libarchive primer of printing byte offsets/sizes for uncompressed entries in formats: TAR, ZIP, ISO

On 29 august 2024 libarchive does not support built-in calculation of byte offsets of uncompressed entries. So a hack is needed. This feature might hopefully be added natively to libarchive in the future (so the hack in this primer would become not needed), see issues referenced below. Hack was developed with help of libarchive maintainer in discussion in libarchive/libarchive#2283

libarhivedd.c is modeled after https://github.com/libarchive/libarchive/blob/master/examples/untar.c and https://github.com/libarchive/libarchive/wiki/Examples#A_Complete_Extractor, another useful example is https://github.com/libarchive/libarchive/blob/master/contrib/untar.c

# configures and build libarchive static lib and ./libarchivedd executable
make

# prepare test archives
# sudo apt install -y zip mkisofs
make libarchivedd.tar libarchivedd.zip libarchivedd.iso

./libarchivedd libarchivedd.tar
#dd if="libarchivedd.tar" of="libarchivedd.c" bs=1 skip=512 count=3756
#dd if="libarchivedd.tar" of="Makefile" bs=1 skip=5120 count=616

./libarchivedd libarchivedd.zip
#dd if="libarchivedd.zip" of="libarchivedd.c" bs=1 skip=72 count=3756
#dd if="libarchivedd.zip" of="Makefile" bs=1 skip=3894 count=616

./libarchivedd libarchivedd.iso
#false #. AE_IFDIR : 16384
#dd if="libarchivedd.iso" of="libarchivedd.c" bs=1 skip=63488 count=3756
#dd if="libarchivedd.iso" of="Makefile" bs=1 skip=67584 count=616

References

• libarchive/libarchive#2295
• libarchive/libarchive#2283

Bonus: helpers of extracting an entry to fd, FILE* and mem buffer

size_t extract_to_fd(struct archive* a, const void* firstblock_buff, size_t firstblock_size, int64_t firstblock_offset, int fd)
{
    (void)firstblock_offset;
    size_t size = write(fd, firstblock_buff, firstblock_size);
    for(;;)
    {
        int r = archive_read_data_block(a, &firstblock_buff, &firstblock_size, &firstblock_offset);
        if (r == ARCHIVE_EOF || r != ARCHIVE_OK)
	    break;
        assert(r == ARCHIVE_OK);
        size += write(fd, firstblock_buff, firstblock_size);
    }
    return size;
}

size_t extract_to_file(struct archive* a, const void* firstblock_buff, size_t firstblock_size, int64_t firstblock_offset, FILE* stream)
{
    (void)firstblock_offset;
    size_t size = fwrite(firstblock_buff, 1, firstblock_size, stream);
    for(;;)
    {
        int r = archive_read_data_block(a, &firstblock_buff, &firstblock_size, &firstblock_offset);
        if (r == ARCHIVE_EOF || r != ARCHIVE_OK)
	    break;
        assert(r == ARCHIVE_OK);
        size += fwrite(firstblock_buff, 1, firstblock_size, stream);
    }
    return size;
}

size_t extract_to_mem(struct archive* a, const void* firstblock_buff, size_t firstblock_size, int64_t firstblock_offset, void* buf, size_t cnt)
{
    (void)firstblock_offset;
    size_t size = firstblock_size <= cnt ? firstblock_size : cnt;
    memcpy(buf, firstblock_buff, size);
    cnt -= size;
    for(;;)
    {
        int r = archive_read_data_block(a, &firstblock_buff, &firstblock_size, &firstblock_offset);
        if (r == ARCHIVE_EOF || r != ARCHIVE_OK)
	    break;
        assert(r == ARCHIVE_OK);
        size_t w = firstblock_size <= cnt ? firstblock_size : cnt;
        memcpy((char*)buf + size, firstblock_buff, w);
        cnt -= w;
    }
    return size;
}

// ... 

//extract_to_fd(a, firstblock_buff, firstblock_len, firstblock_offset, 2);
//extract_to_file(a, firstblock_buff, firstblock_len, firstblock_offset, stderr);
//char tmp[1024]; size_t size = extract_to_mem(a, firstblock_buff, firstblock_len, firstblock_offset, tmp, 1024); tmp[size <= 1023 ? size : 1023] = '\0'; fprintf(stderr, "%s", tmp);