examples/dictionary_decompression.c

/*
 * Copyright (c) Meta Platforms, Inc. and affiliates.
 * All rights reserved.
 *
 * This source code is licensed under both the BSD-style license (found in the
 * LICENSE file in the root directory of this source tree) and the GPLv2 (found
 * in the COPYING file in the root directory of this source tree).
 * You may select, at your option, one of the above-listed licenses.
 */


#include <stdio.h>     // printf
#include <stdlib.h>    // free
#include <zstd.h>      // presumes zstd library is installed
#include "common.h"    // Helper functions, CHECK(), and CHECK_ZSTD()

/* createDict() :
   `dictFileName` is supposed to have been created using `zstd --train` */
static ZSTD_DDict* createDict_orDie(const char* dictFileName)
{
    size_t dictSize;
    printf("loading dictionary %s \n", dictFileName);
    void* const dictBuffer = mallocAndLoadFile_orDie(dictFileName, &dictSize);
    ZSTD_DDict* const ddict = ZSTD_createDDict(dictBuffer, dictSize);
    CHECK(ddict != NULL, "ZSTD_createDDict() failed!");
    free(dictBuffer);
    return ddict;
}

static void decompress(const char* fname, const ZSTD_DDict* ddict)
{
    size_t cSize;
    void* const cBuff = mallocAndLoadFile_orDie(fname, &cSize);
    /* Read the content size from the frame header. For simplicity we require
     * that it is always present. By default, zstd will write the content size
     * in the header when it is known. If you can't guarantee that the frame
     * content size is always written into the header, either use streaming
     * decompression, or ZSTD_decompressBound().
     */
    unsigned long long const rSize = ZSTD_getFrameContentSize(cBuff, cSize);
    CHECK(rSize != ZSTD_CONTENTSIZE_ERROR, "%s: not compressed by zstd!", fname);
    CHECK(rSize != ZSTD_CONTENTSIZE_UNKNOWN, "%s: original size unknown!", fname);
    void* const rBuff = malloc_orDie((size_t)rSize);

    /* Check that the dictionary ID matches.
     * If a non-zstd dictionary is used, then both will be zero.
     * By default zstd always writes the dictionary ID into the frame.
     * Zstd will check if there is a dictionary ID mismatch as well.
     */
    unsigned const expectedDictID = ZSTD_getDictID_fromDDict(ddict);
    unsigned const actualDictID = ZSTD_getDictID_fromFrame(cBuff, cSize);
    CHECK(actualDictID == expectedDictID,
          "DictID mismatch: expected %u got %u",
          expectedDictID,
          actualDictID);

    /* Decompress using the dictionary.
     * If you need to control the decompression parameters, then use the
     * advanced API: ZSTD_DCtx_setParameter(), ZSTD_DCtx_refDDict(), and
     * ZSTD_decompressDCtx().
     */
    ZSTD_DCtx* const dctx = ZSTD_createDCtx();
    CHECK(dctx != NULL, "ZSTD_createDCtx() failed!");
    size_t const dSize = ZSTD_decompress_usingDDict(dctx, rBuff, rSize, cBuff, cSize, ddict);
    CHECK_ZSTD(dSize);
    /* When zstd knows the content size, it will error if it doesn't match. */
    CHECK(dSize == rSize, "Impossible because zstd will check this condition!");

    /* success */
    printf("%25s : %6u -> %7u \n", fname, (unsigned)cSize, (unsigned)rSize);

    ZSTD_freeDCtx(dctx);
    free(rBuff);
    free(cBuff);
}


int main(int argc, const char** argv)
{
    const char* const exeName = argv[0];

    if (argc<3) {
        printf("wrong arguments\n");
        printf("usage:\n");
        printf("%s [FILES] dictionary\n", exeName);
        return 1;
    }

    /* load dictionary only once */
    const char* const dictName = argv[argc-1];
    ZSTD_DDict* const dictPtr = createDict_orDie(dictName);

    int u;
    for (u=1; u<argc-1; u++) decompress(argv[u], dictPtr);

    ZSTD_freeDDict(dictPtr);
    printf("All %u files correctly decoded (in memory) \n", argc-2);
    return 0;
}