How to resolve C printf %b format warnings

Question

I am getting warning for below program

#include <stdio.h>

int main(void)
{
    int num = 0xff;
    printf("%B\n", num);
    printf("%b\n", ~num);
    printf("%b\n", ~num+1);
    return 0;
}

warning

gcc binary-format-print.c
binary-format-print.c: In function ‘main’:
binary-format-print.c:6:18: warning: unknown conversion type character ‘B’ in format [-Wformat=]
    6 |         printf("%B\n", num);
      |                  ^
binary-format-print.c:6:16: warning: too many arguments for format [-Wformat-extra-args]
    6 |         printf("%B\n", num);
      |                ^~~~~~
binary-format-print.c:7:18: warning: unknown conversion type character ‘b’ in format [-Wformat=]
    7 |         printf("%b\n", ~num);
      |                  ^
binary-format-print.c:7:16: warning: too many arguments for format [-Wformat-extra-args]
    7 |         printf("%b\n", ~num);
      |                ^~~~~~
binary-format-print.c:8:18: warning: unknown conversion type character ‘b’ in format [-Wformat=]
    8 |         printf("%b\n", ~num+1);
      |                  ^
binary-format-print.c:8:16: warning: too many arguments for format [-Wformat-extra-args]
    8 |         printf("%b\n", ~num+1);

ldd version,

❯ ldd --version
ldd (Ubuntu GLIBC 2.35-0ubuntu3.6) 2.35
Copyright (C) 2022 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
Written by Roland McGrath and Ulrich Drepper.

gcc version

❯ gcc --version
gcc (Ubuntu 11.4.0-1ubuntu1~22.04) 11.4.0
Copyright (C) 2021 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE

I suppose gnu C supports binary print format. Then why such warnings?

How to resolve these warning?

Thanks.

Does this answer your question? Is there a printf converter to print in binary format? — wohlstad
– wohlstad, Commented Feb 18, 2024 at 12:37
I have checked this answer before asking this question, this answer is of interest stackoverflow.com/a/70993946/2270386 but if the support is added then why warnings? — mrigendra
– mrigendra, Commented Feb 18, 2024 at 12:40
@Some programmer dude, Your info is outdated. It is currently standard "b,B,o,u,x,X The unsigned int argument is converted to unsigned binary (b or B), unsigned octal (o), unsigned decimal (u), or unsigned hexadecimal notation (x or X) in the style dddd; [...]" — ikegami
– ikegami, Commented Feb 18, 2024 at 12:46
@ikegami: The C 2023 draft is not the standard yet. ISO shows it as under development. — Eric Postpischil
– Eric Postpischil, Commented Feb 18, 2024 at 15:36

ikegami · Accepted Answer · 2024-02-18 12:39:35Z

3

You need gcc 12+

<source>: In function 'main':
<source>:6:14: warning: ISO C does not support the '%B' gnu_printf format [-Wformat=]
    6 |     printf("%B\n", num);
      |              ^
<source>:7:14: warning: ISO C17 does not support the '%b' gnu_printf format [-Wformat=]
    7 |     printf("%b\n", ~num);
      |              ^
<source>:8:14: warning: ISO C17 does not support the '%b' gnu_printf format [-Wformat=]
    8 |     printf("%b\n", ~num+1);
      |              ^

11111111
11111111111111111111111100000000
11111111111111111111111100000001

With -std=c2x,

<source>: In function 'main':
<source>:6:14: warning: ISO C does not support the '%B' gnu_printf format [-Wformat=]
    6 |     printf("%B\n", num);
      |              ^

11111111
11111111111111111111111100000000
11111111111111111111111100000001

answered Feb 18, 2024 at 12:39

ikegami

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1 Comment

mrigendra Over a year ago

with gcc-12 I see no warnings.

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