# C flexible array members

**C flexible array members (FAM)** allow a struct to have a variable-length array as its last member with size specified at runtime. Declare as `type array[]` with no size; allocate the struct plus extra space for array data.

Use FAM for variable-sized data structures without separate pointer indirection.

## Example

```c
// compile: gcc -o fam fam.c
// run: ./fam
// description: flexible array members for variable-size structs

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

struct Vector {
    int size;
    int capacity;
    int data[];  // Flexible array member
};

struct Vector *create_vector(int capacity) {
    struct Vector *v = malloc(sizeof(struct Vector) + capacity * sizeof(int));
    v->size = 0;
    v->capacity = capacity;
    return v;
}

void push(struct Vector *v, int value) {
    if (v->size < v->capacity) {
        v->data[v->size++] = value;
    }
}

int main() {
    struct Vector *v = create_vector(10);
    push(v, 1);
    push(v, 2);
    push(v, 3);
    
    printf("Vector size: %d\n", v->size);
    printf("Data: %d %d %d\n", v->data[0], v->data[1], v->data[2]);
    
    free(v);
    return 0;
}
```

## Common patterns

**Declaration**:
- `type array[]` as last member, no size specified
- Only valid as last member (C99)

**Allocation**:
- `malloc(sizeof(struct) + n * sizeof(type))`
- Extra space allocated for array elements
- All in one contiguous allocation

**Advantages over separate pointer**:
- Single allocation, better cache locality
- No extra pointer dereferencing
- Cleaner semantics for fixed-size arrays

**Limitations**:
- Must be last member
- Can't be nested in other structs
- Requires careful size calculation