> ## Documentation Index > Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt > Use this file to discover all available pages before exploring further. # C _Bool `_Bool` is a built-in unsigned integer type introduced in the C99 standard, designed specifically to represent boolean truth values. Unlike standard integer types, `_Bool` enforces strict boolean semantics at the language level: it is only capable of storing the values `0` or `1`. ## Syntax and Implicit Conversion The most defining characteristic of `_Bool` is its implicit conversion rules. When any scalar value (integer, floating-point, or pointer) is assigned to a `_Bool` variable, the compiler automatically normalizes the value. Any non-zero value or non-null pointer evaluates to `1`, while a zero value or null pointer evaluates to `0`. ```c theme={"dark"} _Bool flag_zero = 0; // Stores 0 _Bool flag_one = 1; // Stores 1 // Implicit conversion examples _Bool flag_int = 42; // Evaluates to 1 _Bool flag_neg = -99; // Evaluates to 1 _Bool flag_float = 0.001f; // Evaluates to 1 _Bool flag_null = '\0'; // Evaluates to 0 int* ptr = 0; // Null pointer _Bool flag_ptr = ptr; // Evaluates to 0 ``` This behavior prevents overflow issues that occur when using standard `int` or `char` types for boolean logic, where assigning a value like `256` to an 8-bit integer might truncate to `0`. ## Memory Representation The C standard dictates that `_Bool` must be large enough to store the values `0` and `1`. While the exact memory footprint is implementation-defined, `sizeof(_Bool)` is typically `1` byte (8 bits) on modern architectures. This is because `1` byte is the smallest individually addressable unit of memory for most CPUs. ```c theme={"dark"} #include int main(void) { // Typically outputs: Size of _Bool: 1 byte(s) printf("Size of _Bool: %zu byte(s)\n", sizeof(_Bool)); return 0; } ``` ## The `` Abstraction While `_Bool` is the native language keyword, C99 also introduced the `` header to provide a more conventional syntax that aligns with C++ and other modern languages. This header defines three macros that map directly to the `_Bool` type and its binary states. ```c theme={"dark"} #include /* Underlying macro definitions in : #define bool _Bool #define true 1 #define false 0 */ bool is_ready = true; bool is_done = false; ``` ## Integer Promotion Because `_Bool` is classified as an integer type, it is subject to standard integer promotion rules. When a `_Bool` is used in an arithmetic expression, it is promoted to an `int` before the operation is evaluated. ```c theme={"dark"} _Bool a = 1; _Bool b = 1; // 'a' and 'b' are promoted to int (1 + 1). // The result is 2, which is stored in 'result'. int result = a + b; // If assigned back to a _Bool, the result (2) is implicitly converted back to 1. _Bool combined = a + b; ```

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