / Swift]







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