Question

I need to be able to be able to send a numeric value to a remote socket server and so I need to encode possible numbers as bytes.

The numbers are up to 64 bit, ie requiring up to 8 bytes. The very first byte is the type, and it is always a number under 255 so fits in 1 byte.

For example, if the number was 8 and the type was a 32 bit unsigned integer then the type would be 7 which would be copied to the first (leftmost) byte and then the next 4 bytes would be encoded with the actual number (8 in this case).

So in terms of bytes:

byte1: 7
byte2: 0
byte3: 0
byte4: 0
byte5: 8

I hope this is making sense.

Does this code to perform this encoding look like a reasonable approach?

int type = 7;
uint32_t number = 8;

unsigned char* msg7 = (unsigned char*)malloc(5);
unsigned char* p = msg7;

*p++ = type;

 for (int i = sizeof(uint32_t) - 1; i >= 0; --i) 
    *p++ = number & 0xFF << (i * 8);  

Answer 1

You’ll want to explicitly cast type to avoid a warning:

*p++ = (unsigned char) type;

You want to encode the number with most significant byte first, but you’re shifting in the wrong direction. The loop should be:

for (int i = sizeof(uint32_t) - 1; i >= 0; --i)
    *p++ = (unsigned char) ((number >> (i * 8)) & 0xFF);

It looks good otherwise.