Big endian conversion in Arduino

Projects Programming
1

Hi All,

I'm retrieving some numbers from a modbus connection and need to convert them to big endian for the proper values. What will be optimum way of doing this on an Arduino Uno. I tried htonl in below code but it doesn't compile saying there is no function called "htonl"

#include <Ethernet.h>


void setup() {
  Serial.begin(115200);
}

void loop() {
  unsigned long host_number = 0x12345678; // Example decimal number
  unsigned long big_endian_number = htonl(host_number);

  Serial.print("Host number (decimal): ");
  Serial.println(host_number, HEX);

  Serial.print("Big-endian number (hex): ");
  Serial.println(big_endian_number, HEX);

  delay(2000);
}
2

How did you come up with that function name? What is it supposed to do?

Did you just copy some code from the web?

unsigned long host_number = 0x12345678; // Example decimal number

The comment above is incorrect. The "0x" indicates hexadecimal representation.

3

It is a standard C function to convert from host to network order. Network order is big endian.

4

Try:

#include <arpa/inet.h>

or;

#include <netinet/in.h>

See also: htonl(3) - Linux man page

Must admit, not tried it on an Arduino.

5

Yup this was copied from the google AI generated seqrch snippet. I have seen in many places mentioning this fuction available in network.h or someother standard network library. However it seems not in Arduino. Any other suggestions for this?

6

Nope. Not working.

7

Indeed. Just tried myself. They come up automatically in the editor, but do not actually compile, which results in 'No such file or directory'. Sorry about that.

8

It is no problem to swap bytes in a 32 bit integer, but why not tell us what you are really trying to do?

1 Like
9

This is htonl

#ifndef htonl
#define htonl(x) ( ((x)<<24 & 0xFF000000UL) | \
                   ((x)<< 8 & 0x00FF0000UL) | \
                   ((x)>> 8 & 0x0000FF00UL) | \
                   ((x)>>24 & 0x000000FFUL) )
#endif

Found it in w5100.h which I have installed. You can simply put this in your code, no additional libraries required.

See Ethernet/src/utility/w5100.h at master Ā· arduino-libraries/Ethernet Ā· GitHub

2 Likes
10 
uint32_t __builtin_bswap32 (uint32_t x)
11

Thanks. This did work.