c - Unzip a zip file using zlib

Question

I have an archive.zip which contains two crypted ".txt" files. I would like to decompress the archive in order to retrieve those 2 files.

Here's what I've done so far:

FILE *FileIn = fopen("./archive.zip", "rb");
if (FileIn)
    printf("file opened
");
else
    printf("unable to open file
");

fseek(FileIn, 0, SEEK_END);
unsigned long FileInSize = ftell(FileIn);
printf("size of input compressed file : %u
", FileInSize);

void *CompDataBuff = malloc(FileInSize);
void *UnCompDataBuff = NULL;

int fd = open ("archive.zip", O_RDONLY);
CompDataBuff = mmap(NULL, FileInSize, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
printf("buffer read : %s
", (char *)CompDataBuff);

uLongf UnCompSize = (FileInSize * 11/10 + 12);
UnCompDataBuff = malloc(UnCompSize);

int ret_uncp ;

ret_uncp = uncompress((Bytef*)UnCompDataBuff, &UnCompSize, (const Bytef*)CompDataBuff,FileInSize);
printf("size of uncompressed data : %u
", UnCompSize);

if (ret_uncp == Z_OK){
    printf("uncompression ok
");
    printf("uncompressed data : %s
",(char *)UnCompDataBuff);
    }
if (ret_uncp == Z_MEM_ERROR)
    printf("uncompression memory error
");
if (ret_uncp == Z_BUF_ERROR)
    printf("uncompression buffer error
");
if (ret_uncp == Z_DATA_ERROR)
    printf("uncompression data error
");

I always get "uncompression data error" and I don't know why. And then I would like to know how to retrieve the 2 files with my data uncompressed.

Answer 1

zip is a file format that wraps header and trailer information around compressed data streams in order to represent a set of files and directories. The compressed data streams are almost always deflate data streams, which can in fact be generated and decoded by zlib. zlib also provides the crc32 function which can be used to generate and check the crc values in the zip wrapper information.

What zlib does not do by itself is decode and deconstruct the zip structure. You can either write your own code to do that using the specification (not very hard to do), or you can use the minizip routines in the contrib/minizip directory of the zlib distribution, which provides functions to open, access, and close zip files.