`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...

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? — Juraj
– Juraj ♦, Commented Mar 24 at 7:44
@Juraj Thanks, You Got It!! The code sprintf (s, "%04ld(0x%03lx) : ", id, id ); work fine now! Don't need id2 anymore. :D — James
– James, Commented Mar 24 at 8:17

Edgar Bonet · Accepted Answer · 2025-03-24 08:05:43Z

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) : "

Yes! after change the code to sprintf (s, "%04ld(0x%03lx) : ", id, id ); , it work fine now! Thanks a lot. — James
– James, Commented Mar 24 at 8:21

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