When any scalar value is converted to _Bool, the result is 0 if the value compares equal
to 0; otherwise, the result is 1.
NaNs do not compare equal to 0 and thus convert to 1.
[edit] Example
#include <stdio.h>
int main(void)
{
_Bool a = 0.0;
_Bool b = 8;
_Bool c = 0.0/0.0;
printf("%d\n", a);
printf("%d\n", b);
printf("%d\n", c);
return 0;
}Possible output:
