CMP is a C implementation of the MessagePack serialization format. It currently implements version 5 of the MessagePack Spec.

CMP's goal is to be lightweight and straightforward, forcing nothing on the programmer.

While I'm a big believer in the GPL, I license CMP under the MIT license.

The following examples use a file as the backend, and are modeled after the examples included with the msgpack-c project.

#include <stdio.h>
#include <stdlib.h>

#include "cmp.h"

static bool read_bytes(void *data, size_t sz, FILE *fh) {
    return fread(data, sizeof(uint8_t), sz, fh) == (sz * sizeof(uint8_t));
}

static bool file_reader(cmp_ctx_t *ctx, void *data, size_t limit) {
    return read_bytes(data, limit, (FILE *)ctx->buf);
}

static bool file_skipper(cmp_ctx_t *ctx, size_t count) {
    return fseek((FILE *)ctx->buf, count, SEEK_CUR);
}

static size_t file_writer(cmp_ctx_t *ctx, const void *data, size_t count) {
    return fwrite(data, sizeof(uint8_t), count, (FILE *)ctx->buf);
}

static void error_and_exit(const char *msg) {
    fprintf(stderr, "%s\n\n", msg);
    exit(EXIT_FAILURE);
}

int main(void) {
    FILE *fh = NULL;
    cmp_ctx_t cmp = {0};
    uint32_t array_size = 0;
    uint32_t str_size = 0;
    char hello[6] = {0};
    char message_pack[12] = {0};

    fh = fopen("cmp_data.dat", "w+b");

    if (fh == NULL) {
        error_and_exit("Error opening data.dat");
    }

    cmp_init(&cmp, fh, file_reader, file_skipper, file_writer);

    if (!cmp_write_array(&cmp, 2)) {
        error_and_exit(cmp_strerror(&cmp));
    }

    if (!cmp_write_str(&cmp, "Hello", 5)) {
        error_and_exit(cmp_strerror(&cmp));
    }

    if (!cmp_write_str(&cmp, "MessagePack", 11)) {
        error_and_exit(cmp_strerror(&cmp));
    }

    rewind(fh);

    if (!cmp_read_array(&cmp, &array_size)) {
        error_and_exit(cmp_strerror(&cmp));
    }

    /* You can read the str byte size and then read str bytes... */

    if (!cmp_read_str_size(&cmp, &str_size)) {
        error_and_exit(cmp_strerror(&cmp));
    }

    if (str_size > (sizeof(hello) - 1)) {
        error_and_exit("Packed 'hello' length too long\n");
    }

    if (!read_bytes(hello, str_size, fh)) {
        error_and_exit(cmp_strerror(&cmp));
    }

    /*
     * ...or you can set the maximum number of bytes to read and do it all in
     * one call
     */

    str_size = sizeof(message_pack);
    if (!cmp_read_str(&cmp, message_pack, &str_size)) {
        error_and_exit(cmp_strerror(&cmp));
    }

    printf("Array Length: %u.\n", array_size);
    printf("[\"%s\", \"%s\"]\n", hello, message_pack);

    fclose(fh);

    return EXIT_SUCCESS;
}

See the examples folder.

CMP uses no internal buffers; conversions, encoding and decoding are done on the fly.

CMP's source and header file together are ~4k LOC.

CMP makes no heap allocations.

CMP uses standardized types rather than declaring its own, and it depends only on stdbool.h, stdint.h and string.h.

CMP is written using C89 (ANSI C), aside, of course, from its use of fixed-width integer types and bool.

On the other hand, CMP's test suite requires C99.

CMP only requires the programmer supply a read function, a write function, and an optional skip function. In this way, the programmer can use CMP on memory, files, sockets, etc.

CMP is portable. It uses fixed-width integer types, and checks the endianness of the machine at runtime before swapping bytes (MessagePack is big-endian).

CMP provides a fairly comprehensive error reporting mechanism modeled after errno and strerror.

CMP is thread aware; while contexts cannot be shared between threads, each thread may use its own context freely.

CMP is tested using the MessagePack test suite as well as a large set of custom test cases. Its small test program is compiled with clang using -Wall -Werror -Wextra ... along with several other flags, and generates no compilation errors in either clang or GCC.

CMP's source is written as readably as possible, using explicit, descriptive variable names and a consistent, clear style.

CMP's source is written to be as secure as possible. Its testing suite checks for invalid values, and data is always treated as suspect before it passes validation.

CMP's API is designed to be clear, convenient and unsurprising. Strings are null-terminated, binary data is not, error codes are clear, and so on.

CMP provides optional backwards compatibility for use with other MessagePack implementations that only implement version 4 of the spec.

There is no build system for CMP. The programmer can drop cmp.c and cmp.h in their source tree and modify as necessary. No special compiler settings are required to build it, and it generates no compilation errors in either clang or gcc.

CMP's versions are single integers. I don't use semantic versioning because I don't guarantee that any version is completely compatible with any other. In general, semantic versioning provides a false sense of security. You should be evaluating compatibility yourself, not relying on some stranger's versioning convention.

I only guarantee stability for versions released on the releases page. While rare, both master and develop branches may have errors or mismatched versions.

Version 4 of the MessagePack spec has no BIN type, and provides no STR8 marker. In order to remain backwards compatible with version 4 of MessagePack, do the following:

Avoid these functions:

• cmp_write_bin
• cmp_write_bin_marker
• cmp_write_str8_marker
• cmp_write_str8
• cmp_write_bin8_marker
• cmp_write_bin8
• cmp_write_bin16_marker
• cmp_write_bin16
• cmp_write_bin32_marker
• cmp_write_bin32

Use these functions in lieu of their v5 counterparts:

• cmp_write_str_marker_v4 instead of cmp_write_str_marker
• cmp_write_str_v4 instead of cmp_write_str
• cmp_write_object_v4 instead of cmp_write_object

Thanks to tdragon it's possible to disable floating point operations in CMP by defining CMP_NO_FLOAT. No floating point functionality will be included. Fair warning: this changes the ABI.

CMP will honor WORDS_BIGENDIAN. If defined to 0 it will convert data to/from little-endian format when writing/reading. If defined to 1 it won't. If not defined, CMP will check at runtime.