How do you read in a 3 byte size value as an integer in c++?

Question

I'm reading in an id3 tag where the size of each frame is specified in 3 bytes. How would I be able to utilize this value as an int?

Solution 1

Read each byte and then put them together into your int:

int id3 = byte0 + (byte1 << 8) + (byte2 << 16);

Make sure to take endianness into account.

Solution 2

Read the bytes in individually, and put them into the correct places in an int:

int value = 0;

unsigned char byte1 = fgetc(ID3file);
unsigned char byte2 = fgetc(ID3file);
unsigned char byte3 = fgetc(ID3file);

value = (byte1 << 16) | (byte2 << 8) | byte3;

Edit: it appears that ID3 uses network (big-endian) byte order -- changed code to match.

Solution 3

If your process is computation intensive, and you want to save time (esp. if you are doing embedded systems), my suggestion will be to use union/struct. This takes away the load of left shift and right shift.

union{
unsigned int ui32;
struct{
    unsigned char ll;
    unsigned char lh;
    unsigned char hl;
    unsigned char hh;
    }splitter;
}combine;

combine.splitter.ll = 0x78;
combine.splitter.lh = 0x56;
combine.splitter.hl = 0x34;
combine.splitter.hh = 0x12;

printf("This is the combined Value: %d", combine.ui32);

This Should work. If anyone feels this should not be used, please let me know the reason.

Cheers! Aditya