`sprintf` of %04d and %03x at the same time on Arduino

Question

I have this code, work fine!

uint32_t id , id2;
char s[64]; // Should be enough
... 
id2 = id = CAN.getCanId();
sprintf (s, "%04d : ", id ); // !!! HERE !!!
Serial.print(s);

the output work FINE :

0031 : ...
2047 : ...
0031 : ...
2047 : ...
...

change to this line :

sprintf (s, "(0x%03x) : ", id2 );

work FINE , TOO :

(0x01f) : ...
(0x7ff) : ...
(0x01f) : ...
(0x7ff) : ...

but, IF I WROTE this :

sprintf (s, "%04d(0x%03x) : ", id , id2 ); 

the output:

0031(0x000) : ...
2047(0x000) : ...
0031(0x000) : ...
2047(0x000) : ...

WHY? Why the %03x disappear ??

NOTE
I had include these :

#include <stdio.h>
#include <print.h>
#include <string.h>
#include <SoftwareSerial.h>
#include <Arduino.h> // HC-05 BT
#include "DFRobot_MCP2515.h"

---

I know I can wote like this to solve :

 sprintf (s, "%04d", id );
 Serial.print(s);
 sprintf (s, "(0x%03x) : ", id2 );
 Serial.print(s);

I'm not newbie on C,
It just make me feels... weird...

• Juraj: why do you use uint32_t instead of int? do you plan to use larger values? do your tests work for values larger than 16 bit?
• James: @Juraj Thanks, You Got It!! The code sprintf (s, "%04ld(0x%03lx) : ", id, id ); work fine now! Don't need id2 anymore. :D

Answer 1

Juraj give you a good hint at the solution. If you enable compiler warnings, the compiler will also tell you what is wrong:

warning: format ‘%d’ expects argument of type ‘int’, but argument 3
has type ‘uint32_t {aka long unsigned int}’ [-Wformat=]
 sprintf(s, "%04d(0x%03x) : ", id, id2);
 ^
warning: format ‘%x’ expects argument of type ‘unsigned int’, but
argument 4 has type ‘uint32_t {aka long unsigned int}’ [-Wformat=]

You are providing sprintf() two 4-byte integers. You are also telling it (via the format string) to read two 2-byte integers. The result is that it reads 4 bytes (the first integer) and interprets it as two numbers. For example, the number 31 (0x0000001f) is interpreted as 0x001f (i.e. 31) and 0x0000, in LSB-first order.

Solutions: Either you use plain int variables, or you tell sprintf() that you are giving it long integers instead, using the l prefix to the format: "%04ld(0x%03lx) : "

• James: Yes! after change the code to sprintf (s, "%04ld(0x%03lx) : ", id, id ); , it work fine now! Thanks a lot.