How Does `array[100] = {0};` Initialize an Entire Array to Zero in C/C ?

Understanding the Magic Behind Array[100] = {0}; Initialization

When encountering the C/C code snippet array[100] = {0}, developers may wonder how the compiler initializes all array elements to zero effortlessly.

Compiler's Initialization Mechanism

This behavior is not some arcane magic but rather a well-defined aspect of the C and C specifications.

In C

As per section 6.7.8.21 of the C specification, uninitialized elements within an array will be initialized as follows:

• Pointers are set to NULL.
• Arithmetic types (including characters, which are promoted to int) are initialized to zero.

This applies recursively to nested arrays and structures.

In C

Section 8.5.1.7 of the C specification states that uninitialized elements within an array will be aggregate-initialized. Aggregate initialization initializes the element values to their default values, which for characters is zero.

Empty Initializer List

In C , you can also use an empty initializer list array[100] = {}; to trigger aggregate-initialization for all array elements.

Compiler Assembly Output

The specific assembly code generated by the compiler may vary based on the specific implementation. However, it typically involves initializing a portion of the array and copying the initialized values to the remaining elements.

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