What is the printf format specifier for bool?

549
votes

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:

true
8
c++cbooleanprintf
You can read this for further information cplusplus.com/reference/cstdio/printf You can always make it though! – Varvarigos Emmanouil
possible duplicate of Best way to print the result of a bool as 'false' or 'true' in c? – billinkc
@billinkc, my question is not really about what the best way is to print bool values - it is about a concrete printf specifier. Which does not seem to exist. Another angle to a nice answer would be: perhaps there is a way to add a custom format specifier to printf that does the bool conversion ... – maxschlepzig
Fair enough, although I don't seem to have the ability to uncast the VtC so I'll just have to wait for my vote to expire. – billinkc
@maxschlepzig: the only way to solve the problem is to check documentation. If you use GNU/Linux(as example, since you did not tell us about your system), you can read an up-to-date printf manual on [Linux man pages](man7.org). If you want get "true"/"false" strings printed, you can construct them manually, it is pretty easy. – Bulat M.

8 Answers

843
votes

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);

instead?

49
votes

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");
35
votes

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]

Output:

$ ./main
The result is: false
(written 21 characters)
$ ./main 1
The result is: true
(written 20 characters)
12
votes

In the tradition of itoa():

#define btoa(x) ((x)?"true":"false")

bool x = true;
printf("%s\n", btoa(x));
5
votes

You can't, but you can print 0 or 1

_Bool b = 1;
printf("%d\n", b);

source

3
votes

To just print 1 or 0 based on the boolean value I just used:

printf("%d\n", !!(42));

Especially useful with Flags:

#define MY_FLAG (1 << 4)
int flags = MY_FLAG;
printf("%d\n", !!(flags & MY_FLAG));
1
votes

If you like C++ better than C, you can try this:

#include <ios>
#include <iostream>

bool b = IsSomethingTrue();
std::cout << std::boolalpha << b;
0
votes

I prefer an answer from Best way to print the result of a bool as 'false' or 'true' in c?, just like

printf("%s\n", "false\0true"+6*x);
  • x == 0, "false\0true"+ 0" it means "false";
  • x == 1, "false\0true"+ 6" it means "true";