What is the printf format specifier for bool?

Since ANSI C99 there is _Bool or bool via stdbool.h. But is there also a printf format specifier for bool?

I mean something like in that pseudo code:

bool x = true;
printf("%B\n", x);

which would print:



There is no format specifier for bool types. However, since any integral type shorter than int is promoted to int when passed down to printf()'s variadic arguments, you can use %d:

bool x = true;
printf("%d\n", x); // prints 1

But why not:

printf(x ? "true" : "false");

or, better:

printf("%s", x ? "true" : "false");

or, even better:

fputs(x ? "true" : "false", stdout);


There is no format specifier for bool. You can print it using some of the existing specifiers for printing integral types or do something more fancy:

 printf("%s", x?"true":"false");

ANSI C99/C11 don't include an extra printf conversion specifier for bool.

But the GNU C library provides an API for adding custom specifiers.

An example:

#include <stdio.h>
#include <printf.h>
#include <stdbool.h>

static int bool_arginfo(const struct printf_info *info, size_t n,
    int *argtypes, int *size)
  if (n) {
    argtypes[0] = PA_INT;
    *size = sizeof(bool);
  return 1;
static int bool_printf(FILE *stream, const struct printf_info *info,
    const void *const *args)
  bool b =  *(const bool*)(args[0]);
  int r = fputs(b ? "true" : "false", stream);
  return r == EOF ? -1 : (b ? 4 : 5);
static int setup_bool_specifier()
  int r = register_printf_specifier('B', bool_printf, bool_arginfo);
  return r;
int main(int argc, char **argv)
  int r = setup_bool_specifier();
  if (r) return 1;
  bool b = argc > 1;
  r = printf("The result is: %B\n", b);
  printf("(written %d characters)\n", r);
  return 0;

Since it is a glibc extensions the GCC warns about that custom specifier:

$ gcc -Wall -g    main.c   -o main
main.c: In function ‘main’:
main.c:34:3: warning: unknown conversion type character ‘B’ in format [-Wformat=]
   r = printf("The result is: %B\n", b);
main.c:34:3: warning: too many arguments for format [-Wformat-extra-args]


$ ./main
The result is: false
(written 21 characters)
$ ./main 1
The result is: true
(written 20 characters)
Tags: C++ / C / Boolean

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