karliwson karliwson - 9 days ago 6
C Question

LzmaLib: compress / decompress buffer in C

I'm trying to use LzmaLib's

LzmaCompress()
and
LzmaDecompress()
with buffers, adapting the examples provided here.

I'm testing with a ~3MB buffer and the compression function seems to work fine (produces a ~1.2MB compressed buffer), but when I try to decompress, it just extracts ~300 bytes and returns
SZ_ERROR_DATA
.

The few extracted bytes are right, but I don't know why it stops there.

My code:

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

#include "LzmaLib.h"

void compress(
unsigned char **outBuf, size_t *dstLen,
unsigned char *inBuf, size_t srcLen)
{
unsigned propsSize = LZMA_PROPS_SIZE;
*dstLen = srcLen + srcLen / 3 + 128;

*outBuf = (unsigned char*)malloc(propsSize + *dstLen);

int res = LzmaCompress(
(unsigned char*)(*outBuf + LZMA_PROPS_SIZE), dstLen,
inBuf, srcLen,
*outBuf, &propsSize,
-1, 0, -1, -1, -1, -1, -1);

assert(res == SZ_OK);

*dstLen = *dstLen + LZMA_PROPS_SIZE;
}

void uncompress(
unsigned char **outBuf, size_t *dstLen,
unsigned char *inBuf, size_t srcLen
) {
*dstLen = 5000000;
*outBuf = (unsigned char*)malloc(*dstLen);

srcLen = srcLen - LZMA_PROPS_SIZE;
int res = LzmaUncompress(
*outBuf, dstLen,
(unsigned char*)(inBuf + LZMA_PROPS_SIZE), &srcLen,
inBuf, LZMA_PROPS_SIZE);

assert(res == SZ_OK);
}

void do_compress() {
FILE* file = fopen("Module.dll", "r");
size_t size, decSize;
unsigned char *data, *dec = NULL;

fseek(file, 0L, SEEK_END);
size = ftell(file);
fseek(file, 0L, SEEK_SET);

data = (unsigned char*)malloc(size);
fread(data, 1, size, file);
fclose(file);

compress((unsigned char**)&dec, &decSize, data, size);

file = fopen("Module.lzma", "w");
fwrite(dec, 1, decSize, file);
fclose(file);
}

void do_uncompress() {
FILE* file = fopen("Module.lzma", "r");
size_t size, decSize;
unsigned char *data, *dec = NULL;

fseek(file, 0L, SEEK_END);
size = ftell(file);
fseek(file, 0L, SEEK_SET);

data = (unsigned char*)malloc(size);
fread(data, 1, size, file);
fclose(file);

uncompress((unsigned char**)&dec, &decSize, data, size);

file = fopen("Module_DEC.dll", "w");
fwrite(dec, 1, decSize, file);
fclose(file);
}

int main()
{
do_compress();
do_uncompress();

return 0;
}


If this code is not the better way to compress buffers with LzmaLib, I'm happy to accept suggestions.

Answer

I bet the problem lurks in how you read/write your files. You need to open them in binary mode to prevent any substitutions during read/write operations.

Change all instances of:

  • fopen(xxx, "r") -> fopen(xxx, "rb")
  • fopen(xxx, "w") -> fopen(xxx, "wb")
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