Byte order functions on stm32 microcontroller

Question

Are there any written functions for htonll,ntohll,ntohl,htonl like this one i've found here on forum:

uint32_t ntohl(uint32_t const net) {
    uint8_t data[4] = {};
    memcpy(&data, &net, sizeof(data));

    return ((uint32_t) data[3] << 0)
         | ((uint32_t) data[2] << 8)
         | ((uint32_t) data[1] << 16)
         | ((uint32_t) data[0] << 24);
}

or could you just tell me data shifting scheme for each one and would write it just like this one above?

EDIT:

Are those correct:

uint32_t htonl(uint32_t const net) {
    uint8_t data[4] = {};
    memcpy(&data, &net, sizeof(data));

    return ((uint32_t) data[0] << 0)
         | ((uint32_t) data[1] << 8)
         | ((uint32_t) data[2] << 16)
         | ((uint32_t) data[3] << 24);
}

uint64_t htonll(uint64_t const net) {
    uint8_t data[4] = {};
    memcpy(&data, &net, sizeof(data));

    return ((uint64_t) data[0] << 0)
         | ((uint64_t) data[1] << 8)
         | ((uint64_t) data[2] << 16)
         | ((uint64_t) data[3] << 24);
}

uint64_t ntohll(uint64_t const net) {
    uint8_t data[4] = {};
    memcpy(&data, &net, sizeof(data));

    return ((uint64_t) data[3] << 0)
         | ((uint64_t) data[2] << 8)
         | ((uint64_t) data[1] << 16)
         | ((uint64_t) data[0] << 24);
}

score 3 · Accepted Answer · answered Dec 11 '19 at 09:26

This isn't STM32 specific but can be answered in general for all ARM MCU. Assuming you are using gcc, there are built-in functions like __builtin_bswap32.

So you can implement it as:

uint32_t htonl(uint32_t net)
{
    return __builtin_bswap32(net);
}

uint16_t htons(uint16_t net)
{
    return __builtin_bswap16(net);
}

uint64_t htonll(uint64_t net)
{
    return __builtin_bswap64(net);
}

The resulting assembler code is very efficient:

htonl:
        rev     r0, r0
        bx      lr
htons:
        rev16   r0, r0
        uxth    r0, r0
        bx      lr
htonll:
        rev     r3, r0
        rev     r0, r1
        mov     r1, r3
        bx      lr

If you declare the functions as inline, you can even save the function call.

BTW: In your code, the 64 bit versions are likely incorrect as they only return a 32 bit quantity.

Thanks.. What i need to change in your code to get ntohl and ntohll — mrunje, Dec 11 '19 at 13:32
`ntohl` does exactly the same as `htonl`, namely rearrange the bytes in reverse order. So the code is the same. The same applies to the other pairs. — Codo, Dec 11 '19 at 14:19

Byte order functions on stm32 microcontroller

1 Answers1