> ## 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 double The `double` keyword in C designates a double-precision floating-point data type used to represent fractional numbers. It is the default data type for floating-point literals in C and typically conforms to the IEEE 754 binary64 standard. ## Technical Specifications While the C standard does not strictly mandate the underlying representation, on virtually all modern architectures, `double` possesses the following characteristics: * **Memory Size:** 8 bytes (64 bits). * **Bit Layout (IEEE 754):** * 1 Sign bit * 11 Exponent bits * 52 Mantissa (significand) bits * **Precision:** 15 to 17 significant decimal digits. * **Range:** Approximately $\pm 2.225 \times 10^{-308}$ to $\pm 1.797 \times 10^{308}$. * **Standard Limits:** Defined in the `` header as `DBL_MIN` and `DBL_MAX`. ## Syntax and Initialization Floating-point literals without a suffix are treated as `double` by the compiler. They can be written in standard decimal notation or scientific (exponential) notation. ```c theme={"dark"} double uninitialized_var; double standard_notation = 3.141592653589793; double scientific_notation = 6.022e23; // 6.022 * 10^23 double negative_exponent = 1.6e-19; // 1.6 * 10^-19 ``` ## Format Specifiers Input and output operations via the standard I/O library (``) require specific format specifiers. There is a critical distinction between reading and writing a `double`. **Output (`printf`)** When passed to variadic functions like `printf`, `float` arguments are automatically promoted to `double`. Therefore, the standard specifier for printing a `double` is `%f` (or `%e` for scientific notation, `%g` for the shortest representation). ```c theme={"dark"} double val = 123.456; printf("%f\n", val); // Standard decimal output printf("%e\n", val); // Scientific notation: 1.234560e+02 ``` **Input (`scanf`)** When reading input, you must pass a pointer to the exact type. You **must** use the `%lf` (long float) specifier to read into a `double` variable. Using `%f` will result in undefined behavior, as `scanf` will attempt to write a 4-byte `float` into an 8-byte `double` memory space. ```c theme={"dark"} double input_val; // %lf is strictly required for scanf scanf("%lf", &input_val); ``` ## Type Promotion and Literals Because floating-point literals are `double` by default, assigning a literal to a standard `float` causes an implicit downcast, which may trigger compiler warnings regarding truncation. ```c theme={"dark"} // The literal 3.14 is an 8-byte double. // It is implicitly truncated to a 4-byte float here. float a = 3.14; // The 'f' suffix explicitly types the literal as a float. float b = 3.14f; // The literal matches the variable type perfectly. double c = 3.14; ``` During arithmetic operations involving both `float` and `double`, the `float` operand is implicitly promoted to `double` before the operation is evaluated, yielding a `double` result.

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