Parse Bytes Into Struct Attribute (int16_t) In C

Question

I am trying to parse these bytes:

00000000: 4353 4333 3630 4653 >0200< >0000 1900< 0000

Into uint16_t and uint32_t struct attributes respectively.

The struct at hand:

typedef struct __packed {
uint8_t fs_id [8];
uint16_t block_size;
uint32_t file_system_block_count;
uint32_t fat_start_block;
//and so on

Taking inspiration from Selbie's solution I've written:

read(file_descriptor, &blk.fs_id, sizeof(blk.fs_id));
read(file_descriptor, &blk.block_size, sizeof(blk.block_size));
read(file_descriptor, &blk.file_system_block_count, sizeof(blk.file_system_block_count));
// and so on for additional attributes.

Unfortunately, upon debugging block_size is only holding the value "2" when it is supposed to be holding uint16_t 512. Similarly, file_system_block_count is holding "1638400" when it should be holding "6400" :(

How do I get the read() function to include the "00" after the "02" and therefore read it as 512? I'd imagine it may require memcpy()?

Reading binary data isn't portable, because of different byte orderings. — Barmar, Mar 24 '22 at 22:51
Your file is in big-endian format, but your CPU is little-endian. — Barmar, Mar 24 '22 at 22:54
Yes, I'm aware of that. This will be running exclusively on Intel's architecture. Ahhh thanks for the clarification — Chris Kouts, Mar 24 '22 at 22:54
If you use the same architecture to write and read the file, you should get the expected results. So the question is why it was written as `0200` instad of `0002` if you exclusively use Intel. — Barmar, Mar 24 '22 at 22:55
@Barmar I'm compiling using WSL(ubuntu), I'd imagine that's my issue? — Chris Kouts, Mar 24 '22 at 22:58
It shouldn't matter. If both the reader and writer are compiled with the same compiler, they should be consistent. — Barmar, Mar 24 '22 at 22:59
@Barmar are you aware of how to make the conversion to big-endian? — Chris Kouts, Mar 24 '22 at 23:03
Use `htons()` and `htonl()` for 16-bit and 32-bit numbers, respectively. — Barmar, Mar 24 '22 at 23:04
@Barmar thank you so much you are amazing. Have a great day of coding and may the force be with you! :) — Chris Kouts, Mar 24 '22 at 23:09

score 0 · Accepted Answer · answered Mar 24 '22 at 23:14

Thanks to @barmar a simple call to htons() and htonl() for 16-bit and 32-bit respectivley is required.

memcpy(&blk.block_size, buf+8, 2);
blk.block_size = htons(blk.block_size);

memcpy(&blk.file_system_block_count, buf+10, 4);
blk.file_system_block_count = htonl(blk.file_system_block_count);

This results in blk.block_size and blk.file_system_block_count correctly holding the value "512" and "6400" respectively.

Parse Bytes Into Struct Attribute (int16_t) In C

1 Answers1