Variable Length Arrays (VLA) 变长数组

1. What is a VLA?

• Definition: An array where the size is determined at runtime (using a variable), not at compile-time.

• Standard: Introduced in C99.

• Clarification: "Variable Length" means the size is set when the array is created. Once created, the size cannot change (unlike std::vector in C++).

2. Syntax Comparison

Traditional Array (C89):
Size must be a constant (literal or #define).

#define SIZE 10
int arr[SIZE];   // OK
int arr2[10];    // OK

Variable Length Array (C99+):
Size can be a variable or an expression.

int n;
scanf("%d", &n); // User inputs size
int arr[n];      // OK: Size determined here

3. Key Characteristics

• Storage Location: Allocated on the Stack.

• Lifetime: Automatic. Created when execution enters the block, destroyed when it leaves.

• Initialization: Cannot be initialized in the declaration.

  • int arr[n] = {0}; (Error)

  • You must use a loop or memset to set values.

4. VLA as Function Arguments

VLAs are very useful for passing multidimensional arrays to functions without hardcoding dimensions.

// The dimensions (rows, cols) must come BEFORE the array
void printMatrix(int rows, int cols, int matrix[rows][cols]) {
    for (int i = 0; i < rows; i++) {
        for (int j = 0; j < cols; j++) {
            printf("%d ", matrix[i][j]);
        }
    }
}

5. Pros & Cons

Pros (Advantages)	Cons (Disadvantages)
Simple Syntax: Easier to read than malloc.	Stack Overflow: Large n can crash the program.
Automatic Memory: No need to call free().	No Initialization: Cannot use {1, 2, 3} syntax.
Fast: Stack allocation is faster than Heap.	Portability: Not supported in C++ or older C compilers (C89). C11 made it optional.

6. Comparison: VLA vs. malloc

Feature	VLA	malloc (Dynamic Memory)
Memory	Stack	Heap
Safety	Risky (Stack limit is small)	Safe (Handles large memory)
Cleanup	Automatic	Manual (Must use free)
Resizing	Impossible	Possible (using realloc)

7. Best Practices / Summary

• Use VLA when:

  • The array size is small.

  • You need a temporary buffer for a short calculation.

  • You are doing matrix math in a function.

• Avoid VLA when:

  • The array size might be very large (use malloc instead).

  • You are writing code that needs to be compatible with C++ or Visual Studio (MSVC).