Question

I have something like this.

char header[4096]; //header information from a file

there is a field called startByteOffset which is 8 bytes (offset in header is 8 too) and then there is endByteOffset which is 8 bytes in lenght too.(offset in header is 16).
I need to change the endByteOffset. How to do that? Thanks.
Let me know if it is not clear enough.

Answer 1

To break an integer into characters (bytes) you can use shifting and masking.

uint64_t value = /* ... */;
header[endbyteoffset]   = value         & 0xff;
header[endbyteoffset+1] = (value >> 8)  & 0xff;
header[endbyteoffset+2] = (value >> 16) & 0xff;
header[endbyteoffset+3] = (value >> 24) & 0xff;
// ...
header[endbyteoffset+7] = (value >> 56) & 0xff;

You’ll need to know if the values are to be stored in little-endian or big-endian order. The example I provided above is for little-endian; for big-endian you just reverse the order of the shifts, starting with the largest shift first.