Q: "[W]hich platforms/C-standard-library implementations support "%b"? And - can I test for this support?"

A: glibc has supported %b integer conversion since version 2.35.

I don't have a list of which C libs support %b and which do not. I know that glibc (post ver 2.35) does, and I know that the picolibc does because I use them (i.e. I'm not a know-it-all :)

As far as testing, I guess there are two ways:

1. For glibc, check the version as follows; e.g.:

ldd --version
ldd (Debian GLIBC 2.36-9+rpt2+deb12u3) 2.36
...

2. For other C libraries, perhaps the best way is to compile a simple application in your preferred development environment; e.g.:

#include <stdio.h>
int main() {
    int int_val = 39;
    printf("binary value of int_val: %#b \n", int_val);

//  ANSWER: int_val = 0b100111 ; note no leading zeros

}

The reason for the discrepancy in your results is that the Raspberry Pi C SDK uses a different C library than the one on your Ubuntu system. Specifically, the picolibc library is used by the SDK for code compiled for a Pico target system. This would explain why you get no errors or warnings when the target is a Pico.

OTOH, glibc is the C library used for compilations targeted for Ubuntu. This is a bit of a guess, but I can imagine that since the first version of glibc to support the %b format was 2.35 (the version on your Ubuntu system), there was some unsettled business between gcc and glibc that resulted in a warning. I say this because earlier versions of glibc (e.g. ver 2.31) and gcc (ver 10.2.1) produced an error instead of a warning upon seeing the %b conversion in printf, and compile without error or warning w/ glibc ver 2.36 and gcc ver 12.2.

UPDATE: As of C23, the below function is no longer required. New format specifiers %b and %B have been added to printf() and friends to format integers as binary numbers—the former omitting any leading zeros and the latter including them (similar to %x and %X).

Original answer:

The reason you're having trouble finding a format specifier for printing integers in binary is because there isn't one. You'll have to write you're own function to print numbers in binary.

So for a single unsigned byte:

#include <stdio.h>
#include <limits.h>

void print_bin(unsigned char byte)
{
    int i = CHAR_BIT; /* however many bits are in a byte on your platform */
    while(i--) {
        putchar('0' + ((byte >> i) & 1)); /* loop through and print the bits */
    }
}

And for a standard unsigned int:

#include <stdio.h>
#include <limits.h>

void print_bin(unsigned int integer)
{
    int i = CHAR_BIT * sizeof integer; /* however many bits are in an integer */
    while(i--) {
        putchar('0' + ((integer >> i) & 1)); 
    }
}

Adjust the function for larger integers accordingly. Be wary of signed shifting though because the behavior is undefined and entirely compiler dependent.