#include <stdio.h>
int main()
{_Bool varA;varA = 2;printf("varA:%d.\n",varA);varA = -1;printf("varA:%d.\n",varA);varA = 0;printf("varA:%d.\n",varA);printf("Hello world!\n");return 0;
}$ gcc -o tof tof.c
$ ./tof
varA:1.
varA:1.
varA:0.
Hello world!

​
/* Copyright (C) 1998, 1999, 2000, 2009 Free Software Foundation, Inc.
This file is part of GCC.
GCC is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3, or (at your option)
any later version.
GCC is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.
Under Section 7 of GPL version 3, you are granted additional
permissions described in the GCC Runtime Library Exception, version
3.1, as published by the Free Software Foundation.
You should have received a copy of the GNU General Public License and
a copy of the GCC Runtime Library Exception along with this program;
see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
<Licenses- GNU Project - Free Software Foundation>.  */
/*
* ISO C Standard:  7.16  Boolean type and values  <stdbool.h>
*/
#ifndef _STDBOOL_H
#define _STDBOOL_H
#ifndef __cplusplus
#define bool        _Bool
#define true        1
#define false        0
#else /* __cplusplus */
/* Supporting <stdbool.h> in C++ is a GCC extension.  */
#define _Bool        bool
#define bool        bool
#define false        false
#define true        true
#endif /* __cplusplus */
/* Signal that all the definitions are present.  */
#define __bool_true_false_are_defined        1
#endif        /* stdbool.h */
​

#include <stdio.h>
#include <stdbool.h>
int main()
{bool varA;varA = 2;printf("varA:%d.\n",varA);varA = -1;printf("varA:%d.\n",varA);varA = 0;printf("varA:%d.\n",varA);varA = true;printf("varA:%d.\n",varA);varA = false;printf("varA:%d.\n",varA);printf("Hello world!\n");return 0;
}$ gcc -o tof tof.c
$ ./tof
varA:1.
varA:1.
varA:0.
varA:1.
varA:0.
Hello world!

#include <stdio.h>
int main()
{bool varA;printf("false:%d,true:%d.\n", false, true);varA = 2;printf("varA:%d.\n", varA);varA = -1;printf("varA:%d.\n", varA);varA = 0;printf("varA:%d.\n", varA);printf("Hello world!\n");return 0;
}$ g++ -o tofplus tof.cpp
$ ./tofplus
false:0,true:1.
varA:1.
varA:1.
varA:0.
Hello world!