Unzip a zip file using zlib

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\n");
else
    printf("unable to open file\n");

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

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

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.