MKHPACK — HTTP/2 Compression

A C implementation of HPACK: Header Compression for HTTP/2 as specified in RFC 7541.

Key Features

HPACK integer encoding and decoding (RFC 7541 §5.1)
Huffman string encoding, decoding, and length calculation (RFC 7541 §5.2)
HPACK string literal encoding and decoding with automatic Huffman optimisation
Shared library (.so), static library (.a), and pkg-config support
No runtime dependencies beyond the C standard library

Technology Stack

Component	Detail
Language	C (compiled with GCC)
Build system	GNU Make
Code generation	Ruby (Huffman tables)
Packaging	Debian (`.deb`), RPM (`.spec`), tarballs
CI/CD	GitHub Actions
Licence	ISC

Quick Start

make            # build shared and static libraries into lib/
make check      # build and run tests
make install    # install to /usr/local (or set PREFIX)

Link against the library using pkg-config:

gcc -o myapp myapp.c $(pkg-config --cflags --libs mkhpack)

Usage Examples

These examples use static linking:

HPACK integer encoding

#include <stdio.h>
#include <stdint.h>
#include "lib/mkhpack.h"

MKHPACK_INT_T i = 987; /* number to encode */
size_t  nbits = 6;     /* number of bits to occupy in the first byte */
uint8_t pbits = 0x40;  /* initial value of the first byte */

#define BUFFER_SIZE 4321
uint8_t buffer[BUFFER_SIZE];
size_t  written;       /* will hold the number of bytes written */

int error = mkhpack_encode_int(i, nbits, pbits, buffer, BUFFER_SIZE, &written);
if (error) {
    fprintf(stderr, "Error encoding integer: %d\n", error);
} else {
    printf("Encoded integer in %zu bytes\n", written);
}

#include <stdio.h>
#include <stdint.h>
#include "lib/mkhpack.h"

#define CODE_LENGTH 10
uint8_t code[CODE_LENGTH] = "\xFF\xF2\x81\xC0\x80\x01XYZ";
size_t  consumed;      /* will hold the number of bytes read */
size_t  nbits = 6;     /* number of bits to occupy in the first byte */

MKHPACK_INT_T i;       /* decoded integer */
uint8_t pbits;         /* will hold remainder of the first byte */

int error = mkhpack_decode_int(code, CODE_LENGTH, &consumed, nbits, &i, &pbits);
if (error) {
    fprintf(stderr, "Error decoding integer: %d\n", error);
} else {
    printf("Decoded integer[%lu] from %zu bytes, prefix was %02x\n",
        (unsigned long)i, consumed, pbits);
}

Huffman string encoding

#include <stdio.h>
#include <stdint.h>
#include "lib/mkhpack.h"

#define STRING_LENGTH 16
uint8_t string[STRING_LENGTH] = "www.example.com";

#define BUFFER_SIZE 4321
uint8_t buffer[BUFFER_SIZE];
size_t  written;      /* will hold the number of bytes written */

int error = huffman_encode(string, STRING_LENGTH, NULL, buffer, BUFFER_SIZE, &written);
if (error) {
    fprintf(stderr, "Error encoding string: %d\n", error);
} else {
    printf("Encoded %d-byte string in %zu bytes\n", STRING_LENGTH, written);
}

#include <stdio.h>
#include <stdint.h>
#include "lib/mkhpack.h"

#define CODE_LENGTH 16
uint8_t code[CODE_LENGTH] = "\xE7\xCF\x9B\xEB\xE8\x9B\x6F\xB1\x6F\xA9\xB6\xFFXYZ";
size_t  consumed;     /* will hold the number of bytes read */

#define BUFFER_SIZE 4321
uint8_t buffer[BUFFER_SIZE];
size_t  written;      /* will hold the number of bytes written */

int error = huffman_decode(code, CODE_LENGTH, &consumed, buffer, BUFFER_SIZE, &written);
if (error) {
    fprintf(stderr, "Error decoding string: %d\n", error);
} else {
    printf("Decoded %zu-byte string from %zu bytes\n", written, consumed);
}

Project Links

Resource	URL
Repository	github.com/phluid61/mk-hpack
Issue Tracker	GitHub Issues
GitHub Pages	phluid61.github.io/mk-hpack
Documentation	docs/

Documentation Guide

Audience	Start Here
New developers	Developer Guide
Architecture overview	Architecture
API reference	API
Build & release	Deployment
Configuration	Configuration
All documents	Documentation Index

Contributing

We require all contributors to comply with the Developer Certificate of Origin. This ensures that all contributions are properly licensed and attributed.

Contributor Code of Conduct

This repository is subject to a Contributor Code of Conduct adapted from the Contributor Covenant, version 3.0, available at https://www.contributor-covenant.org/version/3/0/

Licence

This project is licensed under the ISC licence. See LICENSE for details.

Copyright (c) 2014, 2021, 2026, Matthew Kerwin <matthew@kerwin.net.au>

Permission to use, copy, modify, and/or distribute this software for any
purpose with or without fee is hereby granted, provided that the above
copyright notice and this permission notice appear in all copies.

THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.

mk-hpack