A Boolean expression or condition is an expression whose value is interpreted as either true or false. Boolean conditions are expected in several places:

• as the controlling expression in a while loop,
• as the controlling expression in a do-while loop,
• as the controlling expression in a for loop,
• as the controlling expression in an if statement,
• as the expression in an #if or #elif directive,
• as the operand of the logical NOT operator !,
• as the operands of the logical AND operator &&,
• as the operands of the logical OR operator ||.

In these cases, any scalar expression may be given. A real value is first converted to an integer by truncation, and the integer value is compared with 0. If equal, the expression is considered false; otherwise true.

int is typically used to store or pass a Boolean value, with zero meaning false, and all other values meaning true. However, from C99, the native type _Bool is defined as a 1-bit unsigned integer type, capable of holding only 0 or 1. Any value of scalar type converted to _Bool is interpreted as 1 if it is non-zero.

The header <stdbool.h> then defines several macros to improve readability of Boolean values and types:

#define bool _Bool
#define false 0
#define true 1
#define __bool_true_false_are_defined 1

In <stdatomic.h>, atomic_bool is an alias for _Atomic _Bool. This type is lock-free always if ATOMIC_BOOL_LOCK_FREE is 2, sometimes if 1, and never if 0.